Smog, also called smoke fog, is a serious type of air pollution. The word "smog" was created in the early 1900s by combining the words "smoke" and "fog." It describes a thick, smoky fog that is hard to see through and has a strong smell. This type of pollution was known as "pea soup fog" in London from the 1800s to the middle of the 1900s. It was often called "London particular" or "London fog." Smog is made of harmful substances like nitrogen oxides, sulfur dioxide, ozone, smoke, and tiny particles. It comes from human activities such as burning coal, car exhaust, factory emissions, wildfires, and chemical reactions in the air.

Smog is usually divided into two types: summer smog and winter smog. Summer smog happens when sunlight and heat cause chemical reactions that create ozone. This type of pollution is common in warm seasons. Winter smog forms during colder months when cold air traps pollution near the ground. This happens because of weather patterns called inversions, which prevent pollution from rising. During winter, people burn more coal and other fuels for heating, adding to the pollution. Smog forms from both primary and secondary pollutants. Primary pollutants are released directly, like sulfur dioxide from burning coal. Secondary pollutants, such as ozone, form when primary pollutants react in the air.

Photochemical smog, like the kind found in Los Angeles, comes from car exhaust and industrial fumes. These pollutants mix with sunlight to create new harmful substances that join the original pollution, forming smog. In cities like Delhi, smog is made worse by burning crop stubble in nearby fields since the 1980s. In many cities, including Los Angeles, Beijing, Delhi, Lahore, Mexico City, Tehran, and others, smog is trapped close to the ground by inversions. This pollution is dangerous to humans and can cause serious health problems, including illness, shorter lives, or early death.

Etymology

The word "smog" was first credited to Henry Antoine Des Voeux in his 1905 paper titled "Fog and Smoke," presented to the Public Health Congress. A newspaper article from the London Daily Graphic, dated 26 July 1905, reported that Des Voeux 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.'" The next day, the newspaper noted that "Dr. Des Voeux did a public service in coining a new word for the London fog."

However, the term "smog" appeared earlier. It was used in the Santa Cruz & Monterey Illustrated Handbook published in 1880. The term also appeared in a newspaper column quoting the handbook on 3 July 1880 in the Santa Cruz Weekly Sentinel. In 1881, the publication Sporting Times stated that an author claimed to have invented the word: "The 'Smog' – a word I have invented, combined smoke and fog, to designate the London atmosphere."

Anthropogenic causes

Coal fires can produce large amounts of smoke that help create winter smog. These fires are sometimes used to heat buildings or generate electricity in power plants. Air pollution from coal has been reported in England since the Middle Ages. London was especially known for its thick coal-caused smog, called "pea-soupers," until the middle of the 20th century. This type of pollution still affects areas where burning coal creates a lot of smoke. In China, emissions from burning coal are a major cause of air pollution. During autumn and winter, when coal is burned more for heating, smoke levels can be so high that some cities in China must close roads, schools, or airports. A well-known example of this happened in Harbin, a city in northeastern China, in 2013.

Traffic emissions, such as those from trucks, buses, and cars, also contribute to smog. Pollutants from vehicle exhaust and air conditioning systems are a major part of smog in large cities. The main pollutants from transportation include carbon monoxide (CO), nitrogen oxides (NO and NO₂), and volatile organic compounds, which include hydrocarbons (a main part of fuels like gasoline and diesel). Transportation also releases sulfur dioxide and particulate matter, but in much smaller amounts. Nitrogen oxides and volatile organic compounds can combine with sunlight, heat, ammonia, moisture, and other substances to create harmful vapors, ground-level ozone, and particles that form smog.

