Smog: Urban Air Pollution

Smog is a toxic haze which forms in and around cities from a mixture of gases and particles emitted from motor vehicles, coal-fired power plants, factories, and other sources, which can react with heat and sunlight in the atmosphere. It is a particular problem in many Asian cities, where air quality can be further reduced by the burning of crop stubble. We explain the main types of smog pollution and the pollutants involved. We also look at their impact on health and their links with climate change.
Smog over city of Krakow, Poland
Wawel Royal Castle in the early morning smog with Krakow’s coal-fired power plant in background. Photo: © CC BY-SA 3.0

What is Smog?

Smog is a common form of air pollution which occurs mostly in urban areas. There are two main types: (a) regular, or sulfur-based smog, sometimes called “London smog”, and (b) photochemical smog.

Up until the 1950s, smog used to be exclusively sulfur-based, and resulted from a mixture of smoke, from burning high-sulfur coal, and fog – hence the name.

Since then, with the introduction of clean air legislation and cleaner domestic fuels, most smog has been caused by photochemical processes. These are driven mainly by chemical compounds from vehicle exhausts, which react with each other in sunlight.

This is why electric vehicles (EVs) are so much better than conventional internal combustion vehicles (ICVs), because EVs emit no gases at all.

Both London and photochemical smog are aggravated by inhalable airborne particles, known as particulate matter (PM), and by certain meteorological and topographical features.

In general, smog formation is driven by both primary and secondary pollutants. Primary pollutants are those emitted directly from a particular source, such as emissions of soot or sulfur dioxide from coal combustion.

Secondary pollutants are formed when primary pollutants undergo chemical reactions in the atmosphere. For example, the secondary pollutant ozone is formed when primary components like nitrogen oxides and volatile organic chemicals (VOCs) react together in the presence of sunlight.

During the warm, sunlit months of summer in the Northern Hemisphere, the dominant type of city pollution is photochemical smog. During the colder winter months, when more coal and other fossil fuels are burned for heat, when thermal inversions are more common, but when there is less sunlight, the main type of city pollution is regular smog.

Thermal Inversion Diagram
Diagram of a thermal inversion (also known as an atmospheric inversion) showing how high pressure can keep cold air trapped at the surface, underneath a layer of warm air. Any smog that forms in the cold layer will remain trapped by the lid of the warm layer overhead. Image credit: (CC BY-NC-ND 2.0)

Smog is a worldwide phenomenon affecting cities such as Santiago, Los Angeles, Denver, Mexico City, Tehran, Delhi, Lahore, Karachi, Beijing, Jinan, Jakarta, Hanoi, and Kuala Lumpur, to name but a few. It is most common in India and China, whose cities regularly top the most-polluted list. Asian air pollution, which has spawned vast toxic hazes like the Asian Brown Cloud, has worsened considerably since the continent became the world’s manufacturing center.

Another growing contributor to smog is naturally occurring air pollutants caused by forest fires dried out by global warming and drought. The recent Arctic fires (2019) and Australian Bushfires (2019-2020) caused smog-like conditions in northern Europe, and in both Asia and South America, respectively.

Exposure to smog can cause a number of serious health conditions, ranging from asthma and other breathing difficulties, to cardiovascular disease and lung cancer.

Sydney enveloped in smog-like haze produced by wildfires in the Blue Mountains. 2019. Photo: Sardaka (CC BY-SA 4.0)

London Smog

The word ‘smog’ was first used towards the end of the 19th century, to refer to regularly occurring toxic London fogs known as ‘pea soupers’. London smog was no more than a regular thick fog made heavier, darker, more toxic and much less breathable, by soot and smoke from burning high-sulfur coal in domestic homes, and in local coal-fired power plants at Fulham, Battersea, Bankside, and Greenwich.

What’s more, if high pressure weather conditions caused cold London air to become trapped by an overhead layer of warm air (a phenomenon known as a thermal or atmospheric inversion), the trapped air pollutants could build up across the city and cause a major health problem.

The Great Smog of 1952, for example, which lasted from Friday 5 December to Tuesday 9 December, killed an estimated 12,000 people in the space of four days. 1 The Great Smog also boosted indoor air pollution to unprecedented levels.

The 1956 Clean Air Act, together with new regulations on the burning of cleaner coals and coke eventually eliminated traditional smog in London, although it is still prevalent in areas of the world that burn large amounts of coal in winter. In India, for example, cheaper coal with its emissions of fly ash and sulfur dioxide, plus added soot from the burning of wood and crop stubble, can create a very sulfurous and highly toxic winter smog.

When Was the Word ‘Smog’ First Used?

