View source for Smog

{{Short description|Smoke-like, fog-like air pollutions}}
{{Other uses}}
{{Use dmy dates|date=September 2020}}
[[File:Fanhe Town 10 day interval contrast.png|alt=A hazy cityscape to the right and a clear one to the left|thumb|Smog and a sunny day within a 10-day interval in [[Fanhe Town, Tieling|Fanhe]], China]]
{{Pollution sidebar|Air}}
{{external media | width = 210px | float = right | headerimage=  | audio1 = [https://www.sciencehistory.org/distillations/podcast/fighting-smog-in-los-angeles "Fighting Smog in Los Angeles"], ''Distillations'' Podcast, 2018 [[Science History Institute]]}}

'''Smog''', or '''smoke fog''', is a type of intense [[air pollution]]. The word "smog" was coined in the early 20th century, and is a [[portmanteau]] of the words ''[[smoke]]'' and ''[[fog]]''<ref>{{cite book|title=Globalistics and Globalization Studies: Global Transformations and Global Future|vauthors=Kornienko O, Grinin L, Ilyin I, Herrmann P, Korotayev A|pages=220–225|url=https://hal-hprints.archives-ouvertes.fr/hprints-01794187/document|isbn=978-5-7057-5026-9|year=2016|place=Volgograd|publisher=[[Uchitel Publishing House]]|chapter=Social and Economic Background of Blending|chapter-url=https://www.socionauki.ru/upload/socionauki.ru/book/files/globalistics_and_globalization_studies_5_en/220-225.pdf}}</ref> to refer to smoky fog due to its opacity, and odour.<ref>{{cite book|title=A Social History of American Technology|last=Schwartz Cowan|first=Ruth|author-link=Ruth Schwartz Cowan |publisher=Oxford University Press|year=1997|isbn=978-0-19-504605-2}}{{page needed|date=October 2013}}</ref>  The word was then intended to refer to what was sometimes known as [[pea soup fog]], a familiar and serious problem in [[London]] from the 19th century to the mid-20th century, where it was commonly known as a '''London particular''' or '''London fog'''. This kind of visible air pollution is composed of [[nitrogen oxide]]s, [[sulfur oxide]], [[ozone]], smoke and other [[Particulate matter|particulates]]. Man-made smog is derived from coal combustion emissions, vehicular emissions, industrial emissions, forest and agricultural fires and photochemical reactions of these emissions.

Smog is often categorized as being either [[summer smog]] or winter smog. Summer smog is primarily associated with the photochemical formation of ozone. During the summer season when the temperatures are warmer and there is more sunlight present, photochemical smog is the dominant type of smog formation. During the winter months when the temperatures are colder, and [[Inversion (meteorology)|atmospheric inversions]] are common, there is an increase in coal and other fossil fuel usage to heat homes and buildings. These combustion emissions, together with the lack of pollutant [[Atmospheric dispersion modeling|dispersion]] under inversions, characterize winter smog formation. Smog formation in general relies on both primary and secondary pollutants. Primary pollutants are emitted directly from a source, such as emissions of [[sulfur dioxide]] from coal combustion. Secondary pollutants, such as ozone, are formed when primary pollutants undergo chemical reactions in the atmosphere.

Photochemical smog, as found for example in Los Angeles, is a type of air pollution derived from [[vehicular emission]] from [[internal combustion engine]]s and industrial fumes. These pollutants react in the atmosphere with sunlight to form secondary pollutants that also combine with the primary emissions to form [[photochemical smog]]. In certain other cities, such as Delhi, smog severity is often aggravated by [[stubble burning]] in neighboring agricultural areas since the 1980s. The atmospheric pollution levels of [[Los Angeles]], [[Beijing]], [[Delhi]], [[Lahore]], [[Mexico City]], [[Tehran]] and other cities are often increased by an inversion that traps pollution close to the ground. The developing smog is [[toxic]] to humans and can cause severe sickness, a shortened life span, or premature death.

== Etymology ==
Coinage of the term "smog" has been attributed to [[Henry Antoine Des Voeux]] in his 1905 paper, "Fog and Smoke" for a meeting of the [[Public Health Congress]]. The 26 July 1905 edition of the London newspaper ''Daily Graphic'' quoted Des Voeux, "He said it required no science to see that there was something produced in great cities which was not found in the country, and that was smoky fog, or what was known as 'smog'."<ref>{{cite book |url=http://www.psi.ch/ceg/PublicationsEN/Piazzesi,_PhD_thesis,_ETH_Zurich,_2006.pdf |first=Gaia |last=Piazzesi |year=2006 |title=The Catalytic Hydrolysis of Isocyanic Acid (HNCO) in the Urea-SCR Process |type=PhD Thesis |publisher=[[ETH Zurich]] |access-date=25 October 2013 |archive-date=1 December 2017 |archive-url=https://web.archive.org/web/20171201131651/https://www.psi.ch/ceg/PublicationsEN/Piazzesi,_PhD_thesis,_ETH_Zurich,_2006.pdf }}</ref> The following day the newspaper stated that "Dr. Des Voeux did a public service in coining a new word for the London fog."

However, the term appeared twenty-five years earlier than Voeux's paper, in the Santa Cruz & Monterey Illustrated Handbook published in 1880<ref>{{cite book|url=https://books.google.com/books?id=AcZBAQAAMAAJ&pg=PA8|first=Henry|last=Meyrick|year=1880|title=Santa Cruz & Monterey Illustrated Handbook|publisher=San Francisco News Publishing Co.|pages=7–8|quote=It is really not fog at all, but cloud of pure white mist, warmer and much less wetting than a "Scotch mist," and differing entirely from the true British fog, facetiously spelled "smog," because always colored and strongly impregnated with smoke, a mixture as unwholesome as it is unpleasant.}}</ref>  and also appears in print in a column quoting from the book in the 3 July 1880, Santa Cruz Weekly Sentinel.<ref>{{cite news|url=https://cdnc.ucr.edu/?a=d&d=SCWS18800703.1.3&srpos=41&e=-------en--20--41-byDA-txt-txIN-smog-------1|newspaper=Santa Cruz Weekly Sentinel|title=The morning fog|date=3 July 1880|access-date=18 September 2019|page=3|quote=It is really not fog at all, but cloud of pure white mist. warmer and much less wetting than a "Scotch Mist," not differing entirely from the true British fog, facetiously spelled "smog" because always colored and strongly impregnated with smoke, a mixture as unwholesome as it is unpleasant.|archive-date=14 April 2021|archive-url=https://web.archive.org/web/20210414035444/https://cdnc.ucr.edu/?a=d&d=SCWS18800703.1.3&srpos=41&e=-------en--20--41-byDA-txt-txIN-smog-------1}}</ref> On 17 December 1881, in the publication ''Sporting Times,'' the author claims to have invented the word: "The 'Smog'{{snd}}a word I have invented, combined of smoke and fog, to designate the London atmosphere..."<ref>Playhouses without Plays, Sporting Times, London, 17 December 1881, p6. Accessed 12 September 2020, The British Newspaper Archive.</ref>

==Anthropogenic causes==
===Coal===
Coal fire can emit significant clouds of smoke that contribute to the formation of winter smog. Coal fires can be used to heat individual buildings or to provide energy in a power-producing plant. Air pollution from this source has been reported in England since the [[Middle Ages]].<ref name="middleages3">{{cite web|url=http://www.environmentalgraffiti.com/offbeat-news/environmentalism-in-1306/725|title=Environmentalism in 1306|author=Chris|year=2007|publisher=By Environmental Graffiti|archive-url=https://web.archive.org/web/20080725090103/http://www.environmentalgraffiti.com/offbeat-news/environmentalism-in-1306/725|archive-date=25 July 2008}}</ref><ref name="middleages22">{{cite web|url=http://scribol.com/art-and-design/green-design/environmentalism-in-1306/|title=Environmentalism in 1306|author=Karl|year=2008|publisher=By Environmental Graffiti|access-date=17 February 2017|archive-date=13 November 2019|archive-url=https://web.archive.org/web/20191113013723/https://scribol.com/art-and-design/green-design/environmentalism-in-1306/}}</ref> London, in particular, was notorious up through the mid-20th century for its coal-caused smogs, which were nicknamed "[[Pea soup fog|pea-soupers]]". Air pollution of this type is still a problem in areas that generate significant smoke from burning coal. The emissions from coal combustion are one of the main causes of [[air pollution in China]].<ref>{{cite book|url=https://www.merics.org/en/merics-analysis/chinas-political-system/|title=China's Political System|editor-last=Heilmann|editor-first=Sebastian |publisher=Rowman & Littlefield|year=2017|page=360|archive-url=https://web.archive.org/web/20161011093650/http://www.merics.org/en/merics-analysis/chinas-political-system/|archive-date=11 October 2016}}</ref> Especially during autumn and winter when coal-fired heating ramps up, the amount of produced smoke at times forces some Chinese cities to close down roads, schools or airports. One prominent example for this was China's Northeastern city of [[2013 Harbin smog|Harbin in 2013]].

===Transportation emissions===
Traffic emissions – such as from [[truck]]s, [[bus]]es, and [[automobile]]s – also contribute to the formation of smog.<ref>{{cite web|url=http://www.transact.org/report.asp?id=227|title=Clearing the Air|date=19 August 2003|publisher=The Surface Transportation Policy Project|archive-url=https://web.archive.org/web/20070208230135/http://www.transact.org/report.asp?id=227|archive-date=8 February 2007|access-date=26 April 2007}}</ref> Airborne [[by-product]]s from vehicle [[exhaust system]]s and air conditioning cause [[air pollution]] and are a major ingredient in the creation of smog in some large cities.<ref>{{cite press release|title=EPA Tools Available as Summer Smog Season Starts|date=30 April 2008|publisher=United States Environmental Protection Agency|location=Boston, Massachusetts|url=http://yosemite.epa.gov/opa/admpress.nsf/6427a6b7538955c585257359003f0230/228d9bf38da42f7c8525743b006db951!OpenDocument&Start=1&Count=5&Expand=1|archive-url=https://archive.today/20120805235601/http://yosemite.epa.gov/opa/admpress.nsf/6427a6b7538955c585257359003f0230/228d9bf38da42f7c8525743b006db951!OpenDocument&Start=1&Count=5&Expand=1|archive-date=5 August 2012}}</ref><ref>{{cite web|url=http://www.sierraclub.org/sprawl/report01/carsandtrucks.asp|title=Sprawl Report 2001: Measuring Vehicle Contribution to Smog|year=2001|publisher=Sierra Club|access-date=25 October 2013|archive-date=15 November 2001|archive-url=https://web.archive.org/web/20011115044348/http://www.sierraclub.org/sprawl/report01/carsandtrucks.asp}}</ref><ref>{{cite web|url=http://library.thinkquest.org/26026/Environmental_Problems/smog_-_causes.html|title=Smog – Causes|work=The Environment: A Global Challenge|access-date=25 October 2013|archive-date=19 January 2001|archive-url=https://web.archive.org/web/20010119154200/http://library.thinkquest.org/26026/Environmental_Problems/smog_-_causes.html}}</ref><ref name=epa>{{Cite report|date=July 1999|title=Smog — Who Does It Hurt? What You Need to Know About Ozone and Your Health (EPA-452/K-99-001)|url=http://www.epa.gov/air/ozonepollution/pdfs/smog.pdf|publisher=United States Environmental Protection Agency|archive-url=https://web.archive.org/web/20080328164916/http://www.epa.gov/air/ozonepollution/pdfs/smog.pdf|archive-date=28 March 2008}}</ref>

