What Is Air Pollution? Definition, Types, and Environmental Impact

Downtown Los Angeles skyscrapers at smoggy sunrise
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Air pollution occurs when certain gases, droplets, or particles mix with ambient air, rendering the air harmful to living things. There are many different kinds of air pollution, produced from many sources and resulting in many different problems for people, other animals, plants, and the environment.

Ambient air pollution accounts for an estimated 4.2 million annual deaths globally, according to the World Health Organization (WHO). Air pollutants also lead to environmental problems ranging from acid rain and poor visibility to ozone depletion and global climate change.

Pollutants that can become suspended in air include gases, particulates, and organic molecules. They end up in the air in a variety of ways, including human activities such as burning fossil fuels as well as natural sources like dust, wildfires, and volcanoes.

Air Pollution Definition

Both natural and human-induced air pollution can be dangerous, although the latter tends to be more widespread and continuous, like the ongoing combustion of fossil fuels for energy.

In some cases, the distinction is blurring between natural and human-induced air pollution. That’s partly due to carbon dioxide, a natural and vital gas in Earth’s atmosphere that’s also being emitted in unnaturally large amounts by human activities, namely the burning of fossil fuels, resulting in a global greenhouse effect.

That greenhouse effect is now amplifying some natural phenomena like wildfires, resulting in even more air pollution. In addition, people often start wildfires in more direct ways, such as intentionally burning forests for farmland or accidentally sparking dry brush, all of which also create air pollution.

Natural Air Pollution

Aside from wildfires, common natural causes of air pollution include volcanoes, dust storms, methane gas from cattle and other ruminants, and radon gas from underground radium deposits. These tend to be limited to certain locations and time periods, although some can be widespread or chronic.

Ash and sulfur from volcanoes can travel around the planet, for example, and the methane from cattle can be a significant contributor to Earth’s growing greenhouse effect. Radon gas can also become trapped and accumulate in basements and cellars as it seeps up from the ground, posing a long-term health risk to humans.

Human-Induced Air Pollution

Smoky Exhaust Pipes From A Starting Diesel Car.

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Perhaps the most notorious human-induced source of air pollution is the combustion of fossil fuels (coal, petroleum, and natural gas), which can take many forms and can produce a variety of pollutants. This includes the visible plumes rising from smokestacks at factories and power plants, but also many invisible gases and particulates emanating from countless vehicles, facilities, and other sources all around us.

Types of Air Pollution

Some air pollutants are directly dangerous, while others cause trouble in less obvious ways. Noxious gases like nitrogen oxides (NOx) and sulfur dioxide (SO2) are among the former group, along with particulate matter (PM) like sulfates, nitrates, carbon, or mineral dust.

A specific type of very small particulate matter (PM 2.5), which is 30 times thinner than the width of a human hair, poses especially grave concerns. There are also polycyclic aromatic hydrocarbons (PAHs), a group of organic compounds produced by combustion as well as by some industrial processes. And a broad group of air pollutants known as volatile organic compounds (VOCs) are emitted by sources ranging from paints and permanent markers to petroleum fuels.

Other air pollutants are dangerous not necessarily because they harm us when we inhale them, but because of how they interact with other aspects of the environment. Perhaps the most salient example in modern times is carbon dioxide (CO2), the primary greenhouse gas fueling global climate change.

Although carbon dioxide occurs naturally in the air and is vital for life, it’s also a greenhouse gas that traps solar heat in Earth’s atmosphere, and it’s released when people burn fossil fuels for energy. CO2 levels in Earth’s atmosphere are now higher than ever before in human history, and may be at their highest levels since the Pliocene Epoch.

Sources of Air Pollution

There are several ways to classify air pollution beyond natural vs. man-made. There is point-source air pollution, for example, which comes from a single identifiable source, like a factory, farm, or power plant. Nonpoint-source pollution, on the other hand, comes from a more dispersed array of sources that are more difficult to trace individually, like the tailpipes of cars on a highway or charcoal cookstoves spread throughout a community.

