Smog is the term chosen by the Glasgow (Scotland) public health official Henry Antoine Des Voeux at the beginning of the twentieth century to describe the smoky fogs that characterized coal-burning cities of the time. The word, formed by combining the words smoke and fog, has persisted as a description of this type of urban atmosphere. It has frequently been used to describe photochemical smog, the haze that became a characteristic of the Los Angeles Basin (in southern California) from the 1940s. Smog is sometimes even used to describe air pollution in general, even where there is not a reduction in visibility at all. However, the term is most properly used to describe the two distinctive types of pollution that dominated the atmospheres of late-nineteenth-century London and other industrial cities in England, known as winter smog, and twentieth-century Los Angeles, called summer smog.


Smog can be described by using two prominent real-life examples:

Smog over northeastern China, including the cities of Beijing & Tianjin, on December 6, 2016, as captured by the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite.

Smog over northeastern China, including the cities of Beijing & Tianjin, on December 6, 2016, as captured by the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite.
(NASA/Jeff Schmaltz, LANCE/EOSDIS Rapid Response/Science Source)
Nineteenth-century London smog

The city of London (United Kingdom [U.K.]) burned almost 20 million tons (18 million metric tons) of coal annually by the end of the nineteenth century. In addition to what was used in factories, much coal was burned in domestic hearths, and the smoke and sulfur dioxide produced barely rose from the chimneys above the housetops. Only a few rather inaccurate measurements of the pollutant concentrations in the air were made in the nineteenth century, although they hint at concentrations much higher than what scientists might expect in London today.

Morning traffic at Blackfriars, London, almost at a standstill because of the blanket smog in 1952.

Morning traffic at Blackfriars, London, almost at a standstill because of the blanket smog in 1952.
(Hulton Archive/Getty Images)

The smog of nineteenth-century London took the form of dense, vividly colored fogs. The smog was frequently so dense that torches were provided at door entries for people to use to find their way to waiting carriages or to their homes on foot. It is said that visibility became so restricted that fingers on an outstretched arm were invisible. The fog that rolled over window sills and into rooms became such an integral part of what people know as Victorian London that almost any Sherlock Holmes story mentions it. (Scottish author and physician Sir Arthur Conan Doyle (1859–1930) created the fictional detective Sherlock Holmes.)

With London's high humidity and frequent fog, smoke particles from coal-burning formed a nucleus for the condensation of vapor into large fog droplets. This water also served as a site for chemical reactions, in particular the formation of sulfuric acid. Sulfur dioxide dissolved in fog droplets, perhaps aided by the presence of alkaline material such as ammonia or coal ash. Once in solution the sulfur was oxidized, a process often catalyzed by the presence of dissolved metallic ions such as iron and manganese. Dissolution and oxidation of sulfur dioxide gave rise to sulfuric acid droplets, and it was sulfuric acid that made the smog so damaging to the health of Londoners.

London's severe smog occurred throughout the last decades of the nineteenth century. British-Canadian detective writer Robert Barr (1849–1912) even published The Doom of London at the turn of the century, which saw the entire population of London eliminated by an apocalyptic fog.

Twentieth-century London smog

Many residents of Victorian London recognized that the fogs increased death rates, but the most infamous twentieth-century incident occurred in 1952, when a slow-moving anti-cyclone stalled the air over the city. On the first morning, the fog was thicker than many people could ever remember. By the afternoon people noticed the choking smell in the air and started experiencing discomfort. Those who walked about in the fog found their skin and clothing filthy after just a short time. At night, the treatment of respiratory cases was running at twice its normal level. The situation continued for four days.

It was difficult to describe exactly what had happened, because primitive air pollution monitoring equipment could not cope with high and rapidly changing concentrations of pollutants, but it has been argued that for short periods the smoke and sulfur dioxide concentrations may have approached ten thousand micrograms per 1.3 cubic yards (1 cubic meters).

