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 kind of pollution, known as "pea soup fog," was a major problem in London from the 1800s to the middle of the 1900s. It was often called "London particular" or "London fog." Smog is made up of harmful substances like nitrogen oxides, sulfur dioxide, ozone, smoke, and tiny particles. Human activities, such as burning coal, car exhaust, factory smoke, wildfires, and chemical reactions in the air, create smog.

Smog is often divided into two types: summer smog and winter smog. Summer smog happens when sunlight and heat cause chemical reactions that form ozone. This type of smog is common in warm weather. Winter smog occurs during colder months when cold air is trapped near the ground by a weather pattern called an inversion. This traps pollution from burning coal and other fuels used for heating. Both summer and winter smog involve primary pollutants, which are directly released into the air, such as sulfur dioxide from burning coal. Secondary pollutants, like ozone, form when primary pollutants react with sunlight and other chemicals in the atmosphere.

Photochemical smog, such as the kind found in Los Angeles, comes from car exhaust and factory emissions. These pollutants mix with sunlight to create new harmful substances that combine with the original pollution to form smog. In cities like Delhi, smog is worsened by fires in nearby farmland, which have happened since the 1980s. In many cities, including Los Angeles, Beijing, Delhi, Lahore, Mexico City, and Tehran, smog is made worse by inversions that trap pollution near the ground. Smog is dangerous to human health and can lead to serious illness, a shorter life, or early death.

Etymology

The term "smog" is credited to Henry Antoine Des Voeux in his 1905 paper titled "Fog and Smoke," presented to the Public Health Congress. On July 26, 1905, the London newspaper Daily Graphic 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 word "smog" appeared earlier. It was used in the Santa Cruz & Monterey Illustrated Handbook, published in 1880, and later referenced in a column from the Santa Cruz Weekly Sentinel on July 3, 1880. On December 17, 1881, the publication Sporting Times claimed the author invented the word, stating: "The 'Smog' – a word I have invented, combined of smoke and fog, to designate the London atmosphere…"

Anthropogenic causes

Coal fires can create large amounts of smoke that help form winter smog. These fires are used to heat buildings or to produce electricity in power plants. Air pollution from coal has been recorded in England since the Middle Ages. London was especially known for its thick, coal-caused smog until the middle of the 20th century. This smog was called "pea-soupers" because it looked like thick soup made from peas. Air pollution from burning coal is still a problem in areas that produce 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 sometimes force cities to close roads, schools, or airports. For example, in 2013, the city of Harbin in northeastern China experienced this problem.

Traffic emissions, such as those from trucks, buses, and cars, also help create smog. Chemicals from vehicle exhaust and air conditioning systems pollute the air and are a major part of smog in large cities.

The main pollutants from transportation are carbon monoxide (CO), nitrogen oxides (NO and NO₂), and volatile organic compounds, which include hydrocarbons. Hydrocarbons are the main part of fuels like gasoline and diesel. Transportation also releases sulfur dioxide and particulate matter, but these are present in much smaller amounts than the other pollutants. Nitrogen oxides and volatile organic compounds can combine with sunlight, heat, ammonia, moisture, and other chemicals to create harmful vapors, ground-level ozone, and particles that make up 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) combines with oxygen (O₂) in the air. When sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) are released, they eventually become nitric acid and sulfuric acid in the troposphere. These acids mix with water to form acid rain. These chemicals are highly reactive and oxidizing. Photochemical smog is a problem linked to modern industrialization. It is found in all modern cities but is more common in cities with sunny, warm, dry climates and many motor vehicles. Because it moves with the wind, it can also affect sparsely populated areas.

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 discovered that nitrogen oxides from car exhaust and hydrocarbons from cars and oil refineries, when exposed to sunlight, create ozone and photochemical smog. Haagen-Smit worked with Arnold Beckman, who developed tools to detect smog, including an "Apparatus for recording gas concentrations in the atmosphere" patented in 1952 and "air quality monitoring vans" for government and industry use.

Photochemical smog forms through sunlight-driven reactions. Nitrogen dioxide (NO₂) absorbs sunlight and splits 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 removal and allowing its buildup alongside other pollutants like PANs.

Photochemical smog also includes harmful secondary pollutants. PANs are strong irritants to the eyes and respiratory system, formed when VOCs oxidize and create peroxy radicals that react with NO₂. Aldehydes like formaldehyde also form during VOC oxidation, causing irritation and odors.