The major culprits from transportation sources are [[carbon monoxide]] (CO),<ref>{{cite web|url=https://www3.epa.gov/cgi-bin/broker?polchoice=CO&_debug=0&_service=data&_program=dataprog.national_1.sas|title=State and County Emission Summaries: Carbon Monoxide|date=25 October 2013|work=Air Emission Sources|publisher=United States Environmental Protection Agency}}</ref><ref name="queensland2">{{cite web|url=http://www.tmr.qld.gov.au/Community-and-environment/Environmental-management/How-you-can-make-a-difference/Motor-vehicle-pollution.aspx|title=Motor vehicle pollution|date=4 April 2013|publisher=Queensland Government|access-date=25 October 2013|archive-date=5 December 2019|archive-url=https://web.archive.org/web/20191205161028/https://www.tmr.qld.gov.au/Community-and-environment/Environmental-management/How-you-can-make-a-difference/Motor-vehicle-pollution.aspx}}</ref> [[nitrogen oxides]] ([[Nitric oxide|NO]] and [[NO2|NO<sub>2</sub>]])<ref name="epa-no22">{{cite web|url=http://www.epa.gov/air/nitrogenoxides/health.html|archive-url=https://archive.today/20120802140219/http://www.epa.gov/air/nitrogenoxides/health.html|archive-date=2 August 2012|title=Health|date=14 February 2013|work=Nitrogen Dioxide|publisher=United States Environmental Protection Agency}}</ref><ref>{{cite web|url=http://www.epa.gov/air/noxfacts.pdf|archive-url=https://web.archive.org/web/20011110030903/http://www.epa.gov/air/noxfacts.pdf|archive-date=10 November 2001|title=The Regional Transport of Ozone: New EPA Rulemaking on Nitrogen Oxide Emissions (EPA-456/F-98-006)|date=September 1998|publisher=United States Environmental Protection Agency}}</ref><ref name="EPA_NOx2">{{cite web|url=http://www.epa.gov/cgi-bin/broker?_service=data&_debug=0&_program=dataprog.national_1.sas&polchoice=NOX|archive-url=https://web.archive.org/web/20111010005055/http://www.epa.gov/cgi-bin/broker?polchoice=NOX&_debug=0&_service=data&_program=dataprog.national_1.sas|archive-date=10 October 2011|title=State and County Emission Summaries: Nitrogen Oxides|date=25 October 2013|work=Air emission sources|publisher=United States Environmental Protection Agency}}</ref> and volatile organic compounds<ref name="queensland2" /><ref name="epa-no22" /> including [[hydrocarbons]] (hydrocarbons are the main component of [[petroleum fuels]] such as [[gasoline]] and [[diesel fuel]]).<ref name="queensland2" /> Transportation emissions also include [[sulfur dioxides]] and particulate matter but in much smaller quantities than the pollutants mentioned previously. The nitrogen oxides and volatile organic compounds can undergo a series of chemical reactions with sunlight, heat, [[ammonia]], moisture, and other compounds to form the noxious vapors, [[ground level ozone]], and particles that comprise smog.<ref name="queensland2" /><ref name="epa-no22" />

== Photochemical smog ==
[[File:Photochemical_smog_formation.png|thumb|Photochemical smog formation<ref>{{Cite web |url=https://www.mrgscience.com/ess-topic-63-photochemical-smog.html |title=ESS Topic Smog |website=Amazing World of Science With Mr. Green |language=en |access-date=19 September 2019 |archive-url=https://web.archive.org/web/20190610132608/https://www.mrgscience.com/ess-topic-63-photochemical-smog.html |archive-date=10 June 2019 |at=figure 6.3.U3}}</ref>]]

Photochemical smog, often referred to as "summer smog", is the chemical reaction of sunlight, [[nitrogen oxides]] and [[volatile organic compound]]s in the atmosphere, which leaves [[Atmospheric particulate matter|airborne particles]] and [[ground-level ozone]].<ref>{{Cite web|title=Nox/VOC Smog Fact Sheet|url=http://www.ccme.ca/assets/pdf/pn_1257_e.pdf|publisher=Canadian Council of Ministers of the Environment|archive-url=https://web.archive.org/web/20110928160543/http://www.ccme.ca/assets/pdf/pn_1257_e.pdf|archive-date=28 September 2011}}</ref> Photochemical smog depends on primary pollutants as well as the formation of secondary pollutants. These primary pollutants include [[nitrogen oxide]]s, particularly [[nitric oxide]] (NO) and [[nitrogen dioxide]] (NO<sub>2</sub>), and volatile organic compounds. The relevant secondary pollutants include [[Peroxyacetyl nitrates|peroxyacetyl nitrates]] (PANs), [[tropospheric ozone]], and [[aldehyde]]s. An important secondary pollutant for photochemical smog is ozone, which is formed when hydrocarbons (HC) and nitrogen oxides (NO<sub>x</sub>) combine in the presence of sunlight; nitrogen dioxide (NO<sub>2</sub>), which is formed as nitric oxide (NO) combines with oxygen (O<sub>2</sub>) in the air.<ref>{{Cite web|url=https://www.pmfias.com/smog-sulfurous-smog-photochemical-smog-london-smog-los-angeles-smog-effects-geography-upsc-ias/|title=Smog: Photochemical smog & Sulfurous smog|date=4 January 2016}}</ref> In addition, when SO<sub>2</sub> and NO<sub>x</sub> are emitted they eventually are oxidized in the troposphere to [[nitric acid]] and [[sulfuric acid]], which, when mixed with water, form the main components of acid rain.<ref>{{Cite web|url=https://www.englishnotes4all.com/|title=Educate about Smog: What causes acid rain?|website=www.englishnotes4all.com|access-date=5 November 2018|archive-date=2 November 2018|archive-url=https://web.archive.org/web/20181102203022/http://englishnotes4all.com/}}</ref> All of these harsh chemicals are usually highly reactive and oxidizing. Photochemical smog is therefore considered to be a problem of modern industrialization. It is present in all modern cities, but it is more common in cities with sunny, warm, dry climates and a large number of motor vehicles.<ref>{{cite book|title=Living in the Environment: Principles, Connections, and Solutions|last=Miller|first=George Tyler Jr.|publisher=[[The Thomson Corporation]]|year=2018|isbn=978-0-534-37697-0|location=Belmont|page=423|edition=12th}}</ref> Because it travels with the wind, it can affect sparsely populated areas as well.

[[File:PLANE (SUPPLIED BY NASA) USES SPACE TECHNIQUES IN SMOG RESEARCH. SCIENTISTS FROM STATEWIDE AIR - NARA - 542670.jpg|thumb|right|Airplane used to collect airborne hydrocarbons, May 1972]]
The composition and chemical reactions involved in photochemical smog were not understood until the 1950s. In 1948, flavor chemist [[Arie Jan Haagen-Smit|Arie Haagen-Smit]] adapted some of his equipment to collect chemicals from polluted air, and identified ozone as a component of Los Angeles smog. Haagen-Smit went on to discover that nitrogen oxides from automotive exhausts and gaseous hydrocarbons from cars and oil refineries, exposed to sunlight, were key ingredients in the formation of ozone and photochemical smog.<ref name="hundred2">{{cite book|url=https://books.google.com/books?id=edtL_hIi8M0C&pg=PA219|title=Arnold O. Beckman: one hundred years of excellence|author-last1=Thackray|author-first1=Arnold|author-last2=Myers Jr.|author-first2=Minor|publisher=Chemical Heritage Foundation|year=2000|isbn=978-0-941901-23-9|location=Philadelphia, Pa.|name-list-style=amp}}</ref>{{rp|219–224}}<ref name="Gardner2">{{cite web|url=http://www.englishnotes4all.com/|title=Smog: the battle against air pollution|last=Gardner|first=Sarah|date=14 July 2018|website=Marketplace.org|publisher=American Public Media|access-date=6 November 2015|archive-date=24 January 2018|archive-url=https://web.archive.org/web/20180124115059/http://englishnotes4all.com/}}</ref><ref name="Kean2">{{cite journal|last1=Kean|first1=Sam|date=2015|title=The Flavor of Smog|url=https://www.sciencehistory.org/distillations/magazine/the-flavor-of-smog|journal=Distillations|volume=2|issue=3|page=5|access-date=22 March 2018}}</ref> Haagen-Smit worked with [[Arnold Beckman]], who developed various equipment for detecting smog, ranging from an "Apparatus for recording gas concentrations in the atmosphere" patented on 7 October 1952, to "air quality monitoring vans" for use by government and industry.<ref name="hundred2" />{{rp|224–226}}

=== Formation and reactions ===
Photochemical smog forms through a series of sunlight-driven reactions: [[nitrogen dioxide]] (NO₂) absorbs [[Sunlight|solar radiation]] and splits into [[nitric oxide]] (NO) and a free [[oxygen]] [[atom]], which then combines with molecular [[oxygen]] (O₂) to form [[ozone]] (O₃). [[Volatile organic compound|Volatile organic compounds]] (VOCs) participate by reacting with NO, preventing the removal of [[ozone]] and allowing its concentration to build up alongside other secondary [[Oxidizing agent|oxidants]] such as [[peroxyacetyl nitrate]]s (PANs). [https://evs.institute/environmental-chemistry/photochemical-smog-formation-reactions/#google_vignette]

In addition to [[ozone]], photochemical smog contains other harmful [[Air pollution|secondary pollutants]]. [[Peroxyacetyl nitrate]]s (PANs) are potent eye and respiratory irritants formed when [[Volatile organic compound|VOC oxidation]] leads to [[Hydroperoxyl|peroxy radicals]] that react with NO₂. [[Aldehyde|Aldehydes]] such as [[formaldehyde]] also arise during [[Volatile organic compound|VOC oxidation]], contributing to irritation and odors associated with smog episodes.

During the morning [[rush hour]], a high concentration of [[nitric oxide]] and [[Hydrocarbon|hydrocarbons]] are emitted to the [[atmosphere]], mostly via on-road traffic but also from industrial sources. Some [[Hydrocarbon|hydrocarbons]] are rapidly [[Redox|oxidized]] by OH· and form [[Hydroperoxyl|peroxy radicals]], which convert [[nitric oxide]] (NO) to [[nitrogen dioxide]] (NO<sub>2</sub>).

{{olist
| {{chem2 | R• + O2 + M -> RO2• + M }}
| {{chem2 | RO2• + NO -> NO2 + RO• }}
| {{chem2 | HO2• + NO -> NO2 + OH• }}
}}

Nitrogen dioxide (NO<sub>2</sub>) and nitric oxide (NO) further react with ozone (O<sub>3</sub>) in a series of chemical reactions:

{{olist |start=4
| {{chem2 | NO2 + ''hν'' -> O(^{3}P) + NO }}{{spaces|4}}λ < 400 nm
| {{chem2 | O(^{3}P) + O2 + M -> O3 + M(heat) }}
| {{chem2 | O3 + NO -> NO2 + O2 }}
}}

This series of equations is referred to as the [[photostationary state]] (PSS). However, because of the presence of Reaction 2 and 3, NO<sub>x</sub> and ozone are not in a perfectly steady state. By replacing Reaction 6 with Reaction 2 and Reaction 3, the O<sub>3</sub> molecule is no longer destroyed. Therefore, the concentration of ozone keeps increasing throughout the day. This mechanism can escalate the formation of ozone in smog. Other reactions such as the photooxidation of formaldehyde (HCHO), a common secondary pollutant, can also contribute to the increased concentration of ozone and NO<sub>2</sub>. Photochemical smog is more prevalent during summer days since incident solar radiation fluxes are high, which favors the formation of ozone (reactions 4 and 5). The presence of a temperature inversion layer is another important factor. That is because it prevents the vertical convective mixing of the air and thus allows the pollutants, including ozone, to accumulate near the ground level, which again favors the formation of photochemical smog.