While London smog conditions generally improved with the decades of the twentieth century, such was not the case between December 5 and December 9, 1952, when the Great Smog of ‘52 or Big Smoke, as it was called, occurred. During these four days, windless cold weather combined with factory emissions to render visibility practically nil. Peasoup conditions were not unusual for London, but these conditions were proven to be unusually severe. Later medical reports found that about four thousand people had died prematurely, probably because of the air pollution. The government, barraged with questions, set up an investigative committee, and its findings eventually served as the basis for the U.K. Clean Air Act of 1956. This law was gradually adopted through many towns and cities of the United Kingdom and was seen by many as a model piece of legislation. In addition, wide use of electricity in homes reduced, if it did not completely eliminate, the use of coal-burning heaters, a chief source of urban smoke.

Los Angeles summer smog

Old factory polluting the atmosphere with smoke and smog.

Old factory polluting the atmosphere with smoke and smog.

It was some time before Dutch biochemist Arie Jan Haagen-Smit (1900–1977) recognized that damage to crops in the Los Angeles area arose not from familiar pollutants, but from a reaction that takes place in the presence of petroleum vapors and sunlight. His observations focused unwelcome attention on the automobile as an important factor in the generation of summer smog.

The Los Angeles basin proved an almost perfect place for generating smog of this type. It had a huge number of cars and heavy commuter traffic, long hours of sunshine, gentle sea breezes to help pollutants accumulate up against the mountains, and high level inversions preventing the pollutants from dispersing vertically.

Studies through the 1950s revealed that the smog was generated through a photolytic cycle. Sunlight split nitrogen dioxide into nitric oxide and atomic oxygen that could subsequently react and form ozone (O3). This was the key pollutant that clearly distinguished the Los Angeles smog from that occuring in London. Although the highly reactive ozone reacts rapidly with nitric oxide, converting it back into nitrogen dioxide, organic radicals produced from petroleum vapor react with nitric oxide very quickly. The nitrogen dioxide is again split by the sunlight, leading to the formation of more ozone. The cycle continues to build ozone concentrations to higher levels throughout the day.

The reactions that were recognized in the Los Angeles smog are now known to occur over wide areas of the industrialized world. The production of smog of this kind is not limited to urban or suburban areas but may occur for many hundreds of miles to the lee of cities using large quantities of liquid fuel. The importance of hydrocarbons in sustaining the processes that generate photochemical smog has given rise to control policies that recognize the need to lower the emission of hydrocarbons into the atmosphere. Hence, these policies have emphasized the use of catalytic converters and low volatility fuels as part of air pollution control strategies.


Summer smog occurs in areas where heavy traffic involving motorized vehicles are present, along with climate conditions that include high temperatures (during the summer months) and calm winds. Serious smog problems exist in many metropolitan areas around the world. In the United States, smog exists in California from San Francisco in the north to San Diego in the south. On the East Coast, smog exists from above Boston, Massachusetts, to Washington, DC. Many other large cities in the United States also experience smog. In fact, most U.S. cities with populations over 250,000 people have problems with smog. The U.S. Environmental Protection Agency reports that over half of all Americans live in areas where pollution levels, including smog, exceed U.S. air quality standards. In addition, Mexico City, Mexico, and Beijing, China, are two well-known cities that regularly experience smog conditions.

Causes and symptoms

Smog is caused by a series of photochemical reactions involving volatile organic compounds (VOCs), nitrogen oxides and sunlight. These complex reactions form ground-level ozone, which produce smog. Pollutants from power plants, factories, motorized vehicle exhaust, and consumer products help to form smog. Smog can cause symptoms respiratory problems such as dyspnea (difficulty breathing), hypopnea (shallow breathing), hyperpnea (deep breathing), tachypnea (rapid breathing), and, in extreme cases, apnea (absence of breathing). Eye irritation is one symptom that is common to all smog-related illnesses.


Common diseases and disorders

Smog can cause such respiratory disorders as asthma, chronic bronchitis, and emphysema. People most susceptible to such respiratory problems—and thus who are at risk for smog conditions where they live—are:

Public health role and response

Federal and state governments in the United States promote many solutions to smog. Laws encourage automobile manufacturers to develop motorized vehicles that produce fewer emissions. Chemical companies are being regulated and inspected more carefully given that they produce potentially harmful substances that can produce health risks to humans and other living creatures.