Photochemical smog

Photochemical smog, also called "summer smog," forms when sunlight reacts with nitrogen oxides and volatile organic compounds in the air. This reaction creates airborne particles and ground-level ozone. Photochemical smog depends on both primary pollutants and secondary pollutants. Primary pollutants include nitrogen oxides, such as nitric oxide (NO) and nitrogen dioxide (NO₂), and volatile organic compounds. Secondary pollutants include peroxyacetyl nitrates (PANs), tropospheric ozone, and aldehydes. Ozone is a key secondary pollutant formed when hydrocarbons (HC) and nitrogen oxides (NOx) combine in sunlight. Nitrogen dioxide (NO₂) forms when nitric oxide (NO) reacts with oxygen (O₂) in the air. When sulfur dioxide (SO₂) and nitrogen oxides (NOx) are released, they eventually form nitric acid and sulfuric acid in the troposphere. These acids mix with water to create acid rain. These chemicals are highly reactive and oxidizing. Photochemical smog is a problem linked to modern industrialization. It occurs in all modern cities but is more common in sunny, warm, dry areas with many motor vehicles. Because it spreads with the wind, it can affect even sparsely populated regions.

Scientists did not fully understand the composition and reactions of photochemical smog until the 1950s. In 1948, chemist Arie Haagen-Smit used equipment to collect chemicals from polluted air and identified ozone as part of Los Angeles smog. Haagen-Smit later found that nitrogen oxides from car exhaust and hydrocarbons from vehicles and oil refineries, when exposed to sunlight, form ozone and photochemical smog. He worked with Arnold Beckman, who created tools to detect smog, such as devices for measuring gas concentrations and air quality monitoring vehicles.

Photochemical smog forms through sunlight-driven reactions. Nitrogen dioxide (NO₂) absorbs sunlight and breaks into nitric oxide (NO) and a free oxygen atom. This oxygen atom combines with molecular oxygen (O₂) to form ozone (O₃). Volatile organic compounds (VOCs) react with NO, preventing ozone from being removed and allowing its concentration to rise alongside other pollutants like PANs.

Photochemical smog also contains harmful secondary pollutants. PANs are strong irritants to the eyes and respiratory system, formed when VOCs react with nitrogen dioxide. Aldehydes, such as formaldehyde, form during VOC oxidation and contribute to smog-related irritation and odors.

During morning rush hours, high levels of nitric oxide and hydrocarbons are released into the air, mainly from traffic and industry. Some hydrocarbons react with OH· radicals to form peroxy radicals, which convert nitric oxide (NO) into nitrogen dioxide (NO₂).

Nitrogen dioxide (NO₂) and nitric oxide (NO) react with ozone (O₃) in a series of chemical processes. These reactions, called the photostationary state (PSS), are not perfectly balanced because of additional reactions that prevent ozone from being destroyed. This allows ozone levels to increase throughout the day. Other reactions, like the photooxidation of formaldehyde, can also raise ozone and NO₂ levels. Photochemical smog is more common in summer because strong sunlight promotes ozone formation. A temperature inversion layer, which traps pollutants near the ground, also helps smog accumulate.

Some reactions can reduce ozone formation in smog. The main limiting reaction in polluted areas is:

This reaction removes nitrogen dioxide (NO₂), which limits ozone production from its breakdown. Nitric acid (HNO₃) is sticky and can be removed from the air by sticking to surfaces or dissolving in rain. These processes help remove pollutants from the atmosphere.

Volcanoes can release large amounts of sulfur dioxide and particulate matter, which are key to forming smog. However, smog from volcanic eruptions is called "vog" to distinguish it as a natural event. The chemical reactions that form volcanic smog differ from those that create photochemical smog. The term "smog" refers to visible haze caused by gas molecules and particulate matter in the air. The source of emissions can vary, but the result is often smog.

Plants naturally release hydrocarbons, such as isoprene and terpenes, which can react in the atmosphere to form smog. Globally, plants and soil contribute significantly to hydrocarbon production. Plant-released hydrocarbons can be more reactive than man-made ones. For example, isoprene reacts quickly with hydroxyl radicals in the air, forming hydroperoxides that increase ozone levels.

Health effects

Smog is a serious problem in many cities and continues to harm human health. Other harmful substances formed in photochemical smog, such as ozone, PANs, and aldehydes, can cause irritation in the respiratory system, eyes, and reduce lung function, especially in people who are more vulnerable, like children, the elderly, or those with heart or lung conditions such as emphysema, bronchitis, and asthma. These pollutants can inflame the airways, reduce the lungs' ability to function, cause difficulty breathing, pain when inhaling deeply, wheezing, and coughing. They can also irritate the eyes and nose, dry out the protective lining of the nose and throat, and weaken the body's ability to fight infections, making people more likely to get sick. During times when ozone levels are high, hospital visits and deaths from respiratory issues often increase.

