Smog is a type of air pollutant. The word “smog” was coined in the early 20th century as a blend of the words smoke and fog to refer to smoky fog, its opacity, and odour.[1] 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. This kind of visible air pollution is composed of nitrogen oxides, sulphur oxides, ozone, smoke or dirt particles and also less visible particles such as CFC’s. Human-made smog is derived from coal emissions, vehicular emissions, industrial emissions, forest and agricultural fires and photochemical reactions of these emissions.

Modern smog, as found for example in Los Angeles, is a type of air pollution derived from vehicular emission from internal combustion engines and industrial fumes that 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. The atmospheric pollution levels of Los Angeles, Beijing, Delhi, Lahore, Mexico City, Tehran and other cities are increased by inversion that traps pollution close to the ground. It is usually very highly toxic to humans and can cause severe sickness, shortened life or death.


Smog in New York City as viewed from the World Trade Center in 1988


Coinage of the term “smog” is generally attributed to Dr. 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’.”[2] The following day the newspaper stated that “Dr. Des Voeux did a public service in coining a new word for the London fog.” However, this is predated by a Los Angeles Times article of January 19, 1893, in which the word is attributed to “a witty English writer.”



Coal fires, used to heat individual buildings or in a power-producing plant, can emit significant clouds of smoke that contributes to smog. Air pollution from this source has been reported in England since the Middle Ages.[3][4] 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 combustions are one of the main causes of air pollution in China.[5] Especially during autumn and winter when coal-fired heating ramps up, the amount of produced smoke forces some Chinese cities to close down roads, schools or airports. One prominent example for this was China’s Northeastern city Harbin in 2013.

Transportation emissions

Traffic emissions – such as from trucks, buses, and automobiles – also contribute.[6] Airborne by-products from vehicle exhaust systems cause air pollution and are a major ingredient in the creation of smog in some large cities.[7][8][9][10]

The major culprits from transportation sources are carbon monoxide (CO),[11][12] nitrogen oxides (NO and NOx),[13][14][15] volatile organic compounds,[12][13] sulphur dioxide,[12]and hydrocarbons.[12] (Hydrocarbons are the main components of petroleum fuels such as gasoline and diesel fuel.) These molecules react with sunlight, heat, ammonia, moisture, and other compounds to form the noxious vapors, ground level ozone, and particles that comprise smog.[12][13]

Photochemical smog

Formation of Photochemical Smog

Photochemical smog is the chemical reaction of sunlight, nitrogen oxides and volatile organic compounds in the atmosphere, which leaves airborne particles and ground-level ozone.[16] This noxious mixture of air pollutants may include the following:

  • Aldehydes
  • Nitrogen oxides, particularly nitric oxide and nitrogen dioxide
  • Peroxyacyl nitrates
  • Tropospheric ozone
  • Volatile organic compounds

A primary pollutant is an air pollutant emitted directly from a source. A secondary pollutant is not directly emitted as such, but forms when other pollutants (primary pollutants) react in the atmosphere. Examples of a secondary pollutant include 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 in the air; and acid rain, which is formed when sulfur dioxide or nitrogen oxides react with water.[17] 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.[18] Because it travels with the wind, it can affect sparsely populated areas as well.

The composition and chemical reactions involved in photochemical smog were not understood until the 1950s. In 1948, flavor chemist 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 nitrous 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.[19]:219–224[20][21] 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 October 7, 1952, to “air quality monitoring vans” for use by government and industry.[19]:224–226

Characteristic coloration for smog in California in the beige cloud bank behind the Golden Gate Bridge. The brown coloration is due to the NOx in the photochemical smog.

Natural causes

An erupting volcano can also 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 radiocarbon content of some plant life has been linked to the distribution of smog in some areas. For example, the creosote bush in the Los Angeles area has been shown to have an effect on smog distribution that is more than fossil fuel combustion alone.[22]

Health effects

Highland Park Optimist Clubwearing smog-gas masks at banquet, Los Angeles, circa 1954

Smog is a serious problem in many cities and continues to harm human health.[23][24] Ground-level ozone, sulphur 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.[25] 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. Hospital admissions and respiratory deaths often increase during periods when ozone levels are high.[26][27] 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.[28] 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.[29]