There are certain reactions that can limit the formation of O<sub>3</sub> in smog. The main limiting reaction in polluted areas is:

{{olist |start=7
| {{chem2 | NO2 + HO• + M -> HNO3 + M }}
}}

This reaction removes NO<sub>2</sub> which limits the amount of O<sub>3</sub> that can be produced from its [[photolysis]] (reaction 4). HNO<sub>3</sub>, nitric acid, is a sticky compound that can easily be removed onto surfaces (dry deposition) or dissolved in water and be rained out (wet deposition). Both ways are common in the atmosphere and can efficiently remove radicals and nitrogen dioxide.

[[File:Lightmatter_Golden_gate_bridge.jpg|center|thumb|600x600px|The presence of smog in [[California]] is shown near the [[Golden Gate Bridge]]. The brown coloration is due to the NO<sub>2</sub> formed from photochemical smog reactions.<nowiki/>]]

===Natural causes===
====Volcanoes====
An erupting volcano can emit high levels of [[sulfur dioxide]] along with a large quantity of particulate matter; two key components to the creation of smog. However, the smog created as a result of a volcanic eruption is often known as [[vog]] to distinguish it as a natural occurrence. The chemical reactions that form smog following a volcanic eruption are different than the reactions that form photochemical smog. The term smog encompasses the effect when a large number of gas-phase molecules and particulate matter are emitted to the atmosphere, creating a visible [[haze]]. The event causing a large number of emissions can vary but still result in the formation of smog.

====Plants====
Plants are a natural source of hydrocarbons that can undergo reactions in the atmosphere and produce smog. Globally both plants and soil contribute a substantial amount to the production of hydrocarbons, mainly by producing [[isoprene]] and [[terpene]]s.<ref>{{Cite book |url=https://www.elsevier.com/books/chemistry-of-the-natural-atmosphere/warneck/978-0-12-735632-7 |title=Chemistry of the Natural Atmosphere, Volume 71 - 2nd Edition |publisher=Academic Press |first=Peter |last=Warneck |date=29 October 1999 |isbn=978-0-12-735632-7 |language=en |access-date=15 November 2018}}</ref> Hydrocarbons released by plants can often be more reactive than man-made hydrocarbons. For example when plants release isoprene, the isoprene reacts very quickly in the atmosphere with [[Hydroxyl radical|hydroxyl radicals]]. These reactions produce [[Hydroperoxide|hydroperoxides]] which increase ozone formation.<ref>{{Cite journal|last1=Sharkey|first1=T. D.|last2=Wiberley|first2=A. E.|last3=Donohue|first3=A. R.|date=17 October 2007|title=Isoprene Emission from Plants: Why and How|journal=Annals of Botany|language=en|volume=101|issue=1|pages=5–18|doi=10.1093/aob/mcm240|pmid=17921528|pmc=2701830|issn=0305-7364}}</ref>

== Health effects ==

[[File:LA smog masks.jpg|thumb|Highland Park [[Optimist International|Optimist Club]] wearing smog-gas masks at banquet, [[Los Angeles]], circa 1954]]
Smog is a serious problem in many cities and continues to harm human health.<ref>{{cite news | url = https://www.usatoday.com/weather/news/2004-04-15-air-quality-ap_x.htm | date = 16 April 2004 | title = EPA: Half of USA breathing illegal levels of smog | first = Traci | last = Watson | location = Washington | work = USA Today}}</ref><ref>{{cite web|last1=Marziali|first1=Carl|title=L.A.'s Environmental Success Story: Cleaner Air, Healthier Kids|url=https://news.usc.edu/76761/las-environmental-success-story-cleaner-air-healthier-kids/|website=USC News|access-date=16 March 2015|date=4 March 2015}}</ref> Secondary [[Pollutant|pollutants]] produced in photochemical smog such as [[ozone]], [[Peroxyacetyl nitrate|PANs]], and [[Aldehyde|aldehydes]] contribute to respiratory irritation, [[Human eye|eye irritation]], and reduced [[Lung|lung function]], particularly among sensitive populations [https://evs.institute/environmental-chemistry/photochemical-smog-causes-reactions-effects/]. [[tropospheric ozone|Ground-level ozone]], [[sulfur dioxide]], [[nitrogen dioxide]] and [[carbon monoxide]] are especially harmful for senior citizens, children, and people with heart and lung conditions such as [[emphysema]], [[bronchitis]], and [[asthma]].<ref name=epa/> It can inflame breathing passages, decrease the lungs' working capacity, cause shortness of breath, pain when inhaling deeply, wheezing, and coughing. It can cause eye and nose irritation and it dries out the protective membranes of the nose and throat and interferes with the body's ability to fight infection, increasing susceptibility to illness.<ref>{{Cite journal|last1=Kurt|first1=Ozlem Kar|last2=Zhang|first2=Jingjing|last3=Pinkerton|first3=Kent E.|date=March 2016|title=Pulmonary Health Effects of Air Pollution|journal=Current Opinion in Pulmonary Medicine|volume=22|issue=2|pages=138–143|doi=10.1097/MCP.0000000000000248|issn=1070-5287|pmc=4776742|pmid=26761628}}</ref> Hospital admissions and respiratory deaths often increase during periods when ozone levels are high.<ref>{{cite web | url = http://www.cleanwateractioncouncil.org/issues/air-quality/ozone-pollution/ | title = Ozone Pollution | publisher = Clean Water Action Council of Northeast Wisconsin}}</ref><ref>{{cite web | url = http://www.epa.gov/apti/ozonehealth/population.html | archive-url = https://web.archive.org/web/20110430005643/http://www.epa.gov/apti/ozonehealth/population.html | archive-date = 30 April 2011 | title = Health Effects of Ozone in the General Population | work = Ozone and Your Patients' Health: Training for Health Care Providers | publisher = United States Environmental Protection Agency | date = 10 September 2013 | quote = In addition to these effects, evidence from observational studies strongly indicates that higher daily ozone concentrations are associated with increased asthma attacks, increased hospital admissions, increased daily mortality, and other markers of morbidity.}}</ref>

There is a lack of knowledge on the long-term effects of air pollution exposure and the origin of asthma. An experiment was carried out using intense air pollution similar to that of the 1952 Great Smog of London. The results from this experiment concluded that there is a link between early-life pollution exposure that leads to the development of asthma, proposing the ongoing effect of the Great Smog.<ref>{{Cite journal |first1=Prashant |last1=Bharadwaj |first2=Joshua Graff |last2=Zivin |first3=Jamie T. |last3=Mullins |first4=Matthew |last4=Neidelllast |date=8 July 2016 |title=Early life exposure to the Great Smog of 1952 and the Development of Asthma|journal= American Journal of Respiratory and Critical Care Medicine|volume=194 |pages=1475–1482 |number=12|doi=10.1164/rccm.201603-0451OC |pmid=27392261 |pmc=5440984 }}</ref>
Modern studies continue to find links between mortality and the presence of smog. One study, published in [[Nature magazine]], found that smog episodes in the city of [[Jinan]], a large city in eastern China, during 2011–15, were associated with a 5.87% (95% CI 0.16–11.58%) increase in the rate of overall mortality. This study highlights the effect of exposure to air pollution on the rate of mortality in China.<ref>Ambient air pollution, smog episodes and mortality in Jinan, China: Jun Zhang, Yao Liu, Liang-liang Cui, Shou-qin Liu, Xi-xiang Yin & Huai-chen Li
Scientific Reports 7, Article number: 11209 (2017)
doi:10.1038/s41598-017-11338-2</ref> A similar study in Xi'an found an association between ambient air pollution and increased mortality associated with respiratory diseases.<ref>{{Cite journal|last1=Mokoena|first1=Kingsley Katleho|last2=Ethan|first2=Crystal Jane|last3=Yu|first3=Yan|last4=Shale|first4=Karabo|last5=Liu|first5=Feng|date=2019-07-05|title=Ambient air pollution and respiratory mortality in Xi'an, China: a time-series analysis|url= |journal=Respiratory Research|volume=20|issue=1|page=139|doi=10.1186/s12931-019-1117-8|issn=1465-993X|pmc=6612149|pmid=31277656 |doi-access=free }}</ref>

===Levels of unhealthy exposure===
The [[United States Environmental Protection Agency|U.S. EPA]] has developed an [[air quality index]] to help explain air pollution levels to the general public. 8 hour average ozone concentrations of 85 to 104 [[parts per notation|ppbv]] are described as "Unhealthy for Sensitive Groups", 105 ppbv to 124 ppbv as "unhealthy" and 125 ppb to 404 ppb as "very unhealthy".<ref name=epa/> The "very unhealthy" range for some other pollutants are: 355 μg m<sup>−3</sup> – 424 μg m<sup>−3</sup> for [[PM10]]; 15.5 ppm – 30.4ppm for CO and 0.65 ppm – 1.24 ppm for NO<sub>2</sub>.<ref>{{Cite web | url = http://www.epa.gov/ttn/oarpg/t1/memoranda/rg701.pdf | title = Guidelines for the Reporting of Daily Air Quality – the Air Quality Index (AQI) | date = May 2006 | publisher = United States Environmental Protection Agency, Office of Air Quality Planning and Standards | id = EPA-454/B-06-001 | archive-url = https://web.archive.org/web/20060725183242/http://www.epa.gov/ttn/oarpg/t1/memoranda/rg701.pdf | archive-date = 25 July 2006 }}</ref>

===Premature deaths due to cancer and respiratory disease===
In 2016, the [[Ontario Medical Association]] announced that smog is responsible for an estimated 9,500 premature deaths in the province each year.<ref>{{cite web|url=http://wheels.ca/reviews/article/256058 |publisher=Wheels.ca |title=$3.83 to power hybrid plug-in for 6 days |first=Tyler |last=Hamilton |date=9 June 2016 |archive-url=https://web.archive.org/web/20100818225927/http://www.wheels.ca/reviews/article/256058 |archive-date=18 August 2010 }}</ref>

A 20-year [[American Cancer Society]] study found that cumulative exposure also increases the likelihood of premature death from respiratory disease, implying the 8-hour standard may be insufficient.<ref>[https://www.npr.org/templates/transcript/transcript.php?storyId=101694787 Smoggy Skies May Cause Respiratory Death] NPR.org</ref>

===Alzheimer risk===
Tiny magnetic particles from air pollution have for the first time been discovered to be lodged in human brains– and researchers think they could be a possible cause of [[Alzheimer's disease]].
Researchers at Lancaster University found abundant [[magnetite]] [[nanoparticle]]s in brain tissue from 37 individuals aged three to 92-years-old who lived in Mexico City and Manchester. This strongly magnetic mineral is toxic and has been implicated in the production of reactive oxygen species ([[free radicals]]) in the human brain, which is associated with neurodegenerative diseases including Alzheimer's disease.<ref>{{cite web|url=http://www.lancaster.ac.uk/news/articles/2016/toxic-air-pollution-nanoparticles-discovered-in-the-human-brain/|title=Toxic air pollution nanoparticles discovered in the human brain|date=5 September 2016|publisher=Lancaster University}}</ref><ref>{{cite web|last=Reuben|first=Aaron|title=The terrifying truth about air pollution and dementia|url=https://www.motherjones.com/environment/2015/06/air-pollution-dementia-alzheimers-brain/|date=24 June 2015|website=Mother Jones|language=en-US}}</ref>