The EPA also maintains the National Ambient Air Quality Standards (NAAQS) as part of its directive under the U.S. Clean Air Act of 1963. This Act and the NAAQS were created to protect human health from the adverse effects of pollutants.


In July 2010, the Environmental Protection Agency proposed the Cross-State Air Pollution Rule (CSAPR) to protect the health of Americans by reducing the air emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx). The EPA estimated that from 2012 (the year the rule would go into effect) to 2014, the transport rule would reduce power-plant emissions of SO2 and NOx by 64% and 35%, respectively. However, in August 2012, the U.S. Court of Appeals struck down the EPA transport rule, saying the EPA overstepped its authority in requiring states to excessively reduce their emissions of pollutants. Consequently, the EPA reverted to its 2005 Clean Air Interstate Rule (CAIR). The CAIR covers 27 eastern states and the District of Columbia by using a cap-and-trade system to reduce the pollutants of sulfur dioxide and nitrogen oxides. One of the goals of this rule is to provide solutions to the problem of pollution drifting from one state to another from the emissions of power plants.


Air pollution may be prevented only if individuals and businesses stop using toxic substances that cause air pollution in the first place. This change would require the cessation of all fossil fuel-burning processes, from industrial manufacturing to home use of air conditioners. This is a very unlikely scenario at the present. However, smog can be diminished by reducing the amount of air pollution. This can be done by reducing the use of all toxic substances that cause air pollution. With current technology, this would involve reducing the use of fossil fuelburning processes. The use of more efficient and less polluting fossil fuel-burning devices is possible. Motor vehicles are a major source of pollution, so converting to cleaner running internal combustion engines or replacing such engines with cleaner running motors (such as electric) is one way to reduce smog. Driving fewer miles with existing motor vehicles or carpooling are other ways to cut down on smog. Using public transportation rather than individual vehicles also reduces smog conditions in major metropolitan areas. Instead of using coal to generate electricity, wind turbines, solar energy, geothermal, and other cleaner forms of electricity-generating means can help to reduce smog.

Cap and trade—
Part of environmental policy that uses a mandatory cap on emissions while providing flexibility on how to comply with the rules.
Carbon monoxide—
With the chemical formula CO (where C stands for carbon, and O for oxygen), a colorless, odorless, and tasteless gas that is toxic to humans in higher than normal concentrations.
Fossil fuels—
Any type of fuel, such as coal, natural gas, peat, and petroleum, derived from the decomposed remains of prehistoric plants and animals.
Nitrogen dioxide—
With the chemical formula NO2 (where I stands for nitrogen, and O for oxygen), a reddish-brown toxic gas that is one of several nitrogen oxides.
Sulfur dioxide—
With the chemical formula SO2 (where S stands for sulfur, and O for oxygen), a toxic gas with a strong, irritating smell; in nature it is released from spewing volcanoes and it is also released by human-producing industrial processes.

Many organizations have formed to reduce smog. For instance, the Galveston-Houston Association for Smog Prevention (GHASP) was formed to “persuade government and corporate officials to prevent smog.” According to the GHASP website, the organization “seeks to accomplish its mission by being the most credible advocate for clean air in the Houston region; by supporting efforts to educate the public; and by directly engaging government officials, community leaders, the media and industry on regional air pollution issues.”

See also Air pollution ; Asthma ; Bronchitis ; Chronic obstructive pulmonary disease ; Clean Air Act (1963, 1970, 1977, 1990) ; Ozone .



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American Lung Association, 1301 Pennsylvania Ave. NW, Ste. 800, Washington, DC, 20004, (202) 785-3355, Fax: (202) 452-1805, (800) 621-8335, .

Environmental Protection Agency, 1200 Pennsylvania Ave. NW, Ariel Rios Bldg., Washington, DC, 20460, (202) 272-0167, .

Peter Brimblecombe, PhD
Revised by William A. Atkins, BB, BS, MBA

  This information is not a tool for self-diagnosis or a substitute for professional care.