Levels of unhealthy exposure

The 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 ppbv are described as “Unhealthy for Sensitive Groups”, 105 ppbv to 124 ppbv as “unhealthy” and 125 ppb to 404 ppb as “very unhealthy”.[25] The “very unhealthy” range for some other pollutants are: 355 μg m−3 – 424 μg m−3 for PM10; 15.5 ppm – 30.4ppm for CO and 0.65 ppm – 1.24 ppm for NO2.[30]

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.[31]

A 20-year American Cancer Society study found that cumulative exposure also increases the likelihood of premature death from a respiratory disease, implying the 8-hour standard may be insufficient.[32]

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 nanoparticles 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 are associated with neurodegenerative diseases including Alzheimer’s disease.[33] [34]

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 Valleyarea of California was linked to two types of neural tube defects: 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).[35]

Low birth weight

According to a study published in The Lancet, even a very small (5 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.[36]

Areas affected

Beijing air on a day after rain (left) and a smoggy day (right)

Smog can form in almost any climate where industries or cities release large amounts of air 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.


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 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 which decreases air dispersion and leads to smog concentration.[37]

Delhi, India

During the autumn and winter months, some 500 million tons of crop residue are burnt, and winds blow from India’s north and northwest towards east.[38][39][40] 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 quite drastic 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.[41]

Delhi is the most polluted[42] city in the world and according to one estimate, air pollution causes the death of about 10,500 people in Delhi every year.[43][44][45] During 2013–14, peak levels of fine 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.[43][46][47][48] Delhi has the highest level of the airborne particulate matter, PM2.5 considered most harmful to health, with 153 micrograms.[49] Rising air pollution level has significantly increased lung-related ailments (especially asthma and lung cancer) among Delhi’s children and women.[50][51] The dense smog in Delhi during winter season results in major air and rail traffic disruptions every year.[52] According to Indian meteorologists, the average maximum temperature in Delhi during winters has declined notably since 1998 due to rising air pollution.[53]

Dense smog blankets Connaught Place, New Delhi

Environmentalists have criticised the Delhi government for not doing enough to curb air pollution and to inform people about air quality issues.[44] Most of Delhi’s residents are unaware of alarming levels of air pollution in the city and the health risks associated with it.[47][48] Since the mid-1990s, Delhi has undertaken some measures to curb air pollution – Delhi has the third highest quantity of trees among Indian cities[54] and the Delhi Transport Corporation operates the world’s largest fleet of environmentally friendly compressed natural gas (CNG) buses.[55] 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”.[55] The Delhi Metro has also been credited for significantly reducing air pollutants in the city.[56]

However, according to several authors, most of these gains have been lost, especially due to stubble burning, rise in market share of diesel cars and a considerable decline in bus ridership.[57][58] 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.[59][60] 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.[61][62]

Beijing, China

Joint research between American and Chinese researchers in 2006 concluded that much of the city’s pollution comes from surrounding cities and provinces. On average 35–60% of the ozone can be traced to sources outside the city. Shandong Province and Tianjin Municipality have a “significant influence on Beijing’s air quality”,[63] partly due to the prevailing south/southeasterly flow during the summer and the mountains to the north and northwest.

United Kingdom


Victorian London 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 period costume drama

In 1306, concerns over air pollution were sufficient for Edward I to (briefly) ban coal fires in London.[3] In 1661, John Evelyn‘s Fumifugium 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.” The Great Smog of 1952 darkened the streets of London and killed approximately 4,000 people in the short time of 4 days (a further 8,000[64] 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 started legally enforcing smokeless zones in the capital. There were areas where no soft coal was allowed to be burned in homes or in businesses, only 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 colour of the façade was considered so iconic that the bricks were painted black to preserve the image.[65][66] Smog caused by traffic pollution, however, does still occur in modern London.

Other areas

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.[67]

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 in order to stay on the road.[68]

On 29 April 2015, the UK Supreme Court ruled that the government must take immediate action to cut air pollution,[69] following a case brought by environmental lawyers at ClientEarth.[70]

Mexico City, Mexico

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 worst pollution, with pollutants like nitrogen dioxide being double or even triple international standards.[71]

Photochemical smog over Mexico City, December 2010

Santiago, Chile

Similar to Mexico City, the air pollution of Santiago valley, located between the Andes and the Chilean 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.