===Risk of certain birth defects===
A study examining 806 women who had babies with birth defects between 1997 and 2006, and 849 women who had healthy babies, found that smog in the [[San Joaquin Valley]] area of [[California]] was linked to two types of [[neural tube defect]]s: [[spina bifida]] (a condition involving, among other manifestations, certain malformations of the [[spinal column]]), and [[anencephaly]] (the underdevelopment or absence of part or all of the brain, which if not fatal usually results in profound impairment).<ref>{{cite journal |pmid=23045474 |year=2012 |last1=Padula |first1=AM |last2=Mortimer |first2=K |last3=Hubbard |first3=A |last4=Lurmann |first4=F |last5=Jerrett |first5=M |last6=Tager |first6=IB |title=Exposure to traffic-related air pollution during pregnancy and term low birth weight: Estimation of causal associations in a semiparametric model |volume=176 |issue=9 |pages=815–24 |doi=10.1093/aje/kws148 |pmc=3571254 |journal=American Journal of Epidemiology}}</ref> An emerging cohort study in China linked early-life smog exposure to an increased risk for adverse pregnancy outcomes, in particular [[oxidative stress]].<ref>{{Cite journal|last1=Song|first1=Jing|last2=Chen|first2=Yi|last3=Wei|first3=Ling|last4=Ma|first4=Ying|last5=Tian|first5=Ning|last6=Huang|first6=Shi Yun|last7=Dai|first7=Yin Mei|last8=Zhao|first8=Li Hong|last9=Kong|first9=Yuan Yuan|date=2017-09-03|title=Early-life exposure to air pollutants and adverse pregnancy outcomes: protocol for a prospective cohort study in Beijing|journal=BMJ Open|volume=7|issue=9|article-number=e015895|doi=10.1136/bmjopen-2017-015895|doi-access=free|issn=2044-6055|pmc=5588991|pmid=28871018}}</ref>

===Low birth weight===
According to a study published in [[The Lancet]], even a very small (5&nbsp;μg) change in [[PM2.5]] exposure was associated with an increase (18%) in risk of a low birth weight at delivery, and this relationship held even below the current accepted safe levels.<ref>{{cite journal |doi=10.1016/S2213-2600(13)70192-9 |pmid=24429273 |title=Ambient air pollution and low birthweight: A European cohort study (ESCAPE) |year=2013 |last1=Pedersen |first1=Marie |last2=Giorgis-Allemand |first2=Lise |last3=Bernard |first3=Claire |last4=Aguilera |first4=Inmaculada |last5=Andersen |first5=Anne-Marie Nybo |last6=Ballester |first6=Ferran |last7=Beelen |first7=Rob M J |last8=Chatzi |first8=Leda |last9=Cirach |first9=Marta |last10=Danileviciute |first10=Asta |last11=Dedele |first11=Audrius |last12=Eijsden |first12=Manon van |last13=Estarlich |first13=Marisa |last14=Fernández-Somoano |first14=Ana |last15=Fernández |first15=Mariana F |last16=Forastiere |first16=Francesco |last17=Gehring |first17=Ulrike |last18=Grazuleviciene |first18=Regina |last19=Gruzieva |first19=Olena |last20=Heude |first20=Barbara |last21=Hoek |first21=Gerard |last22=Hoogh |first22=Kees de |last23=Van Den Hooven |first23=Edith H |last24=Håberg |first24=Siri E |last25=Jaddoe |first25=Vincent W V |last26=Klümper |first26=Claudia |last27=Korek |first27=Michal |last28=Krämer |first28=Ursula |last29=Lerchundi |first29=Aitana |last30=Lepeule |first30=Johanna  |journal=The Lancet Respiratory Medicine |volume=1 |issue=9 |pages=695–704 |display-authors=5|arxiv=0706.4406 }}</ref>

== Other negative effects ==
Although severe health effects caused by smog are the chief issue, intense air pollution caused by [[haze]] from [[air pollution]], [[dust storm]] particles, and [[Wildfire|bush fire]] smoke, cause a reduction in [[irradiance]] that negatively affects both solar [[Photovoltaics|photovoltaic]]<ref>{{Cite journal |last1=Sadat |first1=Seyyed Ali |last2=Hoex |first2=Bram |last3=Pearce |first3=Joshua M. |date=2022 |title=A Review of the Effects of Haze on Solar Photovoltaic Performance |url= |journal=Renewable and Sustainable Energy Reviews |language=en |volume=167 |article-number=112796 |doi=10.1016/j.rser.2022.112796|bibcode=2022RSERv.16712796S |s2cid=251430613 }}</ref> production as well as [[Crop yield|agricultural yield]].<ref>{{Cite journal |last1=Chameides |first1=W. L. |last2=Yu |first2=H. |last3=Liu |first3=S. C. |author3-link=Liu Shaw-chen |last4=Bergin |first4=M. |last5=Zhou |first5=X. |last6=Mearns |first6=L. |last7=Wang |first7=G. |last8=Kiang |first8=C. S. |last9=Saylor |first9=R. D. |last10=Luo |first10=C. |last11=Huang |first11=Y. |last12=Steiner |first12=A. |last13=Giorgi |first13=F. |date=1999-11-23 |title=Case study of the effects of atmospheric aerosols and regional haze on agriculture: An opportunity to enhance crop yields in China through emission controls? |journal=Proceedings of the National Academy of Sciences |language=en |volume=96 |issue=24 |pages=13626–13633 |doi=10.1073/pnas.96.24.13626 |issn=0027-8424 |pmc=24115 |pmid=10570123 |bibcode=1999PNAS...9613626C |doi-access=free }}</ref>

== Areas affected ==
Smog can form in almost any climate where industries or cities release large amounts of [[air pollution]], such as smoke or gases. Precursors of photochemical smog can be transported downwind from urban centers, allowing smog formation in suburban and rural areas as pollutants react over time during transport. This means that smog episodes can affect regions far from the original emission sources. However, it is worse during periods of warmer, sunnier weather when the upper air is warm enough to inhibit vertical circulation. It is especially prevalent in geologic basins encircled by hills or mountains. It often stays for an extended period of time over densely populated cities or urban areas and can build up to dangerous levels.

=== Asia ===
==== India ====
{{See also|Environmental issues in Delhi}}
[[File: Aerial view of Air Pollution in North India, Agriculture Fires, November 2013.jpg|300px|thumb|During the autumn and spring months, some 500 million tons of [[stubble burning|rice and wheat crop residues are burnt]], and [[western disturbances|winds blow from India's north and northwest towards east]].<ref>Badarinath, K. V. S., Kumar Kharol, S., & Rani Sharma, A. (2009), Long-range transport of aerosols from agriculture crop residue burning in Indo-Gangetic Plains—a study using LIDAR, ground measurements and satellite data. Journal of Atmospheric and Solar-Terrestrial Physics, 71(1), 112–120</ref><ref>Sharma, A. R., Kharol, S. K., Badarinath, K. V. S., & Singh, D. (2010), Impact of agriculture crop residue burning on atmospheric aerosol loading—a study over Punjab State, India. Annales Geophysicae, 28(2), pp 367–379</ref><ref>{{cite journal|first1=Tina|last1=Adler|title=Respiratory Health: Measuring the Health Effects of Crop Burning|journal=Environmental Health Perspectives|date=November 2010|volume=118|issue=11|page=A475|doi=10.1289/ehp.118-a475|pmc=2974718|pmid=21465742}}</ref> This aerial view shows India's annual crop burning, resulting in smoke and air pollution over Delhi and adjoining areas.]]
For the past few years, cities in northern [[India]] have been covered in a thick layer of [[winter]] smog. The situation has turned severe in the national capital, [[Delhi]]. This smog is caused by the collection of [[particulate matter]] (a very fine type of dust and toxic gases) in the air due to stagnant movement of air during winters.<ref>{{Cite news|url=https://www.honeywellsmarthomes.com/blog/causes-of-smog-and-how-to-protect/|title=Causes of Smog {{!}} How to Protect Yourself from Smog - Honeywell Blog|date=26 November 2017|work=Official Blog Updates - Honeywell Air Purifiers|access-date=7 March 2018|language=en-US}}</ref> Moreover, during the post-monsoon to winter transition, air quality in the [[Indo-Gangetic Plain]] (IGP) worsens significantly due to shifts in weather patterns, such as changes in wind, temperature, and boundary layer mixing.<ref name=":0">{{Cite journal |last1=Hassan |first1=Muhammad Azher |last2=Mehmood |first2=Tariq |last3=Liu |first3=Junjie |last4=Luo |first4=Xiaosan |last5=Li |first5=Xinghua |last6=Tanveer |first6=Mohsin |last7=Faheem |first7=Muhammad |last8=Shakoor |first8=Awais |last9=Dar |first9=Afzal Ahmed |last10=Abid |first10=Muhammad |date=2023-02-01 |title=A review of particulate pollution over Himalaya region: Characteristics and salient factors contributing ambient PM pollution |url=https://linkinghub.elsevier.com/retrieve/pii/S1352231022005374 |journal=Atmospheric Environment |volume=294 |article-number=119472 |doi=10.1016/j.atmosenv.2022.119472 |bibcode=2023AtmEn.29419472H |issn=1352-2310|url-access=subscription }}</ref> The impact of emissions from both [[Biomass (energy)|biomass]] burning and [[Urbanization|urban]] activities has intensified, leading to a rise in [[Aerosol|aerosols]] mainly particulate matters.<ref>{{Cite journal |last1=Ojha |first1=Narendra |last2=Naja |first2=Manish |last3=Singh |first3=K. P. |last4=Sarangi |first4=T. |last5=Kumar |first5=R. |last6=Lal |first6=S. |last7=Lawrence |first7=M. G. |last8=Butler |first8=T. M. |last9=Chandola |first9=H. C. |date=2012-10-27 |title=Variabilities in ozone at a semi-urban site in the Indo-Gangetic Plain region: Association with the meteorology and regional processes |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2012JD017716 |journal=Journal of Geophysical Research: Atmospheres |language=en |volume=117 |issue=D20 |article-number=2012JD017716 |doi=10.1029/2012JD017716 |bibcode=2012JGRD..11720301O |issn=0148-0227|url-access=subscription }}</ref> The nearby [[Himalayas|Himalayan region]] is also affected,<ref name=":1">{{Cite journal |last1=Hassan |first1=Muhammad Azher |last2=Mehmood |first2=Tariq |last3=Liu |first3=Junjie |last4=Luo |first4=Xiaosan |last5=Li |first5=Xinghua |last6=Tanveer |first6=Mohsin |last7=Faheem |first7=Muhammad |last8=Shakoor |first8=Awais |last9=Dar |first9=Afzal Ahmed |last10=Abid |first10=Muhammad |date=2023-02-01 |title=A review of particulate pollution over Himalaya region: Characteristics and salient factors contributing ambient PM pollution |url=https://linkinghub.elsevier.com/retrieve/pii/S1352231022005374 |journal=Atmospheric Environment |volume=294 |article-number=119472 |doi=10.1016/j.atmosenv.2022.119472 |bibcode=2023AtmEn.29419472H |issn=1352-2310|url-access=subscription }}</ref> where mountainous topography trap [[Air pollution|air pollutants]] and increase the air quality issues specifically in northern India.<ref>{{Cite journal |last1=Mehra |first1=Manisha |last2=Panday |first2=Arnico K. |last3=Puppala |first3=Siva Praveen |last4=Sapkota |first4=Vikrant |last5=Adhikary |first5=Bhupesh |last6=Pokheral |first6=Chiranjibi P. |last7=Ram |first7=Kirpa |date=2019-11-01 |title=Impact of local and regional emission sources on air quality in foothills of the Himalaya during spring 2016: An observation, satellite and modeling perspective |url=https://linkinghub.elsevier.com/retrieve/pii/S1352231019305369 |journal=Atmospheric Environment |volume=216 |article-number=116897 |doi=10.1016/j.atmosenv.2019.116897 |bibcode=2019AtmEn.21616897M |issn=1352-2310|url-access=subscription }}</ref>