The word smog was first used in a column on page 3 of the Santa Cruz Weekly Sentinel on July 3, 1880. Another reference appeared in the Sporting Times (London) on December 17, 1881. However, the term was first popularized by Dr. Henry Antoine Des Voeux, treasurer of the Smoke Abatement League, in a speech written for a meeting of the Public Health Congress in July 1905. The speech was duly reported in the Journal of the American Medical Association, in its edition of August 26, 1905.

Des Voeux again referred to ‘smog’ in 1911, during his report to the Manchester Conference of the Smoke Abatement League of Great Britain, when describing the 1,000 “smoke-fog” deaths that occurred in Edinburgh and Glasgow during the late autumn of 1909.

Diagram showing primary and secondary pollutants in photochemical smog.
Diagram showing primary and secondary pollutants in photochemical smog. Secondary pollutants include ozone and Peroxyacetyl nitrates (PANs). © Li-Wei Chao (CC BY-SA-4.0)

Formation and Composition of Photochemical Smog

Photochemical smog typically forms when three sets of air pollutants build up in the troposphere, in the presence of sunlight.

Primary Pollutants

The first set of pollutants (primary pollutants) are volatile organic compounds (emitted from petroleum combustion, paints, and cleaning solvents) and nitrogen oxides (from car exhausts, coal-fired power plants).

Secondary Pollutants

These VOCs and NOx chemicals are broken down by the sunlight and in the process form the second set of air pollutants (secondary), including ground-level ozone, peroxylacyl nitrates (PAN), and masses of tiny particles from combusted gases, including sulfur dioxide and ammonium nitrate.

Airborne Particulate Matter

The third set of pollutants (primary) consists of additional airborne particulates (PM10 and PM2.5) that are released directly into the air during the combustion of fossil fuels in vehicles and power plants. These particles include aerosols of fly ash, soot and black carbon, as well as aeolian dust, road dust, windblown soil and other material. These airborne particles give the smog its characteristic brownish-yellow colour.

The Science of Tropospheric Ozone
The formation of ozone goes something like this.

Nitrogen dioxide (NO2) from vehicle tailpipes is broken down by sunlight. This generates nitrogen oxide (NO) and a lone oxygen atom (O).

The single oxygen atom (O) then combines with an oxygen molecule (O2) to produce ozone (O3).

Usually, most ozone molecules (O3) oxidize (give oxygen to) nitrogen oxide (NO) turning it back into nitrogen dioxide (NO2), which results in only a very slight build-up of ozone close to the ground.

However, when volatile organic compounds (VOCs) build up in the atmosphere, the equation changes completely. The highly reactive VOCs oxidize nitrogen oxide (NO) into nitrogen dioxide (NO2), without breaking down any ozone molecules in the process. This results in a rapid increase of ozone at ground level and the consequent formation of smog.

The formation of smog is also affected by three other factors: (a) The burning of biomass and other materials. (b) Thermal inversion. (c) Absence of wind and rain.

(a) Biomass Burning

In some Asian cities, like Delhi for example, urban smog is aggravated by the burning of crop stubble or fuelwood, or by the use of fireworks during important festivals. 2

(b) Thermal Inversion

Thermal inversions (also called atmospheric inversions) that trap air pollution close to the ground exert a major influence on the severity of photochemical smog levels in Los Angeles, Sacramento, Salt Lake City, Beijing, Delhi, Lahore, Mexico City, Tehran and other metro areas.

Smog over Mexico City
Aerial View of Photochemical Smog Pollution Over Mexico City. Photo: © Fidel Gonzalez (CC BY-SA 3.0)

(c) Absence of Wind and Rain

Smog is dispersed mostly by wind and rain. Wind blows it away; rain washes it out of the atmosphere. The Asian Brown Cloud only materializes during the winter months because these are the driest months of the year in India and China.

Generally speaking, whenever there is a build-up of the above-mentioned groups of pollutants, in dry, calm conditions, then smog will form. Furthermore, when sulfur dioxide and nitrogen oxides are released into the atmosphere, they may be converted to nitric acid and sulfuric acid. If this acidic matter then mixes with rainwater, it falls to Earth as acid rain.

In many respects, photochemical smog is a problem of modern industrialization. As industrial productivity rises and economic fortunes prosper, cities become choked with pollution and their air becomes unbreathable. But this should come as no surprise. When more and more dirty fossil fuels are consumed, when tens of thousands of bigger, faster cars fill the air with clouds of toxic exhaust gases, and other industrial contaminants including microscopic particles of soot and dust, the process is bound to end in tears.

NOTE: It wasn’t until the early 1950s that the chemical reactions between fossil fuel gases, PM2.5, sunlight and ozone, were properly understood. In the end it was thanks to the research of Dutch chemist Arie Haagen-Smit (1900-77) in southern California, that identified ozone as an important component in photochemical pollution. Haagen-Smit collaborated with American chemist Arnold Beckman (1900-2004), who invented various instruments and air quality measuring equipment for both the government and industry.