During morning rush hour, 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) to nitrogen dioxide (NO₂).

Nitrogen dioxide (NO₂) and nitric oxide (NO) react with ozone (O₃) in a series of chemical reactions. These reactions are called the photostationary state (PSS). However, because of other reactions, ozone levels continue to rise throughout the day. Reactions involving formaldehyde (HCHO), a common pollutant, also increase ozone and NO₂ levels. Photochemical smog is more common in summer due to strong sunlight, which promotes ozone formation. A temperature inversion layer, which traps pollutants near the ground, further supports smog formation.

Some reactions can reduce ozone levels in smog. The main limiting reaction removes nitrogen dioxide (NO₂), which limits ozone production. 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.

Volcanic eruptions can release sulfur dioxide and particulate matter, which are key to smog formation. However, this type of smog is called "vog" to distinguish it as a natural event. The chemical reactions in volcanic smog differ from those in photochemical smog. The term "smog" describes a visible haze caused by gas-phase molecules and particulate matter in the air. The source of emissions can vary but still leads to smog formation.

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

Health effects

Smog is a serious problem in many cities and continues to harm human health. Harmful substances formed in smog, such as ozone, PANs, and aldehydes, can cause irritation in the lungs and eyes, and reduce lung function, especially in people who are more at risk, like children, older adults, and those with heart or lung conditions such as emphysema, bronchitis, or asthma. These pollutants can inflame the airways, make it harder to breathe, cause chest pain, wheezing, and coughing. They can also irritate the eyes and nose, dry out the protective tissues in the nose and throat, and weaken the body’s ability to fight illness. During times when ozone levels are high, more people are admitted to hospitals, and more deaths from respiratory issues occur.

Scientists do not fully understand the long-term effects of air pollution or the causes of asthma. One study used air pollution levels similar to those during the 1952 Great Smog of London. It found that exposure to pollution during early life may increase the risk of developing asthma, showing the lasting effects of the Great Smog. Modern research continues to show that smog is linked to higher death rates. A study in Nature magazine found that during smog episodes in Jinan, China, between 2011 and 2015, the overall death rate increased by 5.87%. Another study in Xi'an found that air pollution was connected to higher death rates from respiratory diseases.

The U.S. Environmental Protection Agency (EPA) created an air quality index to help the public understand pollution levels. Ozone levels between 85 and 104 ppbv are considered "Unhealthy for Sensitive Groups," 105 to 124 ppbv are "Unhealthy," and 125 to 404 ppb are "Very Unhealthy." For other pollutants, "Very Unhealthy" levels include 355 to 424 μg/m³ for PM10, 15.5 to 30.4 ppm for CO, and 0.65 to 1.24 ppm for NO₂.

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

Scientists discovered tiny magnetic particles from air pollution in human brains for the first time. These particles, called magnetite, were found in brain tissue from people in Mexico City and Manchester, England. Magnetite is toxic and may contribute to the production of harmful substances in the brain linked to diseases like Alzheimer’s.

A study of 806 women who had babies with birth defects and 849 women who had healthy babies found that smog in California’s San Joaquin Valley was connected to two types of birth defects in the brain and spinal cord: spina bifida and anencephaly. A study in China found that exposure to smog during early life was linked to a higher risk of problems during pregnancy, such as oxidative stress.

According to a study in The Lancet, even a small increase in exposure to PM2.5 (a type of fine particle pollution) by 5 μg was linked to an 18% higher risk of having a baby with low birth weight. This relationship was true even when pollution levels were below the current safety standards.

Other negative effects

Although the main problem is the serious health effects caused by smog, air pollution from haze, dust storms, and bush fire smoke also reduces the amount of sunlight that reaches the ground. This reduction in sunlight hurts both solar energy production and crop growth.

Areas affected

Smog can form in many climates where industries or cities release large amounts of air pollution, such as smoke or gases. Pollutants that lead to smog can travel from cities to other areas, like suburbs or rural places, as they mix in the air over time. This means smog can affect areas far from where the pollution first started. However, smog is worse during warm, sunny weather because the air near the ground is too warm to rise and mix with higher air. Smog is especially common in areas surrounded by hills or mountains. It often stays over crowded cities for long periods and can reach dangerous levels.