Coal Burning

Coal-fired power plants have long been a major source of many types of air pollution. Burning coal to generate electricity is notorious for releasing carbon dioxide, accounting for an estimated 30% of global CO2 emissions.

Coal combustion can also release SO2, NOx, particulates, and heavy metals like mercury, and while some power plants now use special equipment to control some of those emissions, coal remains a leading source of air pollution around the world.

Natural Gas

Natural gas has become a popular substitute for coal in the electricity-generation sector in recent years, largely due to its reputation as a cleaner-burning fossil fuel. It does release less CO2 than coal, although while coal releases about 200 pounds of CO2 per million British thermal units (MMBtu), an equivalent amount of natural gas still releases about 117 pounds of CO2.

Natural gas is mostly methane, itself a potent greenhouse gas, and it’s responsible for methane that escapes into the atmosphere not just when natural gas is burned for energy, but also the “fugitive” methane that escapes during extraction and transportation.

Petroleum Fuels

Petroleum fuels are another source of air pollution, whether they’re burned at industrial facilities or, more commonly, to propel cars, trucks, and other vehicles.

This nonpoint-source pollution from burning gasoline and other petroleum fuels is a major source of air pollution in many cities around the world, releasing a blend of airborne contaminants including carbon monoxide, carbon dioxide, nitrogen oxides, sulfur oxides, VOCs, PAHs, and particulate matter. It plays a key role in the formation of smog, and also adds a substantial amount of CO2 into the atmosphere.

Overall, transportation accounts for 29% of U.S. CO2 emissions and 14% of global CO2 emissions. About 90% of all fuel used for transportation is petroleum-based, mainly gasoline and diesel.


Brown Layer of Los Angeles Smog
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Smog is created by chemical reactions in which nitrogen oxides mix with VOCs in the presence of sunlight to form ozone. Ozone is beneficial high in the atmosphere, where it forms the planet’s protective ozone layer, but it can be dangerous to human health at ground level.

Unlike some types of air pollution, smog is visible; while its exact composition and appearance vary, it often appears as a brownish or orange haze, which often forms in urban areas on sunny days.

While we often think of air pollution as an outdoor problem, many people unwittingly inhale harmful indoor air pollution, too. This often comes from VOCs, which waft up from products such as paint, lacquer, solvents, building materials, and various household cleaners and other chemicals.

Older buildings may contain other kinds of potentially air-polluting building materials, such as those made with asbestos. Some indoor air pollution even comes from naturally occurring sources—in the form of mildew and black mold, for example, or radon gas seeping up from the ground and accumulating in basements, cellars, and other lower levels of buildings.

Effects of Air Pollution

Air pollution can affect humans, other animals, plants, and the broader environment in many ways.

Carbon Dioxide

Carbon dioxide emissions may not be directly dangerous to humans, but they represent some of this century’s most important air pollution due to CO2’s influence on climate.

CO2 is known as a greenhouse gas because it traps solar heat within Earth’s atmosphere, fueling the global climate crisis we face today, which entails widespread threats to humans and wildlife.

Concentrations of CO2 in the atmosphere are now well above 400 parts per million (ppm), a level unseen since long before our species existed, and international efforts to rein in growing CO2 emissions have made little progress for decades. Methane is a more potent greenhouse gas, but CO2 lingers longer in the atmosphere, potentially trapping heat for centuries.

Particulate Matter

Particulate matter is a broad category of air pollution, including all sorts of tiny solids and liquids suspended in the air, often as a result of combustion. It could come from wildfires, power plants, or vehicle traffic, and those tiny particulates can cause big problems when they’re inhaled, especially the very smallest ones.

Particles less than 10 micrometers wide pose the most risk, according to the EPA, because they’re small enough to become embedded deep in the lungs, and might even reach the bloodstream.