Effects on Human Health

Smog’s effect on human health varies from mild symptoms such as coughing, sneezing and irritation of the eyes, to potentially fatal conditions, like heart disease and lung cancer. Pollutants like ozone, sulphur dioxide, nitrogen dioxide and carbon monoxide are especially harmful for children, senior citizens, and people with heart and lung conditions such as asthma, bronchitis, and emphysema. 3 For more, see: Health Effects of Air Pollution.

Ozone, in particular, can irritate the eyes, throat and airways. If inhaled, it can trigger a variety of health problems including coughing, pharyngitis, breathing difficulties and asthma attacks. It exacerbates bronchitis and lung problems, and is especially harmful to people with respiratory and cardiopulmonary problems.

Because ozone is highly reactive, it has the capacity to damage lung tissue. Long-term exposure to ozone causes a decrease in lung elasticity, premature lung aging and heightens the risk of respiratory failure and lung cancer.

An 18-year study of 450,000 people living in U.S. urban areas showed that inhabitants of cities with high ozone levels, such as Los Angeles, Sacramento or Houston, had a 30 percent increased risk of dying from lung disease. Prolonged exposure to ozone also leads to a permanent reduction in lung function, and an increased risk of developing asthma.

Smog contains large quantities of dangerous microscopic particles (PM2.5 and ultrafine matter) which can pass through the body’s filters and penetrate the lungs and other organs. In 2016, for example, an international study under Professor Barbara Maher of Lancaster University, found microscopic particles lodged inside the brains of 37 individuals who had lived in Manchester or Mexico City – both air pollution hotspots. These tiny particles are thought to be a possible cause of Alzheimer’s disease. 4

There are also well-documented links between smog and overall mortality. One study, published in Nature magazine, revealed that smog in the Chinese city of Jinan (pop: 8,700,000), during the years 2011–15, accounted for a 5.87 percent rise in the rate of overall mortality. 5

Delhi is the world’s most toxic city, with the highest level of PM2.5. Its smog and other forms of air pollution account for roughly 10,500 premature deaths every year. 6 High concentrations of air pollutants in the city have led to a significant rise in lung-related conditions (especially asthma and lung cancer) among Delhi’s women and children.

In April 2020, following the coronavirus shutdowns, Asian skies cleared temporarily as industrial plants ceased operations. See: Effect of COVID-19 on Climate Change.

In the United States, according to “The State of Global Air 2019” report compiled by the Health Effects Institute, smog pollution kills 24,000 Americans per year. On a scale from the cleanest to the dirtiest, the U.S. is ranked 123rd out of 195 countries measured. 7

How is Smog Linked to Climate Change?

The main connection between smog and climate change is that both are driven by the burning of fossil fuels (coal and petroleum). Vehicular greenhouse gas emissions, in particular, are a major cause of both global warming and smog.

Secondly, ground level ozone is a heat-trapping greenhouse gas with relatively strong, albeit very short-term effects.

Its annual global warming potential (GWP) is between 918 and 1022, which means that it traps roughly 1,000 times more heat than carbon dioxide (CO2). However, due to its instability, it only lasts for about 3 weeks in the atmosphere. So, its GWP over 20 years is no more than 62-69. As a result, it does not have a significant global effect, although it can have strong radiative effects on a local or regional level – up to one and a half times that of CO2.

That said, ozone is likely to have a greater impact on our climate system in the years to come, for two main reasons. First, rising temperatures tend to boost ozone production. For example, scientists estimate that by the year 2050, warming alone may increase the number of high ozone days across the eastern United States by more than two-thirds.

Thirdly, global warming is also expected to reduce the overall frequency of surface cyclones. This in turn will create more stagnant atmospheric conditions, which will result in the production of more ozone.


  1. “A Retrospective Assessment of Mortality from the London Smog Episode of 1952: The Role of Influenza and Pollution”. Bell, M.L.; Davis, D.L.; Fletcher, T. (2004). Environ Health Perspect. 112 (1, January): 6–8. []
  2. “Holiday fireworks can bring extreme pollution, India finds.” []
  3. Source: U.S. Environmental Protection Agency []
  4. “Magnetite pollution nanoparticles in the human brain.” Barbara A. Maher, et al. PNAS September 6, 2016. []
  5. “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). (9) []
  6. “Delhi’s Air Has Become a Lethal Hazard and Nobody Seems to Know What to Do About It”. Time magazine. 10 February 2014. []
  7. “State of Global Air 2019.” HEI. []
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