Delhi is the most polluted<ref>{{cite news|url=http://www.hindustantimes.com/india-news/delhi-has-dirtiest-air-china-data-foggy-who/article1-1216605.aspx |archive-url=https://web.archive.org/web/20140508004750/http://www.hindustantimes.com/india-news/delhi-has-dirtiest-air-china-data-foggy-who/article1-1216605.aspx |archive-date=8 May 2014 |title=Delhi is most polluted city in world, Beijing much better: WHO study |work=Hindustan Times|access-date=8 May 2014}}</ref> city in the world and according to one estimate, air pollution causes the death of about 10,500 people in Delhi every year.<ref name="TimePollution">{{cite magazine|url=http://world.time.com/2014/02/10/smog-in-new-delhi/ |archive-url=https://web.archive.org/web/20140302085642/http://world.time.com/2014/02/10/smog-in-new-delhi/ |archive-date=2 March 2014 |title=Delhi's Air Has Become a Lethal Hazard and Nobody Seems to Know What to Do About It |magazine=Time magazine |access-date=10 February 2014}}</ref><ref name="VOAPollution">{{cite web |url=https://www.voanews.com/a/indias-air-pollution-triggers-comparisons-with-china/1855331.html |title=India's Air Pollution Triggers Comparisons with China |date=20 February 2014 |publisher=Voice of America |access-date=20 February 2014 |archive-url=https://web.archive.org/web/20140221104928/http://www.voanews.com/content/indias-air-pollution-triggers-comparisons-with-china/1855331.html |archive-date=21 February 2014 |url-status=live }}</ref><ref name="EconomistPollution">{{cite news|url=https://www.economist.com/blogs/banyan/2012/11/air-pollution-india |title=A Delhi particular |newspaper=The Economist|access-date=6 November 2012}}</ref> During 2013–14, peak levels of fine [[Particle|particulate]] matter (PM) in Delhi increased by about 44%, primarily due to high vehicular and industrial emissions, construction work and crop burning in adjoining states.<ref name="TimePollution" /><ref name="WSJPollution">{{cite news|url=https://blogs.wsj.com/indiarealtime/2014/02/15/how-crop-burning-affects-delhis-air-pollution/ |title=How Crop Burning Affects Delhi's Air |newspaper=Wall Street Journal |access-date=15 February 2014}}</ref><ref name=Gardiner>{{cite news|last=Harris|first=Gardiner|title=Beijing's Bad Air Would Be Step Up for Smoggy Delhi|url=https://www.nytimes.com/2014/01/26/world/asia/beijings-air-would-be-step-up-for-smoggy-delhi.html?emc=edit_tnt_20140126&tntemail0=y|access-date=27 January 2014|newspaper=New York Times|date=25 January 2014}}</ref><ref>{{cite news|last=Bearak|first=Max|title=Desperate for Clean Air, Delhi Residents Experiment with Solutions|url=http://india.blogs.nytimes.com/2014/02/07/desperate-for-clean-air-delhi-residents-experiment-with-solutions/?emc=edit_tnt_20140208&tntemail0=y|access-date=8 February 2014|newspaper=New York Times|date=7 February 2014}}</ref> Delhi has the highest level of the airborne particulate matter, [[PM2.5]] considered most harmful to health, with 153 micrograms.<ref>{{cite news|author=Madison Park|title=Top 20 most polluted cities in the world|url=http://www.cnn.com/2014/05/08/world/asia/india-pollution-who/index.htm|publisher=CNN|date=8 May 2014}}</ref> Rising air pollution level has significantly increased lung-related ailments (especially asthma and lung cancer) among Delhi's children and women.<ref name="TodayPollution">{{cite magazine|url=https://www.indiatoday.in/india/north/story/pollution-in-delhi-cng-children-in-delhi-182151-2014-02-21 |title=Children in Delhi have lungs of chain-smokers! |magazine=India Today |access-date=22 February 2014}}</ref><ref name="DNAPollution">{{cite web|url=http://www.dnaindia.com/health/report-pollution-increasing-lung-cancer-in-indian-women-1959054 |title=Pollution increasing lung cancer in Indian women |publisher=DNA |access-date=3 February 2014|date=3 February 2014 }}</ref> The dense smog in Delhi during winter season results in major air and rail traffic disruptions every year.<ref name="ReutersPollution">{{cite news |url=http://in.reuters.com/article/india-delhi-winter-smog-idINDEE9BH0D420131218 |title=Delhi blanketed in thick smog, transport disrupted |work=Reuters |access-date=18 December 2013 |date=18 December 2013 |archive-date=20 December 2013 |archive-url=https://web.archive.org/web/20131220012916/http://in.reuters.com/article/2013/12/18/india-delhi-winter-smog-idINDEE9BH0D420131218 }}</ref> According to Indian meteorologists, the average maximum temperature in Delhi during winters has declined notably since 1998 due to rising air pollution.<ref>{{Cite news|url=https://timesofindia.indiatimes.com/city/delhi/January-days-getting-colder-tied-to-rise-in-pollution/articleshow/29429495.cms|title=January days getting colder, tied to rise in pollution |website=The Times of India|date=27 January 2014 }}</ref>
[[File: Smog in the skies of Delhi, India.jpg|thumb|left|Dense smog blankets [[Connaught Place, New Delhi]]]]

Environmentalists have criticized the Delhi government for not doing enough to curb air pollution and to inform people about air quality issues.<ref name="VOAPollution"/> Most of Delhi's residents are unaware of alarming levels of air pollution in the city and the health risks associated with it.<ref name=Gardiner/> Since the mid-1990s, Delhi has undertaken some measures to curb air pollution – Delhi has the third highest quantity of trees among Indian cities<ref>{{cite web|url=http://www.ndtv.com/news/cities/delhi_third_greenest_city.php |title=Delhi 'third greenest' city |publisher=Ndtv.com |access-date=11 March 2011}}</ref> – and the [[Delhi Transport Corporation]] operates the world's largest fleet of environmentally friendly [[compressed natural gas]] (CNG) buses.<ref name="cities.expressindia.com">{{cite news|url=http://cities.expressindia.com/fullstory.php?newsid=85665 |title=Smog city to clean capital How Delhi did it|newspaper=Express India |access-date=11 March 2011 |archive-url=https://web.archive.org/web/20101231054504/http://cities.expressindia.com/fullstory.php?newsid=85665 |archive-date=31 December 2010 }}</ref> In 1996, the [[Centre for Science and Environment]] (CSE) started a public interest litigation in the [[Supreme Court of India]] that ordered the conversion of Delhi's fleet of buses and taxis to run on CNG and banned the use of leaded petrol in 1998. In 2003, Delhi won the [[United States Department of Energy]]'s first 'Clean Cities International Partner of the Year' award for its "bold efforts to curb air pollution and support alternative fuel initiatives".<ref name="cities.expressindia.com"/> The Delhi Metro has also been credited for significantly reducing air pollutants in the city.<ref>{{Cite web|url=https://www.indiatoday.in/india/north/story/delhi-metro-helps-reduce-vehicular-air-pollution-delhi-school-of-economics-160315-2013-04-28|title=Delhi Metro helps reduce vehicular air pollution, indicates research|first=Neetu|last=Chandra|date=28 April 2013|website=India Today}}</ref>

However, according to several authors, most of these gains have been lost, especially due to [[stubble burning]], rise in market share of [[diesel exhaust|diesel cars]] and a considerable decline in bus ridership.<ref name="kumari">{{cite journal |author1=R. Kumari |author2=A.K. Attri |author3=L. Int Panis |author4=B.R. Gurjar | title = Emission estimates of Particulate Matter and Heavy Metals from Mobile sources in Delhi (India)| journal = J. Environ. Science & Engg.| volume = 55 | issue = 2 | pages = 127–142 |date=April 2013 |url=https://www.researchgate.net/publication/259827470}}</ref><ref name="Pollution">{{cite web|url=http://cseindia.org/node/835 |title=What is the status of air pollution in Delhi? |publisher=CSE, India |access-date=2 March 2014}}</ref> According to CUE and System of Air Quality Weather Forecasting and Research (SAFER), burning of agricultural waste in nearby Punjab, Haryana and Uttar Pradesh regions results in severe intensification of smog over Delhi.<ref name="Antipollution">{{cite news|url=http://articles.economictimes.indiatimes.com/2014-11-06/news/55835957_1_pm-2-5-level-air-quality-weather-forecasting-pollution-levels |archive-url=https://web.archive.org/web/20141111083702/http://articles.economictimes.indiatimes.com/2014-11-06/news/55835957_1_pm-2-5-level-air-quality-weather-forecasting-pollution-levels |archive-date=11 November 2014 |title=Delhi's air quality deteriorating due to burning of agriculture waste |newspaper=Economic Times }}</ref><ref name="CSE2Pollution">{{cite magazine|url=https://www.indiatoday.in/india/north/story/smog-delhi-winter-low-wind-speed-emissions-225394-2014-11-01 |title=Thick blanket of smog envelopes Delhi, northern India |magazine=India Today }}</ref> The state government of adjoining [[Uttar Pradesh]] is considering imposing a ban on crop burning to reduce pollution in Delhi NCR and an environmental panel has appealed to India's Supreme Court to impose a 30% cess on diesel cars.<ref>{{Cite web|url=http://timesofindia.indiatimes.com/city/noida/Straw-burning-ban-soon-to-reduce-smog-in-NCR/articleshow/2835975.cms|title=Straw burning ban soon to reduce smog in NRC, Times of India, 4 January 2014|website=[[The Times of India]]}}</ref><ref>{{Cite news|url=https://timesofindia.indiatimes.com/home/environment/pollution/Impose-30-cess-on-diesel-cars-panel-tells-Supreme-Court/articleshow/30180391.cms|title=Impose 30% cess on diesel cars, panel tells Supreme Court - Times of India|website=The Times of India|date=11 February 2014 }}</ref>

==== China ====
{{main|Beijing#Air quality}}

Joint research between American and Chinese researchers in 2006 concluded that much of [[Beijing]]'s pollution comes from surrounding cities and provinces. On average 35–60% of the [[ozone]] can be traced to sources outside the city. [[Shandong|Shandong Province]] and [[Tianjin|Tianjin Municipality]] have a "significant influence on Beijing's air quality",<ref>David G. Streetsa, Joshua S. Fub, Carey J. Jangc, Jiming Haod, Kebin Hed, Xiaoyan Tange, Yuanhang Zhang, Zifa Wangf, Zuopan Lib, Qiang Zhanga, Litao Wangd, Binyu Wangc, Carolyne Yua, [https://web.archive.org/web/20130724181603/http://www.cee.mtu.edu/~reh/papers/pubs/non_Honrath/streets07.pdf Air quality during the 2008 Beijing Olympic Games]. Accessed 23 April 2012</ref> partly due to the prevailing south/southeasterly flow during the summer and the mountains to the north and northwest.

