Air pollution consists of chemicals or particles in the air that can harm the health of humans, animals and plants. It can even damage buildings. Pollutants in the air take many forms. They can be gasses, solid particles or liquid droplets.

Sources of Air Pollution

Pollution enters the Earth's atmosphere in many different ways. Most air pollution is created by people burning fossil fuels, which include coal, natural gas and oil. Problems with human-caused air pollution began in the mid-1700s, when Europe and North America entered the period of time known as the Industrial Revolution. During this time, the rise in air pollution was largely caused by more people burning coal to heat their homes and to power factories and engines.

Today, the burning of fossil fuels—in vehicles, airplanes, power plants and factories—is still a major source of air pollutants, meaning substances that contaminate the air. Cigarette and e-cigarette (or vape) smoke are also considered air pollution. These human-made sources of pollution are called anthropogenic sources. Some types of air pollution, such as smoke from wildfires, ash from volcanoes and windblown sand or dust, occur naturally. These are called natural sources.

Air pollution is most common in large cities where emissions from many different sources are concentrated. Mountains or tall buildings in and around cities can prevent air pollution from spreading. This air pollution, called smog, often appears as a cloud making the air murky. The word "smog" comes from combining the words "smoke" and "fog."

Large cities in economically developing nations tend to have worse air pollution than cities in economically developed nations. This is largely because of fewer regulations, lower financial resources and lack of access to cleaner fuels. According to the 2023 World Air Quality Report, the world’s top four most polluted cities were all in India. Still, economically developed nations also have air pollution problems. In 2022, Italy and Poland were at the top of a list of European countries with the worst air quality. In the United States, the city of Los Angeles, California is known to be the smoggiest city in the country.

Indoor Air Pollution

Air pollution is usually thought of as smoke from large factories or exhaust from vehicles. But air pollution is also a concern indoors.

Naturally occurring radon gas—a cancer-causing material—is released through the surface of the Earth and can build up in homes. There are many tools and methods to help owners reduce radon levels in their homes and other buildings.

Some construction materials, including insulation, are also dangerous to human health. In addition, ventilation, or air movement, in homes and rooms can lead to the spread of toxic mold. A single colony of mold may exist in a damp, cool place in a house, such as between walls. The mold's spores can then enter the air and spread throughout the house, causing anyone who breathes in the spores to become sick. Smoking is also a source of indoor pollution. Secondhand smoke—breathing in smoke from someone else using a cigarette—can raise the risk of lung cancer.

Heating a house by burning substances, such as kerosene, wood or coal, can expose residents to harmful smoke. The people who suffer the most from using these sources of fuel are women and children in low- to middle-income countries, where they often do not have access to electricity and where other fuels are too expensive or inaccessible.

Effects on Humans

People experience a wide range of health effects from being exposed to air pollution. These effects can be broken down into short-term effects and long-term effects.

Short-term effects, which may be temporary, include illnesses such as pneumonia or bronchitis. They may include discomfort, such as irritation to the nose, throat, eyes or skin. Air pollution can also cause headaches, dizziness and nausea. Bad smells made by factories, garbage or sewer systems are considered air pollution, too. These odors are less serious but still unpleasant.

Long-term effects of air pollution can last for years or for an entire lifetime. They can even lead to a person's death. Long-term health effects from air pollution include heart disease, lung cancer and respiratory disease, such as emphysema. Air pollution can also cause long-term damage to people's nerves, brain, kidneys, liver and other organs. Some scientists suspect air pollutants can raise the risk of birth defects. The State of Global Air Report estimates that, worldwide in 2021, about 8 million people died from exposure to air pollution.

People react differently to different types of air pollution. Young children and older adults—whose immune systems tend to be weaker—are often more sensitive to pollution. Certain conditions, such as asthma, heart disease and lung disease, can be made worse by exposure to air pollution. The length of exposure and amount and type of pollutants are also factors.

The most dangerous kinds of air pollution are fine particulate matter and ultrafine particulate matter. Particulate matter is a mixture of solid particles and liquid droplets that travel through the air in smoke from diesel engines and in windblown dust and dust storms. Wildfire smoke also carries dangerous particulate matter. Fine particulate matter ranges in size from less than 2.5 to 10 micrometers. (This is much smaller than a human hair, which is roughly 50 to 70 micrometers wide.) Ultrafine particulate matter measures at 0.1 micrometers, or 100 nanometers, or even smaller.

