Smog, orsmoke fog, is a type of intenseair pollution. The word "smog" was coined in the early 20th century, and is aportmanteau of the wordssmoke andfog[1] to refer to smoky fog due to its opacity, and odour.[2] The word was then intended to refer to what was sometimes known aspea soup fog, a familiar and serious problem inLondon from the 19th century to the mid-20th century, where it was commonly known as aLondon particular orLondon fog. This kind of visible air pollution is composed ofnitrogen oxides,sulfur oxide,ozone, smoke and otherparticulates. 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 eithersummer 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, andatmospheric 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 pollutantdispersion 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 ofsulfur 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 fromvehicular emission frominternal combustion engines and industrial fumes. These pollutants react in the atmosphere with sunlight to form secondary pollutants that also combine with the primary emissions to formphotochemical smog. In certain other cities, such as Delhi, smog severity is often aggravated bystubble burning in neighboring agricultural areas since the 1980s. The atmospheric pollution levels ofLos 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 usuallytoxic to humans and can cause severe sickness, a shortened life span, or premature death.
Coinage of the term "smog" has been attributed toHenry Antoine Des Voeux in his 1905 paper, "Fog and Smoke" for a meeting of thePublic Health Congress. The 26 July 1905 edition of the London newspaperDaily 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'."[3] 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[4] and also appears in print in a column quoting from the book in the 3 July 1880, Santa Cruz Weekly Sentinel.[5] On 17 December 1881, in the publicationSporting Times, the author claims to have invented the word: "The 'Smog' – a word I have invented, combined of smoke and fog, to designate the London atmosphere..."[6]
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 theMiddle Ages.[7][8] London, in particular, was notorious up through the mid-20th century for its coal-caused smogs, which were nicknamed "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 ofair pollution in China.[9] 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 ofHarbin in 2013.
The major culprits from transportation sources arecarbon monoxide (CO),[15][16]nitrogen oxides (NO andNO2)[17][18][19] and volatile organic compounds[16][17] includinghydrocarbons (hydrocarbons are the main component ofpetroleum fuels such asgasoline anddiesel fuel).[16] Transportation emissions also includesulfur 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.[16][17]
The photochemical smog formation diagram. (Based on U 6.3.3 in mrgsciences.com[20])
Photochemical smog, often referred to as "summer smog", is the chemical reaction of sunlight,nitrogen oxides andvolatile organic compounds in the atmosphere, which leavesairborne particles andground-level ozone.[21] Photochemical smog depends on primary pollutants as well as the formation of secondary pollutants. These primary pollutants includenitrogen oxides, particularlynitric oxide (NO) andnitrogen dioxide (NO2), andvolatile organic compounds. The relevant secondary pollutants includeperoxylacyl nitrates (PAN),tropospheric ozone, andaldehydes. An important secondary pollutant for photochemical smog is ozone, which is formed when hydrocarbons (HC) and nitrogen oxides (NOx) combine in the presence of sunlight; nitrogen dioxide (NO2), which is formed as nitric oxide (NO) combines with oxygen (O2) in the air.[22] In addition, when SO2 and NOx are emitted they eventually are oxidized in the troposphere tonitric acid andsulfuric acid, which, when mixed with water, form the main components of acid rain.[23] 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.[24] Because it travels with the wind, it can affect sparsely populated areas as well.
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 chemistArie 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.[25]: 219–224 [26][27] Haagen-Smit worked withArnold 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.[25]: 224–226
During the morning rush hour, a high concentration of nitric oxide and hydrocarbons are emitted to the atmosphere, mostly via on-road traffic but also from industrial sources. Some hydrocarbons are rapidly oxidized by OH· and form peroxy radicals, which convert nitric oxide (NO) to nitrogen dioxide (NO2).
(1)R• + O2 + M → RO2• + M
(2)RO2• + NO → NO2 + RO•
(3)HO2• + NO → NO2 + OH•
Nitrogen dioxide (NO2) and nitric oxide (NO) further react with ozone (O3) in a series of chemical reactions:
(4)NO2 +hν → O(3P) + NOλ < 400 nm
(5)O(3P) + O2 + M → O3 + M(heat)
(6)O3 + NO → NO2 + O2
This series of equations is referred to as thephotostationary state (PSS). However, because of the presence of Reaction 2 and 3, NOx and ozone are not in a perfectly steady state. By replacing Reaction 6 with Reaction 2 and Reaction 3, the O3 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 NO2. 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 O3 in smog. The main limiting reaction in polluted areas is:
(7)NO2 + HO• + M → HNO3 + M
This reaction removes NO2 which limits the amount of O3 that can be produced from its photolysis (reaction 4). HNO3, 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.