==== Iran ====
In December 2005, schools and public offices were forced to close in [[Tehran]] and 1,600 people were taken to hospital, in a severe smog blamed largely on unfiltered car exhaust.<ref>{{cite news |url=https://news.bbc.co.uk/2/hi/middle_east/4516430.stm |title=Hundreds treated over Tehran smog |work=[[BBC]] News |date=10 December 2005 |access-date=3 August 2006}}</ref>

==== Mongolia ====
In the late 1990s, massive immigration to [[Ulaanbaatar]] from the countryside began. An estimated 150,000 households, mainly living in traditional Mongolian [[Yurt|gers]] on the outskirts of Ulaanbaatar, burn wood and coal (some poor families burn car tires and trash) to heat themselves during the harsh winter, which lasts from October to April, since these outskirts are not connected to the city's central heating system. A temporary solution to decrease smog was proposed in the form of stoves with improved efficiency, although with no visible results.

Coal-fired ger stoves release high levels of ash and other particulate matter (PM). When inhaled, these particles can settle in the lungs and respiratory tract and cause health problems. At 2 to 10 times above Mongolian and international air quality standards, Ulaanbaatar's PM rates are among the worst in the world, according to a December 2009 World Bank report. The Asian Development Bank (ADB) estimates that health costs related to this air pollution account for as much as 4 percent of Mongolia's GDP.<ref>{{cite news | url = http://www.eurasianet.org/departments/news/articles/eav032310e.shtml | title = Mongolia: Ulaanbaatar Grapples with Smog Problem | date = 22 March 2010 | first = Andrew | last = Cullen | publisher = EurasiaNet.org | access-date = 1 October 2012 | archive-url = https://web.archive.org/web/20120828095515/http://www.eurasianet.org/departments/news/articles/eav032310e.shtml | archive-date = 28 August 2012 }}</ref>

==== Southeast Asia ====
{{See also|Asian brown cloud|haze}}
[[File:Downtown Core, Singapore, Oct 06.JPG|thumb|left|[[Singapore]]'s [[Downtown Core]] on 7 October 2006, when it was affected by [[wildfire|forest fires]] in [[Sumatra]], [[Indonesia]]]]
Smog is a regular problem in [[Southeast Asia]] caused by [[wildfire|land and forest fires]] in [[Indonesia]], especially [[Sumatra]] and [[Kalimantan]], although the term [[haze]] is preferred in describing the problem. Farmers and plantation owners are usually the cause of the fires, which they use to clear tracts of land for further plantings. These fires mainly affect [[Brunei]], [[Indonesia]], [[Philippines]], [[Malaysia]], [[Singapore]] and [[Thailand]], and occasionally [[Guam]] and [[Saipan]].<ref>{{cite news|url=http://www.saipantribune.com/newsstory.aspx?cat=1&newsID=61706 |date=5 October 2006 |title=Indon haze spreads to NMI |first=Ferdie |last=de la Torre |work=Saigpan Tribune |archive-url=https://web.archive.org/web/20070218044848/http://www.saipantribune.com/newsstory.aspx?cat=1&newsID=61706 |archive-date=18 February 2007 }}</ref><ref>{{cite news | url = http://www.sun2surf.com/article.cfm?id=15717 | title = 15 areas with unhealthy air (updated) | first = S.Tamarai | last = Chelvi | location = Petaling Jaya | publisher = Sun Media Corporation Sdn. Bhd. | archive-url = https://web.archive.org/web/20090110073337/http://www.sun2surf.com/article.cfm?id=15717 | archive-date = 10 January 2009}}</ref> The economic losses of the fires in 1997 have been estimated at more than US$9 billion.<ref>{{cite web | url = http://haze.asean.org/?wpfb_dl=12 | date = 28 June 2007 | title = Combating Haze in ASEAN: Frequently Asked Questions | author = ASEAN Secretariat, Jl. | publisher = ASEAN Haze Action Online | access-date = 25 October 2013 | archive-date = 29 October 2013 | archive-url = https://web.archive.org/web/20131029195409/http://haze.asean.org/?wpfb_dl=12 }}</ref> This includes damages in agriculture production, destruction of forest lands, health, transportation, tourism, and other economic endeavours. Not included are social, environmental, and psychological problems and long-term health effects. The [[2006 Southeast Asian haze|second-latest bout of haze]] to occur in [[Malaysia]], [[Singapore]] and the [[Malacca Straits]] was in October 2006, and was caused by smoke from fires in [[Indonesia]] being blown across the Straits of Malacca by southwesterly winds. A similar haze occurred in June 2013, with the PSI setting a [[Pollutant Standards Index#PSI in Singapore|new record]] in Singapore on 21 June at 12pm with a reading of 401, which is in the "Hazardous" range.<ref>{{cite news|url=http://www.channelnewsasia.com/news/singapore/psi-hits-new-all-time/719496.html |archive-url=https://web.archive.org/web/20130624015317/http://www.channelnewsasia.com/news/singapore/psi-hits-new-all-time/719496.html |archive-date=24 June 2013 |title=Singapore: PSI hits new all-time high of 401 on Friday |publisher=Channel NewsAsia |date=21 June 2013 }}</ref>

The [[Association of Southeast Asian Nations]] (ASEAN) reacted. In 2002, the [[ASEAN Agreement on Transboundary Haze Pollution|Agreement on Transboundary Haze Pollution]] was signed between all ASEAN nations.<ref>{{cite web | url = http://haze.asean.org/?page_id=185 | title = ASEAN Agreement on Transboundary Haze Pollution | author = ASEAN Secretariat, Jl. | work = ASEAN Haze Action Online | date = 31 May 2013 | access-date = 31 January 2019 | archive-url = https://web.archive.org/web/20150910063035/http://haze.asean.org/?page_id=185 | archive-date = 10 September 2015 }}</ref> ASEAN formed a Regional Haze Action Plan (RHAP) and established a co-ordination and support unit (CSU).<ref>{{cite web | url = http://haze.asean.org/?page_id=244 | title = About Us | author = ASEAN Secretariat, Jl. | work = ASEAN Haze Action Online | date = 3 June 2013 | access-date = 31 January 2019 | archive-url = https://web.archive.org/web/20150912134926/http://haze.asean.org/?page_id=244 | archive-date = 12 September 2015 }}</ref> RHAP, with the help of [[Canada]], established a monitoring and warning system for forest/vegetation fires and implemented a Fire Danger Rating System (FDRS). The Malaysian Meteorological Department (MMD) has issued a daily rating of fire danger since September 2003.<ref>{{cite web | url = http://www.met.gov.my/index.php?option=com_content&task=view&id=4749&Itemid=1157 | title = Fire Danger Rating System (FDRS) for Southeast Asia | author = Malaysian Meteorological Department | publisher = Ministry of Science, Technology, and Innovation (MOSTI), Malaysia | access-date = 25 October 2013 | archive-url = https://web.archive.org/web/20131029192243/http://www.met.gov.my/index.php?option=com_content&task=view&id=4749&Itemid=1157 | archive-date = 29 October 2013 }}</ref> Indonesia has been ineffective at enforcing legal policies on deviant farmers.{{citation needed | date = October 2013}}

==== Pakistan ====
Since the start of the [[Winter|winter season]], heavy smog loaded with pollutants covered major parts of [[Punjab, Pakistan|Punjab]], especially the city of [[Lahore]],<ref>{{Cite web|url=https://scroll.in/article/856628/smoke-signal-winter-smog-is-a-reminder-india-and-pakistan-need-to-talk-about-more-than-geopolitics|title=Smoke signal: Winter smog is a reminder India and Pakistan need to talk about more than geopolitics|website=Scroll.in|date=5 November 2017 }}</ref> causing breathing problems and disrupting normal traffic.<ref>{{Cite news|url=https://www.foxnews.com/world/polluted-smog-covers-pakistans-city-of-lahore/|title=Polluted smog covers Pakistan's city of Lahore|date=5 November 2016|newspaper=Fox News|language=en-US|access-date=6 November 2016}}</ref> A recent study from 2022 shows that the primary cause of pollution in Lahore is from traffic-related PM (both [[Exhaust system|exhausts]] and non exhaust sources).<ref>{{Cite journal|url=https://www.repository.cam.ac.uk/bitstreams/9fab52d7-4aac-4adf-ab2a-28ad0da46cc7/download|title=Biomagnetic Characterization of Air Pollution Particulates in Lahore, Pakistan|last=Sheikh|first=Hassan Aftab|journal=Geochemistry, Geophysics, Geosystems |date=18 January 2022|volume=23 |issue=2 |article-number=e2021GC010293 |doi=10.1029/2021GC010293 |bibcode=2022GGG....2310293S |s2cid=245135298 }}</ref> Air quality in the Punjab, Pakistan deteriorates markedly during the post-[[monsoon]] to winter transition, driven by shifts in weather patterns like alterations in wind, temperature, and [[Boundary-Layer Meteorology|boundary layer]] mixing.<ref name=":0" /> In post-moonsoon, anthropogenic emissions from sources like [[Exhaust gas|vehicle exhaust]], industrial activities, and [[crop burning]] impact air quality across Punjab, Pakistan, affecting the region by 90–100%.<ref name=":1" /><ref>{{Cite journal |last1=Ojha |first1=Narendra |last2=Sharma |first2=Amit |last3=Kumar |first3=Manish |last4=Girach |first4=Imran |last5=Ansari |first5=Tabish U. |last6=Sharma |first6=Som K. |last7=Singh |first7=Narendra |last8=Pozzer |first8=Andrea |last9=Gunthe |first9=Sachin S. |date=2020-04-03 |title=On the widespread enhancement in fine particulate matter across the Indo-Gangetic Plain towards winter |journal=Scientific Reports |language=en |volume=10 |issue=1 |page=5862 |doi=10.1038/s41598-020-62710-8 |pmid=32246046 |issn=2045-2322|pmc=7125076 |bibcode=2020NatSR..10.5862O }}</ref>

Doctors advised residents to stay indoors and wear facemasks outside.<ref>{{Cite news|url=https://www.dawn.com/news/1159190|title=Lahore smog: It's not a natural phenomenon|last=Khan|first=Rina Saeed|date=24 January 2015|newspaper=dawn.com|access-date=6 November 2016}}</ref>

=== United Kingdom ===
====London====
[[File:LondonSmog.jpg|thumbnail|right|[[Victorian era|Victorian]] [[London]] was notorious for its thick smogs, or "[[Pea soup fog|pea-soupers]]", a fact that is often recreated (as here) to add an air of mystery to a period [[costume drama]]]]

In 1306, concerns over air pollution were sufficient for [[Edward I of England|Edward I]] to (briefly) ban coal fires in London.<ref name="middleages3"/> In 1661, [[John Evelyn]]'s ''[[Fumifugium]]'' suggested burning fragrant wood instead of mineral coal, which he believed would reduce coughing. The "[[s: Ballad of Gresham College|Ballad of Gresham College]]" the same year describes how the smoke "does our lungs and spirits choke, Our hanging spoil, and rust our iron."