In recent years, cities in northern India have been covered by thick winter smog. The situation is worst in Delhi, the country’s capital. This smog happens because tiny dust and harmful gases collect in the air when air movement slows during winter. Air quality also worsens in the Indo-Gangetic Plain during the change from monsoon season to winter, due to changes in wind, temperature, and air mixing. Pollution from burning crops and city activities increases, leading to more harmful particles in the air. Nearby mountain areas trap these pollutants, making air quality worse in northern India.

Delhi is the most polluted city in the world. Air pollution is estimated to cause about 10,500 deaths in Delhi each year. Between 2013 and 2014, levels of tiny particles in the air, called PM2.5, rose by about 44% due to car and factory emissions, construction, and burning of crops in nearby areas. Delhi has the highest levels of PM2.5, which is very harmful to health, with 153 micrograms per cubic meter. Poor air quality has increased lung diseases, like asthma and lung cancer, in children and women. Thick smog during winter causes delays in air and train travel every year. Scientists say average winter temperatures in Delhi have dropped since 1998 because of rising pollution.

Environmental experts say the Delhi government has not done enough to reduce pollution or warn people about air quality dangers. Many residents are unaware of the high pollution levels and health risks. Since the 1990s, Delhi has taken some steps, like planting many trees and using buses powered by natural gas. In 1996, a court ordered Delhi to switch buses and taxis to natural gas and stop using leaded gasoline. In 2003, Delhi won an award for reducing pollution. The Delhi Metro has also helped lower pollution levels.

However, recent efforts have not been enough. Stubble burning, more diesel cars, and fewer people using buses have worsened pollution. Smoke from burning crops in nearby states like Punjab and Haryana increases smog in Delhi. Some governments are considering banning crop burning, and environmental groups are asking courts 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 harmful ozone in Beijing comes from outside the city. Nearby areas like Shandong Province and Tianjin have a big impact on Beijing’s air quality.

In 2005, smog in Tehran forced schools and offices to close, and 1,600 people were hospitalized. The smog was mainly caused by car exhaust that was not cleaned.

In the late 1990s, many people moved to Ulaanbaatar, Mongolia’s capital. Many live in traditional homes on the city’s outskirts and burn wood or coal for heat. These homes are not connected to the city’s heating system. Efforts to improve stove efficiency have not reduced smog. Coal stoves release harmful particles that can damage lungs and cause health problems. Air pollution in Ulaanbaatar is among the worst in the world, with pollution levels 2 to 10 times higher than allowed. Health costs from this pollution are estimated to be 4% of Mongolia’s economy.

Smog is a regular problem in Southeast Asia, especially in Indonesia, where fires from land clearing create haze. These fires mainly affect nearby countries like Brunei, Malaysia, Singapore, and Thailand. The economic losses from fires in 1997 were over $9 billion, including damage to agriculture, forests, health, and transportation. In 2006 and 2013, haze from Indonesian fires caused severe air quality issues in Malaysia and Singapore.

In 2002, countries in Southeast Asia signed an agreement to reduce haze pollution. A plan was created to monitor fires and warn about dangers. However, Indonesia has not enforced rules against illegal burning well.

During winter, heavy smog with pollutants 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 with special tools called nephelometers, which help track how clear the air is and how traffic is managed in ports. Haze, which makes the air look cloudy, can also show that air quality is poor. However, this is better understood using specific air quality indexes, such as the American Air Quality Index, the Malaysian Air Pollution Index, and the Singaporean Pollutant Standards Index.

During hazy weather, these indexes usually show the amount of tiny particles floating in the air. In some places, the law requires stating which pollutant is causing the haze.

Cultural references

• The thick smog in London, called "pea-soupers," gave the city the nickname "The Smoke." Edinburgh was known as "Auld Reekie" because of its smoggy air. Smog appears in many 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 scream 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 tells the story of a man working to clean the air after his friend dies from a lung disease caused by smog.
• The history of smog in Los Angeles is described in the book Smogtown by Chip Jacobs and William J. Kelly.
• The 2025 documentary series Clearing the Air: The War on Smog shows how smog first appeared in Los Angeles in 1943, how scientists studied it, how people pushed for change, how the U.S. Environmental Protection Agency was created, and how laws like the Clean Air Act helped reduce pollution. Today, pollution levels in Los Angeles are less than 1% of their highest levels.