When people breathe in smoke or dust, the fine and ultrafine particulate matter can enter their lungs, travel through the bloodstream and affect major organs. Prolonged exposure can lead to serious illness and even death. Again, the length of exposure and the amount and type of pollutants are deciding factors.

Effects on the Environment

Like people, animals and plants, entire ecosystems can suffer the effects of air pollution.

Sulfur dioxide and nitrogen oxide particles in the air can create acid rain when they mix with water and oxygen in the atmosphere. These air pollutants come mostly from power plants and motor vehicles that burn fossil fuels. When acid rain falls to Earth, it damages plants by changing soil composition, degrades water quality in rivers, lakes and streams, and harms crops. It can even cause buildings and monuments to decay.

Air pollution can also directly contaminate the surface of bodies of water and soil. This can damage crops or reduce their yield, and it can harm young trees and other plants. Air pollution can even directly damage buildings. Some pollutants, such as sulfur dioxide, break down stone and other materials.

Like humans, animals can suffer health effects from exposure to air pollution. Disease, DNA damage and harm to the reproductive systems of animals have all been attributed to air pollution.

Air pollution also affects visibility of the environment. Haze, like smog, is a visible type of air pollution that obscures shapes and colors.

Global Warming

Global warming—a phenomenon caused by anthropogenic air pollution—refers to rising air and ocean temperatures around the world due to an increase in greenhouse gas emissions from human activities. Greenhouse gasses trap heat energy in the Earth’s atmosphere. This “greenhouse effect” is how the planet stays warm enough for humans and other life to survive. But human activities are releasing too much of these gasses into the atmosphere, causing Earth’s overall temperature to rise.

Humans have come to rely on burning fossil fuels to power vehicles, heat homes and run factories, at the cost of polluting the air with carbon dioxide. Energy consumption, including heat and electricity generation, is the biggest source of greenhouse gas emissions. Transportation, manufacturing and construction are also major pollution sources.

Other greenhouse gasses emitted by natural and anthropogenic sources include methane, nitrous oxide and fluorinated gasses. Methane is a major emission from coal mines and agricultural processes. Nitrous oxide is a common emission from industrial factories, agriculture and the burning of fossil fuels in cars. Fluorinated gasses, such as hydrofluorocarbons, are emitted by industry. Fluorinated gasses are often used instead of gasses such as chlorofluorocarbons (CFCs). CFCs have been outlawed in many places because they deplete the ozone layer.

Global warming is making air pollution from natural sources worse. As the Earth has become hotter and drier, wildfires have become more frequent occurrences worldwide, bringing with them hazardous smoke. More areas are experiencing desertification, meaning they are turning into hot, dry deserts, which has resulted in sandstorms that send sand and dust into the air.

Most of the world’s greenhouse gas emissions come from just a handful of regions. At the top of the list are China, the United States, India and the European Union (EU).

Regulation

Worldwide, many countries have taken steps to combat global warming by reducing or limiting air pollution and greenhouse gas emissions. In 2015, the Paris Agreement was signed by 195 nations. Each nation committed to work on adopting laws to reduce the air pollution they emit.

In 2021, the World Health Organization (WHO) issued new air quality guidelines to set stricter limits on the amount of air pollution people can safely be exposed to each year. The guidelines recommend limits for particulate matter, ozone, nitrogen dioxide and sulfur dioxide air pollutants. In 2021, the EU began revising its own air quality standards to align more closely with the WHO’s latest recommendations.

One method used by many countries to reduce emissions from industrial activity is called cap and trade. A cap, or limit, is placed on the amount of pollution a company is allowed to emit. Companies that exceed their cap must pay. Companies that pollute less than their cap can trade or sell their remaining pollution allowance to other companies. Cap and trade essentially pays companies to limit their pollution. The EU, Mexico and the U.S. state of California all have cap-and-trade programs in effect.

India is also experimenting with a cost-based system designed to reduce particulate air pollution. In 2019, the state of Gujarat launched a pollution control market for trading in particulate matter emissions. Businesses that emit too much particulate matter must buy permits from those that emit less, making the heaviest polluters pay the most.

Reduction

Anybody can take steps to reduce air pollution by making simple changes in their lives. Taking public transportation or riding a bike instead of traveling in gasoline- or diesel-powered vehicles are a couple of ways to reduce air pollution. Other air pollution reduction measures include taking fewer airplanes to travel, turning off lights when you do not need them, and eating less meat.