During the winter months when the temperatures are colder, and atmospheric inversions are common, there is an increase in coal and other fossil fuel usage to heat homes and buildings.

Photochemical 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.

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'.

[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.

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 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 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.

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.

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.

[39] 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.

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.

[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 the San Joaquin Valley area 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).

[44] 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.

[51] 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.

[57][58][59] 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.

[69] 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".

[70] 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.

[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.

Shandong Province and Tianjin 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 in Tehran and 1,600 people were taken to hospital, in a severe smog blamed largely on unfiltered car exhaust.

[citation needed] Since the start of the winter season, heavy smog loaded with pollutants covered major parts of Punjab, especially the city of Lahore,[87] causing breathing problems and disrupting normal traffic.

[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, and boundary layer mixing.

[52] In post-moonsoon, anthropogenic emissions from sources like vehicle exhaust, industrial activities, and crop burning impact air quality across Punjab, Pakistan, affecting the region by 90–100%.

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] 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.

[101] 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.

Heavy automobile traffic, combined with the additional effects of the San Francisco Bay and Los Angeles/Long Beach port complexes, frequently contribute to further air pollution.

[107] 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.