There is limited understanding of the long-term effects of air pollution on health and the causes of asthma. An experiment was conducted using air pollution levels similar to those during the 1952 Great Smog of London. The results showed a connection between early exposure to pollution and the development of asthma, suggesting that the effects of the Great Smog may still be ongoing. Modern studies continue to find links between smog and increased mortality. One study published in Nature magazine found that smog events in Jinan, a large city in eastern China, between 2011 and 2015 were linked to a 5.87% increase in overall mortality rates. This study highlights how air pollution affects death rates in China. A similar study in Xi'an found a connection between air pollution and higher rates of death from respiratory diseases.

The U.S. Environmental Protection Agency (EPA) created an air quality index to help the public understand pollution levels. Ozone concentrations averaged over 8 hours are described as follows: 85 to 104 parts per billion by volume (ppbv) is "Unhealthy for Sensitive Groups," 105 to 124 ppbv is "Unhealthy," and 125 to 404 ppbv is "Very Unhealthy." For other pollutants, the "Very Unhealthy" range includes: 355 to 424 micrograms per cubic meter (μg/m³) for PM10 (fine particles), 15.5 to 30.4 parts per million (ppm) for carbon monoxide (CO), and 0.65 to 1.24 ppm for nitrogen dioxide (NO₂).

In 2016, the Ontario Medical Association reported that smog causes about 9,500 premature deaths in the province each year. A 20-year study by the American Cancer Society found that long-term exposure to air pollution increases the risk of dying from respiratory diseases, suggesting that the current 8-hour ozone standard may not be enough to protect health.

Tiny magnetic particles from air pollution have been found in human brains for the first time. Researchers believe these particles may be linked to Alzheimer's disease. Scientists at Lancaster University discovered large amounts of magnetite nanoparticles in brain tissue from 37 people aged three to 92 who lived in Mexico City and Manchester. This magnetic mineral is toxic and can create harmful molecules in the brain, which are connected to neurodegenerative diseases like Alzheimer's.

A study comparing 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 of California was linked to two types of birth defects: spina bifida (a condition involving malformations of the spinal column) and anencephaly (a condition where part or all of the brain is missing or underdeveloped, often leading to severe disability). A study in China found that exposure to smog during early life increases the risk of harmful pregnancy outcomes, especially related to oxidative stress.

According to a study published in The Lancet, even a small increase in exposure to PM2.5 (a type of fine particle pollution) by 5 μg/m³ was linked to an 18% higher risk of low birth weight in newborns. This relationship was observed even when pollution levels were below the current standards considered safe.

Other negative effects

Although serious health problems caused by smog are the main concern, strong air pollution from haze, dust storms, and smoke from bush fires also reduces the amount of sunlight reaching the Earth. This reduction in sunlight negatively affects both the ability of solar panels to produce energy and the growth of crops.

Areas affected

Smog can form in many climates where industries or cities release large amounts of air pollution, such as smoke or gases. Pollutants from cities can travel far, reaching suburbs and rural areas. This means smog can affect areas far from where the pollution was first released. Smog is often worse during warm, sunny weather because the air above is too warm for pollution to rise and spread. It is especially common in areas surrounded by hills or mountains. Smog can stay over cities for long periods and build up to dangerous levels.

In northern India, cities have been covered by thick winter smog for several years. The situation is worst in Delhi, where smog forms because air moves very slowly during winter, trapping dust and toxic gases. Air quality in the Indo-Gangetic Plain gets worse after the monsoon season due to changes in wind, temperature, and air mixing. Emissions from burning crops and city activities increase pollution, especially in northern India. The nearby Himalayas trap pollutants, making air quality problems worse.

Delhi is the most polluted city in the world. Air pollution causes about 10,500 deaths each year in Delhi. Between 2013 and 2014, levels of fine dust (PM) in Delhi rose by 44%, mainly from cars, factories, construction, and burning crops in nearby areas. Delhi has the highest levels of harmful PM2.5, which is dangerous to health. High pollution levels increase lung problems, like asthma and lung cancer, in children and women. Thick smog in winter causes traffic delays and rail disruptions. Scientists say winter temperatures in Delhi have dropped since 1998 because of pollution.