Severe episodes of smog continued in the 19th and 20th centuries, mainly in the winter, and were nicknamed "[[Pea soup fog|pea-soupers]]," from the phrase "as thick as pea soup". The [[Great Smog of 1952]] darkened the streets of London and killed approximately 4,000 people in the short time of four days (a further 8,000<ref name="EHP_112_1">{{cite journal |doi=10.1289/ehp.6539 |title=A Retrospective Assessment of Mortality from the London Smog Episode of 1952: The Role of Influenza and Pollution |year=2003 |last1=Bell |first1=Michelle L. |last2=Davis |first2=Devra L. |last3=Fletcher |first3=Tony |journal=Environmental Health Perspectives |volume=112 |pages=6–8 |pmid=14698923 |issue=1 |pmc=1241789}}</ref> died from its effects in the following weeks and months). Initially, a [[flu]] [[epidemic]] was blamed for the loss of life.

In 1956 the [[Clean Air Act 1956|Clean Air Act]] started legally enforcing [[smokeless zone]]s in the capital. There were areas where no soft coal was allowed to be burned in homes or in businesses, only [[coke (fuel)|coke]], which produces no smoke. Because of the smokeless zones, reduced levels of sooty particulates eliminated the intense and persistent London smog.

It was after this that the great clean-up of London began. One by one, historical buildings which, during the previous two centuries had gradually completely blackened externally, had their stone facades cleaned and restored to their original appearance. Victorian buildings whose appearance changed dramatically after cleaning included the [[British Museum of Natural History]]. A more recent example was the [[Palace of Westminster]], which was cleaned in the 1980s. A notable exception to the restoration trend was [[10 Downing Street]], whose bricks upon cleaning in the late 1950s proved to be naturally yellow; the smog-derived black color of the façade was considered so iconic that the bricks were painted black to preserve the image.<ref>{{cite book|last1=Jones|first1=Christopher|title=10 Downing Street: The Story of a House|date=1985|publisher=The Leisure Circle|isbn=978-0-563-20441-1|pages=[https://archive.org/details/no10downingstree0000jone/page/154 154–55]|url=https://archive.org/details/no10downingstree0000jone/page/154}}</ref><ref>{{cite book|last1=Minney|first1=R. J.|title=No. 10 Downing Street: A House in History|date=1963|publisher=Little, Brown & Co.|location=Boston|pages=429–33}}</ref>

====Other areas====
[[File:07 01 2020 - VERONA SMOG - prefiltro (filtro cappa antigrasso, grease filter hood) su VMC (ventilazione meccanizzata controllata) dopo 4 giorni (parte bianca coperta da nastro adesivo) - 2020 01 07 - photo Paolo Villa.png|thumb|Grease filter hood after 4 days in an Italian city with polluted air in winter (whole surface was previously white)]]
Other areas of the United Kingdom were affected by smog, especially heavily industrialised areas.

The cities of Glasgow and Edinburgh, in Scotland, suffered smoke-laden fogs in 1909. Des Voeux, commonly credited with creating the "smog" moniker, presented a paper in 1911 to the Manchester Conference of the Smoke Abatement League of Great Britain about the fogs and resulting deaths.<ref>{{cite web | url = http://www.grida.no/geo/GEO/Geo-3-010.htm | title = The Great Smog Of 1952 | publisher = The Met Office | archive-url = https://web.archive.org/web/20131029194334/http://www.grida.no/geo/GEO/Geo-3-010.htm | archive-date = 29 October 2013 }}</ref>

One [[Birmingham]] resident described near black-out conditions in the 1900s before the Clean Air Act, with visibility so poor that cyclists had to dismount and walk to stay on the road.<ref>{{Cite journal | url = http://www.bbc.co.uk/ww2peopleswar/stories/10/a4914010.shtml | archive-url = https://archive.today/20130114055525/http://www.bbc.co.uk/ww2peopleswar/stories/10/a4914010.shtml | archive-date = 14 January 2013 | publisher = [[BBC]] |title = When smog was a frequent occurrence | journal = WW2 People's War | date = 10 August 2005 | access-date = 3 August 2006 }}</ref>

On 29 April 2015, the [[UK Supreme Court]] ruled that the government must take immediate action to cut air pollution,<ref>{{cite news|url=https://www.bbc.co.uk/news/science-environment-32512152 |title=Court orders UK to cut NO2 air pollution |work=BBC News |publisher=BBC |date=29 April 2015 |access-date=29 April 2015}}</ref> following a case brought by environmental lawyers at ClientEarth.<ref>{{cite news|url=http://www.clientearth.org/news/press-releases/uk-supreme-court-orders-government-to-take-immediate-action-on-air-pollution-2843 |title=UK Supreme Court orders Government to take "immediate action" on air pollution |publisher=ClientEarth |date=29 April 2015 |access-date=29 April 2015 |archive-url=https://web.archive.org/web/20150505041213/http://www.clientearth.org/news/press-releases/uk-supreme-court-orders-government-to-take-immediate-action-on-air-pollution-2843 |archive-date=5 May 2015 }}</ref>

=== Latin America ===
==== Mexico ====
[[File:AerialViewMexicoCity.jpg|thumb|Situated in a valley, and relying heavily on automobiles, Mexico City often suffers from poor air quality.]]
Due to its location in a highland "bowl", cold air sinks down onto the urban area of [[Mexico City]], trapping industrial and vehicle pollution underneath, and turning it into the most infamously smog-plagued city of Latin America. Within one generation, the city has changed from being known for some of the cleanest air of the world into one with some of the most severe pollution, with pollutants like [[nitrogen dioxide]] being double or triple the international standards.<ref>[http://biophysics.sbg.ac.at/mexico/air.htm SBC.ac.at] {{Webarchive|url=https://web.archive.org/web/20110914131727/http://biophysics.sbg.ac.at/mexico/air.htm |date=14 September 2011 }}, Air pollution in Mexico City, [[University of Salzburg]]</ref>
[[File:AerialViewPhotochemicalSmogMexicoCity 2.jpg|thumb|Photochemical smog over Mexico City, December 2010]]

==== Chile ====
Similar to Mexico City, the air pollution of the [[Santiago]] valley in Chile, located between the [[Andes]] and the [[Chilean Coast Range]], turn it into one of the most infamously smog-plagued city of South America. Other aggravates of the situation include the city's high latitude (31 degrees South) and dry weather during most of the year.

=== North America ===
==== Canada ====
According to the Canadian Science Smog Assessment published in 2012, smog is responsible for detrimental effects on human and ecosystem health, as well as socioeconomic wellbeing across the country. It was estimated that the province of [[Ontario]] sustains $201 million in damages annually for selected crops, and an estimated tourism revenue degradation of $7.5 million in [[Vancouver]] and $1.32 million in The [[Fraser Valley]] due to decreased visibility. [[Air pollution in British Columbia]] is of particular concern, especially in the Fraser Valley, because of a meteorological effect called [[Inversion (meteorology)|inversion]] which decreases air dispersion and leads to greater smog concentration.<ref>{{cite book |last1 = Environment Canada |last2 = Health Canada |author-link = Environment and Climate Change Canada |author2-link = Health Canada |title = Canadian Smog Science Assessment – Highlights and Key Messages |publisher = Environment Canada |date = 2011 |location = Ottawa |page = 57 |url = http://www.ec.gc.ca/Publications/AD024B6B-A18B-408D-ACA2-59B1B4E04863/CanadianSmogScienceAssessmentHighlightsAndKeyMessages.pdf |isbn = 978-1-100-19064-8 |access-date = 18 March 2017 |archive-url = https://web.archive.org/web/20160804082323/http://www.ec.gc.ca/Publications/AD024B6B-A18B-408D-ACA2-59B1B4E04863/CanadianSmogScienceAssessmentHighlightsAndKeyMessages.pdf |archive-date = 4 August 2016 }}</ref>

==== United States ====
[[File:2000- Portion of people affected by polluted air (US).svg |thumb |Though the rate of exposure to ground-level ozone ("smog") and small-particulate matter ("soot") has been declining, in 2026, nearly half of people in the US under age{{nbsp}}18 live in an area receiving a failing grade for at least one measure of air pollution.<ref name=WashPost_20260422>American Lung Association ''State of the Air 2026'' as presented by {{cite news |last1=Dennis |first1=Brady |last2=Noll |first2=Ben |title=More Americans are exposed to polluted air in the United States. See where. |url=https://www.washingtonpost.com/climate-environment/2026/04/22/unhealthy-air-pollution-report/ |work=The Washington Post |date=22 April 2026}}</ref>]]
[[File:Upstatenysmog.jpg|thumb|right|A [[NASA]] astronaut's photograph of a smog layer over central [[New York (state)|New York]]]]
[[File:LAcityhalllookingsouth8.JPG|thumb|right|View of smog south from [[Los Angeles City Hall]], September 2011]]
[[File:US Counties Designated Non-attainment according to EPA NAAQS.jpg|thumb|right|Counties in the United States where one or more [[National Ambient Air Quality Standards]] are not met, as of October 2015]]
Smog was brought to the attention of the general U.S. public in 1933 with the publication of the book "Stop That Smoke", by Henry Obermeyer, a New York public utility official, in which he pointed out the effect on human life and even the destruction of {{convert|3000|acre|km2}} of a farmer's spinach crop.<ref>{{Cite web|url=https://books.google.com/books?id=6CcDAAAAMBAJ&q=motor+gun+boat&pg=PA29|title=Popular Science|date=10 October 1933|publisher=Bonnier Corporation|via=Google Books}}</ref> Since then, the [[United States Environmental Protection Agency]] has designated over 300 U.S. counties to be non-attainment areas for one or more pollutants tracked as part of the [[National Ambient Air Quality Standards]].<ref>[https://archive.today/20120802070957/http://www.epa.gov/oar/oaqps/greenbk/ EPA.gov<!-- Bot generated title -->], The Green Book Nonattainment Areas, Green Book |</ref> These areas are largely clustered around large metropolitan areas, with the largest contiguous non-attainment zones in California and the Northeast. Various U.S. and Canadian government agencies collaborate to produce real-time air quality maps and [[Air pollution forecasting|forecasts]].<ref>{{cite web | url = http://www.airnow.gov/index.cfm?action=topics.about_airnow | title = About AIRNow | date = 6 May 2013 | publisher = AIRNow | access-date = 25 October 2013 | archive-url = https://web.archive.org/web/20131029190708/http://www.airnow.gov/index.cfm?action=topics.about_airnow | archive-date = 29 October 2013 }}</ref> To combat smog conditions, localities may declare "smog alert" days, such as in the [[Spare the Air program]] in the [[San Francisco Bay Area]]. By 1970, Congress enacted the [[Clean Air Act (United States)|Clean Air Act]] to regulate air pollutant emissions.<ref>{{Cite web |last=US EPA |first=OP |date=2013-02-22 |title=Summary of the Clean Air Act |url=https://www.epa.gov/laws-regulations/summary-clean-air-act |access-date=2022-03-13 |website=www.epa.gov |language=en}}</ref>

In the United States, smog pollution kills 24,000 Americans every year. The U.S. is among the more polluted countries in terms of smog, ranked 123 out of 195 countries measured, where 1 is cleanest and 195 is most smog polluted.<ref>Associated Press, 5 June 2019, [https://www.apnews.com/a2e7024d43c9409087ec8d5245534092 "AP Fact Check: Trump Didn't Set Records on Clean Air in US"]</ref>

====={{anchor|LA}}Los Angeles and the San Joaquin Valley=====
Because of their locations in low basins surrounded by mountains, [[Los Angeles]] and the [[San Joaquin Valley]] are notorious for their smog. Heavy automobile traffic, combined with the additional effects of the [[San Francisco Bay]] and Los Angeles/[[Long Beach, California|Long Beach]] port complexes, frequently contribute to further air pollution.