The presence of smog inCalifornia is shown near theGolden Gate Bridge. The brown coloration is due to the NO2 formed from photochemical smog reactions.
An erupting volcano can emit high levels ofsulfur 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 asvog 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 visiblehaze. The event causing a large number of emissions can vary but still result in the formation of smog.
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 producingisoprene andterpenes.[28] 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 radicals. These reactions produce hydroperoxides which increase ozone formation.[29]
Smog is a serious problem in many cities and continues to harm human health.[30][31]Ground-level ozone,sulfur dioxide,nitrogen dioxide andcarbon monoxide are especially harmful for senior citizens, children, and people with heart and lung conditions such asemphysema,bronchitis, andasthma.[14] 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.[32] Hospital admissions and respiratory deaths often increase during periods when ozone levels are high.[33][34]
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.[35]Modern studies continue to find links between mortality and the presence of smog. One study, published inNature 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.[36] A similar study in Xi'an found an association between ambient air pollution and increased mortality associated with respiratory diseases.[37]
TheU.S. EPA has developed anair quality index to help explain air pollution levels to the general public. 8 hour average ozone concentrations of 85 to 104ppbv are described as "Unhealthy for Sensitive Groups", 105 ppbv to 124 ppbv as "unhealthy" and 125 ppb to 404 ppb as "very unhealthy".[14] The "very unhealthy" range for some other pollutants are: 355 μg m−3 – 424 μg m−3 forPM10; 15.5 ppm – 30.4ppm for CO and 0.65 ppm – 1.24 ppm for NO2.[38]
Premature deaths due to cancer and respiratory disease
In 2016, theOntario Medical Association announced that smog is responsible for an estimated 9,500 premature deaths in the province each year.[39]
A 20-yearAmerican 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.[40]
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 abundantmagnetitenanoparticles in the 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.[41][42]
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 theSan Joaquin Valley area ofCalifornia was linked to two types ofneural tube defects:spina bifida (a condition involving, among other manifestations, certain malformations of thespinal column), andanencephaly (the underdevelopment or absence of part or all of the brain, which if not fatal usually results in profound impairment).[43] An emerging cohort study in China linked early-life smog exposure to an increased risk for adverse pregnancy outcomes, in particular oxidative stress.[44]
According to a study published inThe Lancet, even a very small (5 μg) change inPM2.5 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.[45]
Smog can form in almost any climate where industries or cities release large amounts ofair pollution, such as smoke or gases. 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.
For the past few years, cities in northernIndia have been covered in a thick layer ofwinter smog. The situation has turned quite drastic in the national capital,Delhi. This smog is caused by the collection ofparticulate matter (a very fine type of dust and toxic gases) in the air due to stagnant movement of air during winters.[51] Moreover, during the post-monsoon to winter transition, air quality in theIndo-Gangetic Plain (IGP) worsens significantly due to shifts in weather patterns, such as changes in wind, temperature, and boundary layer mixing.[52] The impact of emissions from bothbiomass burning andurban activities has intensified, leading to a rise inaerosols mainly particulate matters.[53] The nearbyHimalayan region is also affected,[54] where mountainous topography trapair pollutants and increase the air quality issues specifically in northern India.[55]
Delhi is the most polluted[56] city in the world and according to one estimate, air pollution causes the death of about 10,500 people in Delhi every year.[57][58][59] During 2013–14, peak levels of fineparticulate matter (PM) in Delhi increased by about 44%, primarily due to high vehicular and industrial emissions, construction work and crop burning in adjoining states.[57][60][61][62] Delhi has the highest level of the airborne particulate matter,PM2.5 considered most harmful to health, with 153 micrograms.[63] Rising air pollution level has significantly increased lung-related ailments (especially asthma and lung cancer) among Delhi's children and women.[64][65] The dense smog in Delhi during winter season results in major air and rail traffic disruptions every year.[66] According to Indian meteorologists, the average maximum temperature in Delhi during winters has declined notably since 1998 due to rising air pollution.[67]
Environmentalists have criticized the Delhi government for not doing enough to curb air pollution and to inform people about air quality issues.[58] Most of Delhi's residents are unaware of alarming levels of air pollution in the city and the health risks associated with it.[61] Since the mid-1990s, Delhi has undertaken some measures to curb air pollution – Delhi has the third highest quantity of trees among Indian cities[68] and theDelhi Transport Corporation operates the world's largest fleet of environmentally friendlycompressed natural gas (CNG) buses.[69] In 1996, theCentre for Science and Environment (CSE) started a public interest litigation in theSupreme 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 theUnited 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".[69] The Delhi Metro has also been credited for significantly reducing air pollutants in the city.[70]
However, according to several authors, most of these gains have been lost, especially due tostubble burning, rise in market share ofdiesel cars and a considerable decline in bus ridership.[71][72] 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.[73][74] 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.[75][76]
Joint research between American and Chinese researchers in 2006 concluded that much ofBeijing's pollution comes from surrounding cities and provinces. On average 35–60% of theozone can be traced to sources outside the city.Shandong Province andTianjin Municipality have a "significant influence on Beijing's air quality",[77] partly due to the prevailing south/southeasterly flow during the summer and the mountains to the north and northwest.