Environmental experts say the Delhi government has not done enough to reduce pollution or warn people about health risks. Many residents do not know how bad the pollution is or its dangers. Since the 1990s, Delhi has planted many trees and uses buses powered by natural gas to cut pollution. In 1996, a court ordered buses and taxis to switch to natural gas and banned leaded gasoline. In 2003, Delhi won an award for reducing pollution. The Delhi Metro also helps lower pollution levels.

However, recent efforts have not been effective due to burning crop waste, more diesel cars, and fewer people using buses. Studies show that burning crops in nearby states worsens smog in Delhi. Some governments are considering banning crop burning, and an environmental group has asked the Supreme Court to tax diesel cars.

In 2006, researchers found that much of Beijing's pollution comes from nearby cities and provinces. About 35–60% of the ozone in Beijing comes from outside the city. Areas like Shandong and Tianjin affect Beijing's air quality because of wind patterns and nearby mountains.

In December 2005, smog in Tehran caused schools and offices to close, and 1,600 people were hospitalized. The smog was mainly from car exhaust.

In the late 1990s, many people moved to Ulaanbaatar, Mongolia. Poor families living outside the city burn wood, coal, or even car tires for heat during winter because they lack central heating. Efforts to improve stove efficiency did not help. Coal stoves release harmful particles that can damage lungs. Air pollution in Ulaanbaatar is among the worst in the world, costing 4% of Mongolia's economy in health-related expenses.

Smog is a common problem in Southeast Asia, especially in Indonesia, where land and forest fires create haze. Farmers and companies burn forests to clear land, affecting countries like Brunei, Malaysia, and Singapore. In 1997, fires caused economic losses of over $9 billion. In 2006 and 2013, haze from Indonesia reached Malaysia and Singapore, with pollution levels reaching dangerous levels.

In 2002, Southeast Asian countries signed an agreement to fight haze. A monitoring system was created to track fires, and warnings are issued daily. However, Indonesia has struggled to stop illegal burning.

In winter, heavy smog has covered parts of Punjab, especially Lahore, causing breathing problems and traffic delays. A 2022 study found that traffic-related pollution is the main cause of smog in Lahore.

Pollution index

The seriousness of smog is often measured using special machines called nephelometers, as haze affects how clear the air is and can impact traffic in ports. Haze can also be a sign that the air quality is bad, though this is usually better shown using specific air quality indexes designed for this purpose, such as the American Air Quality Index, the Malaysian API (Air Pollution Index), and the Singaporean Pollutant Standards Index.

During hazy conditions, these indexes often report the level of tiny particles floating in the air. In some areas, laws require that the pollutant responsible for the haze be identified.

Cultural references

• The thick, dark smog in London, called "pea-soupers," gave the city the nickname "The Smoke." Edinburgh was known as "Auld Reekie" because of its own smog. Smog often appears in London stories as a symbol of hidden danger or mystery, such as in Margery Allingham's The Tiger in the Smoke (1952), Charles Dickens's Bleak House (1852), and T.S. Eliot's poem "The Love Song of J. Alfred Prufrock."
• In the 1957 Warner Brothers cartoon What's Opera, Doc?, Elmer Fudd shouts for bad things to happen to Bugs Bunny, ending with a loud cry of "SMOG!!"
• The 1970 made-for-TV movie A Clear and Present Danger was one of the first American television shows to warn about smog and air pollution. It showed a man trying to improve air quality after his friend died from emphysema caused by polluted air.
• The history of smog in Los Angeles is written about in a book called Smogtown by Chip Jacobs and William J. Kelly.
• The 2025 documentary series Clearing the Air: The War on Smog traces the history of smog in Los Angeles, starting with the first smoky clouds over the city in 1943. It covers years of research, public efforts to reduce pollution, the creation of the U.S. Environmental Protection Agency (EPA), and the passing of the Clean Air Act. Today, pollution levels in Los Angeles are less than 1% of their highest levels.