Tehran, Iran

In December 2005, schools and public offices had to close in Tehran and 1600 people were taken to hospital, in a severe smog blamed largely on unfiltered car exhaust.[72]

United States

A NASA astronaut’s photograph of a smog layer over central New York
View of smog south from Los Angeles City Hall, September 2011
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 US 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.[73] 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.[74] 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 forecasts.[75] To combat smog conditions, localities may declare “smog alert” days, such as in the Spare the Air program in the San Francisco Bay Area.

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. The over-reliance on vehicles for transportation in these regions combined with the additional effects of the San Francisco Bayand Los Angeles/Long Beach port complexes frequently contribute to further air pollution.

Los Angeles in particular is strongly predisposed to accumulation of smog, because of peculiarities of its geography and weather patterns. Los Angeles is situated in a flat basin with 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 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.

Though 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,[76] strict regulations by government agencies overseeing this problem, 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. For example, air concentrations of volatile organic compounds declined by a factor of 50 between 1962 and 2012.[77] Concentrations of air pollutants such as nitrous oxides and ozone declined by 70% to 80% over the same period of time.[78]

Major incidents in the US

  • 1943, July 26, Los Angeles, California: A smog so sudden and severe that “Los Angeles residents believe the Japanese are attacking them with chemical warfare.”[79][80]
  • 1948, October 30–31, Donora, Pennsylvania: 20 died, 600 hospitalized, thousands more stricken. Lawsuits were not settled until 1951.[81]
  • 1966, November 24, New York City, New York: Smog kills at least 169[82] people.

Ulaanbaatar, Mongolia[edit]

In the late 1990s, massive immigration to Ulaanbaatar from the countryside began. An estimated 150,000 households, mainly living in traditional Mongolian gers 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.[83]

Southeast Asia

Singapore‘s Downtown Core on 7 October 2006, when it was affected by forest fires in Sumatra, Indonesia

Smog is a regular problem in Southeast Asia caused by 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 responsible for the fires, which they use to clear tracts of land for further plantings. Those fires mainly affect Brunei, Indonesia, Philippines, Malaysia, Singaporeand Thailand, and occasionally Guam and Saipan.[84][85] The economic losses of the fires in 1997 have been estimated at more than US$9 billion.[86] 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 second-latest bout of haze to occur in Malaysia, Singapore and the Malacca Straits is in October 2006, and was caused by smoke from fires in Indonesia being blown across the Straits of Malacca by south-westerly winds. A similar haze has occurred in June 2013, with the PSI setting a new record in Singapore on June 21 at 12pm with a reading of 401, which is in the “Hazardous” range.[87]

The Association of Southeast Asian Nations (ASEAN) reacted. In 2002, the Agreement on Transboundary Haze Pollution was signed between all ASEAN nations.[88] ASEAN formed a Regional Haze Action Plan (RHAP) and established a co-ordination and support unit (CSU).[89] 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.[90] Indonesia has been ineffective at enforcing legal policies on errant farmers.[citation needed]


A road close to historical Badshahi mosque while dense smog engulf the neighborhood of Lahore, Pakistan, Saturday, Nov. 5, 2016

Since start of winter season heavy smog loaded with pollutants covered major part of Punjab especially the city of Lahore,[91] causing breathing problems and disrupting normal traffic.[92]

Doctors advised residents to stay indoors and wear facemasks outside.[93]

Pollution index

Smog in São Paulo, Brazil

The severity of smog is often measured using automated optical instruments such as Nephelometers, 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, 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.

The Malaysian API does not have a capped value; hence its most hazardous readings can go above 500. 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, state of emergency rulings due to hazardous API levels were applied to the Malaysian towns of Port Klang, Kuala Selangor and the state of Sarawak during the 2005 Malaysian haze and the 1997 Southeast Asian haze.

Cultural references

Claude Monet made several trips to London between 1899 and 1901, during which he painted views of the Thames and Houses of Parliamentwhich show the sun struggling to shine through London’s smog-laden atmosphere.
  • The London “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 Dickens‘s Bleak House (1852) and T.S. Eliot‘s “The Love Song of J. Alfred Prufrock.”
  • 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.[94]
  • The history of smog in LA is detailed in Smogtown by Chip Jacobs and William J. Kelly.[95]


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