In December 2005, schools and public offices were forced to close inTehran and 1,600 people were taken to hospital, in a severe smog blamed largely on unfiltered car exhaust.[78]
In the late 1990s, massive immigration toUlaanbaatar from the countryside began. An estimated 150,000 households, mainly living in traditional Mongoliangers on the outskirts of Ulaanbaatar, burn wood and coal (some poor families burn even 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 two 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.[79]
Smog is a regular problem inSoutheast Asia caused byland and forest fires inIndonesia, especiallySumatra andKalimantan, although the termhaze is preferred in describing the problem. Farmers and plantation owners are usually responsible for the fires, which they use to clear tracts of land for further plantings. Those fires mainly affectBrunei,Indonesia,Philippines,Malaysia,Singapore andThailand, and occasionallyGuam andSaipan.[80][81] The economic losses of the fires in 1997 have been estimated at more than US$9 billion.[82] 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. Thesecond-latest bout of haze to occur inMalaysia,Singapore and theMalacca Straits is in October 2006, and was caused by smoke from fires inIndonesia being blown across the Straits of Malacca by south-westerly winds. A similar haze has occurred in June 2013, with the PSI setting anew record in Singapore on 21 June at 12pm with a reading of 401, which is in the "Hazardous" range.[83]
TheAssociation of Southeast Asian Nations (ASEAN) reacted. In 2002, theAgreement on Transboundary Haze Pollution was signed between all ASEAN nations.[84] ASEAN formed a Regional Haze Action Plan (RHAP) and established a co-ordination and support unit (CSU).[85] RHAP, with the help ofCanada, 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.[86] Indonesia has been ineffective at enforcing legal policies on errant farmers.[citation needed]
Since the start of thewinter season, heavy smog loaded with pollutants covered major parts ofPunjab, especially the city ofLahore,[87] causing breathing problems and disrupting normal traffic.[88] A recent study from 2022 shows that the primary cause of pollution in Lahore is from traffic-related PM (bothexhausts and non exhaust sources).[89] 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, andboundary layer mixing.[52] In post-moonsoon, anthropogenic emissions from sources likevehicle exhaust, industrial activities, andcrop burning impact air quality across Punjab, Pakistan, affecting the region by 90–100%.[54][90]
Doctors advised residents to stay indoors and wear facemasks outside.[91]
VictorianLondon was notorious for its thick smogs, or "pea-soupers", a fact that is often recreated (as here) to add an air of mystery to a periodcostume drama
In 1306, concerns over air pollution were sufficient forEdward I to (briefly) ban coal fires in London.[7] In 1661,John Evelyn'sFumifugium suggested burning fragrant wood instead of mineral coal, which he believed would reduce coughing. The "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-soupers," from the phrase "as thick as pea soup". TheGreat 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[92] died from its effects in the following weeks and months). Initially, afluepidemic was blamed for the loss of life.
In 1956 theClean Air Act started legally enforcingsmokeless zones in the capital. There were areas where no soft coal was allowed to be burned in homes or in businesses, onlycoke, 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 theBritish Museum of Natural History. A more recent example was thePalace of Westminster, which was cleaned in the 1980s. A notable exception to the restoration trend was10 Downing Street, whose bricks upon cleaning in the late 1950s proved to be naturallyyellow; the smog-derived black color of the façade was considered so iconic that the bricks were painted black to preserve the image.[93][94] Smog caused by traffic pollution, however, does still occur in modern London.
Grease filter hood after 4 days at Italian city polluted air in winter (all surface was 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.[95]
OneBirmingham 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.[96]
On 29 April 2015, theUK Supreme Court ruled that the government must take immediate action to cut air pollution,[97] following a case brought by environmental lawyers at ClientEarth.[98]
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 ofMexico 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 worst pollution, with pollutants likenitrogen dioxide being double or even triple international standards.[99]
Photochemical smog over Mexico City, December 2010
Similar to Mexico City, the air pollution of theSantiago valley in Chile, located between theAndes and theChilean Coast Range, turn it into the most infamously smog-plagued city of South America. Other aggravates of the situation reside in its high latitude (31 degrees South) and dry weather during most of the year.