Los Angeles, in particular, is strongly predisposed to the accumulation of smog, because of the peculiarities of its geography and weather patterns. Los Angeles is situated in a flat basin with the ocean on one side and mountain ranges on three sides. A nearby cold ocean current depresses surface air temperatures in the area, resulting in an [[Inversion (meteorology)|inversion layer]]: a phenomenon where air temperature increases, instead of decreasing, with altitude, suppressing [[thermals]] and restricting vertical convection. All taken together, this results in a relatively thin, enclosed layer of air above the city that cannot easily escape out of the basin and tends to accumulate pollution.

Los Angeles was one of the best-known cities suffering from transportation smog for much of the 20th century, so much so that it was sometimes said that ''Los Angeles'' was a synonym for ''smog.''<ref>{{cite book|title=The Economics and Politics of the Slowdown in Regulatory Reform|author=Roger G. Noll|year=1999}}</ref> In 1970, when the Clean Air Act was passed, Los Angeles was the most polluted basin in the country, and California was unable to create a State Implementation Plan that would enable it to meet the new air quality standards.<ref>"Early Implementation of the Clean Air Act of 1970 in California." EPA Alumni Association. [http://www.epaalumni.org/history/video/interview.cfm?id=38 Video], [https://www.epaalumni.org/userdata/pdf/2B9E3C6816EC9466.pdf#page=1 Transcript] (see p6). 12 July 2016.</ref> However, ensuing strict regulations by state and federal government agencies overseeing this problem (such as the [[California Air Resources Board]] and the [[United States Environmental Protection Agency]]), including tight restrictions on allowed emissions levels for all new cars sold in California and mandatory regular emission tests of older vehicles, resulted in significant improvements in air quality.<ref>{{Cite web|last=US EPA|first=OAR|date=2016-05-05|title=Vehicle Emissions California Waivers and Authorizations|url=https://www.epa.gov/state-and-local-transportation/vehicle-emissions-california-waivers-and-authorizations|access-date=2020-11-26|website=US EPA|language=en}}</ref> For example, air concentrations of volatile organic compounds declined by a factor of 50 between 1962 and 2012.<ref>{{Cite web|url=https://csl.noaa.gov/news/2012/119_0809.html|title=NOAA CSL: 2012 News & Events: 50-year decline in some Los Angeles vehicle-related pollutants|first=NOAA Chemical Sciences|last=Laboratory (CSL)|website=csl.noaa.gov}}</ref> Concentrations of air pollutants such as nitrous oxides and ozone declined by 70% to 80% over the same period of time.<ref>{{cite web|url=http://hbsciu.com/2015/02/15/is-clean-air-worth-the-cost-a-case-study-for-developing-megacities-2/|title=Is clean air worth the cost? A case study for developing megacities|work=HBSciU |date=15 February 2015}}</ref>

===== Major incidents in the U.S. =====
* 26 July 1943, [[Los Angeles, California]]: A smog so sudden and severe that "Los Angeles residents believed the Japanese were attacking them with chemical warfare."<ref>{{cite magazine | date = 2010 | url = https://www.wired.com/2010/07/0726la-first-big-smog/ | title = July 26, 1943: L.A. Gets First Big Smog | author = Jess McNally | magazine = Wired}}</ref><ref>{{cite book |last=Buntin |first=John |title=L.A. Noir: The Struggle for the Soul of America's Most Seductive City |url=https://books.google.com/books?id=PIHClVp5F4AC |access-date=12 October 2014 |year=2009 |publisher=Harmony Books |location=New York |isbn=978-0-307-35207-1 |oclc=431334523 |page=108}}</ref>
* [[1948 Donora smog|30-31 October 1948]], [[Donora, Pennsylvania]]: 20 died, 600 hospitalized, thousands more stricken. Lawsuits were not settled until 1951.<ref name="autogenerated2">{{cite web | date = 1948 | url = http://www.radford.edu/~wkovarik/envhist/7forties.html | title = World War II and the Postwar Years | publisher = Environmental History Timeline | archive-url = https://web.archive.org/web/20110205085302/http://www.radford.edu/~wkovarik/envhist/7forties.html | archive-date = 5 February 2011 }}</ref>
* [[1966 New York City smog|24 November 1966]], [[New York City, New York]]: Smog kills at least 169<ref>{{cite news|url=https://www.washingtonpost.com/blogs/capital-weather-gang/post/the-killer-london-smog-event-of-december-1952-a-reminder-of-deadly-smog-events-in-us/2012/12/19/452c66bc-498e-11e2-b6f0-e851e741d196_blog.html |title=The Killer London Smog Event of December 1952: A Reminder of Deadly Smog Events in the US |last=Tracton |first=Steve |newspaper=The Washington Post |date=20 December 2012 |access-date=25 February 2015}}</ref> people.

== Pollution index ==
[[File:Pollution over São Paulo (Jardins).jpg|thumb|Smog in [[São Paulo]], [[Brazil]]]]

The severity of smog is often measured using automated optical instruments such as [[nephelometer]]s, as haze is associated with visibility and traffic control in ports. Haze, however, can also be an indication of poor air quality, though this is often better reflected using accurate purpose-built air indexes such as the American [[Air quality index|Air Quality Index]], the Malaysian API (Air Pollution Index), and the Singaporean [[Pollutant Standards Index]].

In hazy conditions, it is likely that the index will report the suspended particulate level. The disclosure of the responsible pollutant is mandated in some jurisdictions.<ref>{{cite web |last1=Green |first1=Fiona |title=A deep dive on smog |url=https://www.clarity.io/blog/a-deep-dive-on-smog |website=Clarity |publisher=Clarity Movement Co. |access-date=30 December 2025}}</ref>

== Cultural references ==

[[File: London, the Houses of Parliament, Sunlight Opening in Fog, by Claude Monet.jpg|thumb|right|[[Claude Monet]] made several trips to London between 1899 and 1901, during which he painted views of the [[River Thames|Thames]] and [[Palace of Westminster|Houses of Parliament]] which show the sun struggling to shine through London's smog-laden atmosphere.]]
* The London "[[Pea soup fog|pea-soupers]]" earned the capital the nickname of "The Smoke". Similarly, [[Edinburgh]] was known as "Auld Reekie". The smogs feature in many London novels as a motif indicating hidden danger or a mystery, perhaps most overtly in [[Margery Allingham]]'s ''The Tiger in the Smoke'' (1952), but also in [[Charles Dickens|Dickens]]'s ''[[Bleak House]]'' (1852) and [[T.S. Eliot]]'s "[[The Love Song of J. Alfred Prufrock]]".
* In the 1957 [[Warner Brothers]] cartoon, ''[[What's Opera, Doc]]'', [[Elmer Fudd]] called for various calamities to befall [[Bugs Bunny]], ending in a screamed "SMOG!!"
* The 1970 [[made-for-TV movie]] ''A Clear and Present Danger'' was one of the first American television network entertainment programs to warn about the problem of smog and air pollution, as it dramatized a man's efforts toward clean air after [[emphysema]] killed his friend.<ref>{{cite news | url = https://www.nytimes.com/movies/movie/124503/A-Clear-and-Present-Danger/overview| archive-url = https://web.archive.org/web/20131113211750/http://www.nytimes.com/movies/movie/124503/A-Clear-and-Present-Danger/overview| archive-date = 13 November 2013| department = Movies & TV Dept.| work = [[The New York Times]]| author = Hal Erickson| author-link = Hal Erickson (author)| date = 2013|title=A Clear and Present Danger}}</ref>
* The history of smog in LA is detailed in ''Smogtown'' by [[Chip Jacobs]] and William J. Kelly.<ref>{{cite book | url = https://archive.org/details/smogtownlungburn00jaco | publisher = Overlook Press | title = Smogtown, The Lung-Burning History of Pollution in Los Angeles | first1 = Chip | last1 = Jacobs | first2 = William J. | last2 = Kelly | isbn = 978-1-58567-860-0 | date = 4 October 2009 | access-date = 31 January 2019 | url-access = registration }}</ref>
* The 2025 documentary series ''Clearing the Air: The War on Smog'' follows the history of smog in Los Angeles, from the first sudden appearance of smoky clouds over the city in 1943, to the decades of scientific investigation and public pressure, to the creation of the [[US EPA]] and passage of the [[Clean Air Act (United States)|Clean Air Act]], to current day with pollution less than 1% of what it was at its worst.<ref>{{Cite web |last1=Yost |first1=Peter |last2=Albuquerque |first2=Edna |date=August 26, 2025 |title=Clearing the Air: The War on Smog {{!}} American Experience |url=https://www.pbs.org/wgbh/americanexperience/films/clearing-air-war-smog/ |access-date=2025-10-14 |website=www.pbs.org |language=en}}</ref><ref>{{Cite web |last=Robinson |first=Jennifer |date=2025-08-21 |title=AMERICAN EXPERIENCE: Clearing the Air: The War on Smog |url=https://www.kpbs.org/news/2025/08/21/american-experience-clearing-the-air-the-war-on-smog |access-date=2025-10-14 |website=KPBS Public Media |language=en}}</ref>

== See also ==
{{Div col}}
* [[Smog tower]] 
* [[Asian brown cloud]]
* [[1997 Southeast Asian haze]]
* [[2005 Malaysian haze]]
* [[2006 Southeast Asian haze]]
* [[2013 Eastern China smog]]
* [[2013 Northeastern China smog]]
* [[2013 Southeast Asian haze]]
* [[2015 Southeast Asian haze]]
* [[Atmospheric chemistry]]
* [[CityTrees]]
* [[Contrail]]
* [[Criteria air contaminants]]
* [[Emission standard]]
* [[Great Smog of London]]
* [[Haze]]
* [[Inversion (meteorology)]]
* [[List of most polluted cities by particulate matter concentration]]
* [[Nitric oxide]]
* [[Ozone]]
* [[Umweltzone]]
* [[Vog]]
{{Div col end}}

==References==
{{Reflist|30em}}
* Upadhyay, Harikrishna (2016-11-07)[https://www.bhaskar.com/news/NAT-NAN-air-pollution-and-smog-in-delhi-news-hindi-5454351-PHO.html "All You Need To Know About Delhi Smog / Air Pollution – 10 Questions Answered"], Dainik Bhaskar. Retrieved on 7 November 2016.

== Further reading ==
* [[Peter Brimblecombe|Brimblecombe, Peter]]. "History of air pollution." in ''Composition, Chemistry and Climate of the Atmosphere'' (Van Nostrand Reinhold (1995): 1–18
* [[Peter Brimblecombe|Brimblecombe, Peter]], and [[László Makra]]. "Selections from the history of environmental pollution, with special attention to air pollution. Part 2*: From medieval times to the 19th century." ''International Journal of environment and pollution'' 23.4 (2005): 351–367.
* Corton, Christine L. ''London Fog: The Biography'' (2015)

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{{Pollution}}
{{Coal}}
{{Authority control}}

[[Category:Smog| ]]
[[Category:1900s neologisms]]
[[Category:Pollution]]
[[Category:Air pollution]]
[[Category:By-products]]