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 well-being across the country. It was estimated that the province ofOntario sustains $201 million in damages annually for selected crops, and an estimated tourism revenue degradation of $7.5 million inVancouver and $1.32 million in TheFraser Valley due to decreased visibility.Air pollution in British Columbia is of particular concern, especially in the Fraser Valley, because of a meteorological effect calledinversion which decreases air dispersion and leads to smog concentration.[100]
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 3,000 acres (12 km2) of a farmer's spinach crop.[101] Since then, theUnited 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 theNational Ambient Air Quality Standards.[102] 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 andforecasts.[103] To combat smog conditions, localities may declare "smog alert" days, such as in theSpare the Air program in theSan Francisco Bay Area. By 1970, Congress enacted the Clean Air Act to regulate air pollutant emissions.[104]
In the United States, smog pollution kills 24,000 Americans every year. The U.S. is among the dirtier countries in terms of smog, ranked 123 out of 195 countries measured, where 1 is cleanest and 195 is most smog polluted.[105]
Because of their locations in low basins surrounded by mountains,Los Angeles and theSan Joaquin Valley are notorious for their smog. Heavy automobile traffic, combined with the additional effects of theSan Francisco Bay and Los Angeles/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 aninversion layer: a phenomenon where air temperature increases, instead of decreasing, with altitude, suppressingthermals 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 thatLos Angeles was a synonym forsmog.[106] 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.[107] However, ensuing strict regulations by state and federal government agencies overseeing this problem (such as theCalifornia Air Resources Board and theUnited 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.[108] For example, air concentrations of volatile organic compounds declined by a factor of 50 between 1962 and 2012.[109] Concentrations of air pollutants such as nitrous oxides and ozone declined by 70% to 80% over the same period of time.[110]
26 July 1943,Los Angeles, California: A smog so sudden and severe that "Los Angeles residents believe the Japanese are attacking them with chemical warfare."[111][112]
The severity of smog is often measured using automated optical instruments such asnephelometers, 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 AmericanAir Quality Index, the Malaysian API (Air Pollution Index), and the SingaporeanPollutant 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.
The Malaysian API does not have a capped value. Hence, its most hazardous readings can go above 500. When the reading goes above 500, a state of emergency is declared in the affected area. Usually, this means that non-essential government services are suspended, and all ports in the affected area are closed. There may also be prohibitions on private sector commercial and industrial activities in the affected area excluding the food sector. So far, the state of emergency rulings due to hazardous API levels was applied to the Malaysian towns of Port Klang, Kuala Selangor, and the state of Sarawak during1997 Southeast Asian haze and the2005 Malaysian haze.[needs update]
Claude Monet made several trips to London between 1899 and 1901, during which he painted views of theThames andHouses of Parliament which show the sun struggling to shine through London's smog-laden atmosphere.
The 1970made-for-TV movieA 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.[115]
The history of smog in LA is detailed inSmogtown byChip Jacobs and William J. Kelly.[116]
^Meyrick, Henry (1880).Santa Cruz & Monterey Illustrated Handbook. San Francisco News Publishing Co. pp. 7–8.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.
^"The morning fog".Santa Cruz Weekly Sentinel. 3 July 1880. p. 3. Archived fromthe original on 14 April 2021. Retrieved18 September 2019.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.
^Playhouses without Plays, Sporting Times, London, 17 December 1881, p6. Accessed 12 September 2020, The British Newspaper Archive.
^"Health Effects of Ozone in the General Population".Ozone and Your Patients' Health: Training for Health Care Providers. United States Environmental Protection Agency. 10 September 2013.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.
^Pedersen, Marie; Giorgis-Allemand, Lise; Bernard, Claire; Aguilera, Inmaculada; Andersen, Anne-Marie Nybo; et al. (2013). "Ambient air pollution and low birthweight: A European cohort study (ESCAPE)".The Lancet Respiratory Medicine.1 (9):695–704.arXiv:0706.4406.doi:10.1016/S2213-2600(13)70192-9.PMID24429273.
^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
^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
Brimblecombe, Peter. "History of air pollution." inComposition, Chemistry and Climate of the Atmosphere (Van Nostrand Reinhold (1995): 1–18
Brimblecombe, Peter, andLászló 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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