Aside from its potential effects on humans and other animals, particulate matter also leads to broader environmental effects depending on its location. It can affect cloud formation and provide reaction centers for other air pollutants in the upper atmosphere, while reducing visibility and influencing weather in the lower atmosphere.

Particulates often contribute to hazy, low-visibility conditions in urban areas, but because they can be carried long distances by wind, they also hinder views in some wilderness areas, including national parks.

Nitrogen Oxides

Nitrogen dioxide (NO2) and other nitrogen oxides (NOx) can irritate airways in the human respiratory system, according to the EPA, and aggravate respiratory diseases like asthma. NOx can also react with other compounds in the atmosphere to form nitrate particulates, which may pose additional dangers.

NOx has been known to help generate nitric acid in the atmosphere, too, which ultimately falls as acid rain. After reaching the surface, acidic runoff eventually washes into waterways or wetlands, reducing pH levels and leaching aluminum from soil along the way, potentially harming fish, insects, and other wildlife. Because it contains nitrogen, this runoff can also contribute to the nutrient pollution behind aquatic dead zones.

Acid rain and acid fog also harm some trees and other plants, both by damaging foliage and by removing nutrients from the soil.

Sulfur Dioxide

Sulfur dioxide can similarly irritate airways and make breathing difficult, according to the EPA. SO2 and SOx can react with other compounds in the air to form particulates, thus reducing visibility and potentially posing the various dangers associated with PM pollution.

SO2 and other sulfur oxides can also contribute to the formation of sulfuric acid in the air, and thus acid rain. 

Heavy Metals

Heavy metals like mercury and lead can be emitted by burning fossil fuels, often falling to the surface relatively close to their source, although they and other air pollutants may travel farther if they’re emitted from taller smokestacks.

Once airborne mercury descends, it commonly washes into waterways and bioaccumulates in animal tissue as it moves up the food web. That’s why large, predatory fish like tuna and swordfish tend to have higher levels of mercury than smaller fish like sardines and anchovies.

Mercury, lead, cadmium, and some other toxic metals can have serious health effects in humans and other animals.

Volatile Organic Compounds

VOCs include a variety of air pollutants both outdoors and indoors. One example is benzene, a sweet-smelling chemical that can be emitted from many different sources, including tobacco smoke, industrial emissions, vehicle exhaust, fuel fumes, wildfires, and volcanic eruptions.

CFCs and HCFCs

Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are not toxic to humans, but like CO2, they still pose significant environmental threats. That’s because they contribute to the depletion of Earth’s natural ozone layer—while ground-level ozone is itself an air pollutant, ozone in the upper atmosphere protects us from excess solar radiation.

Once widely used as refrigerants, aerosols, and solvents, CFCs have been largely phased out under the Montreal Protocol, often heralded as a rare success story in pollution control.

How to Reduce Air Pollution

Asian little girl helping his father to plant the tree
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Use Less Electricity

Because so much air pollution comes from power plants, one of the simplest ways for anyone to help reduce air pollution is to use less electricity, thus reducing the demand for energy from those power plants.

Governments and large corporations have a far greater ability to make an impact with changes like that compared with most individual people, but every little bit helps.

Drive Less

Transportation is another major contributor to air pollution, including CO2 emissions as well as the particulates and ozone that plague many urban and rural areas.

Fewer vehicles on roads generally means less air pollution, so it’s often in the interest of human and ecological health to adopt public policies that incentivize and support working remotely as well as cleaner modes of travel, from walking and cycling to driving electric vehicles, carpooling, and using public transit.

When you do drive a gasoline-powered vehicle, avoid idling any more than necessary, since this creates additional air pollution without the benefit of propulsion. Keep gasoline engines well-tuned and car tires properly inflated. Consider buying an electric or low-emission vehicle.

Avoid Burning Material

Try to limit the amount of wood or other biomass you burn, whether in a burn pile, fire pit, or fireplace.

Mulch or compost yard waste instead of burning it. Never burn plastic.

Plant More Trees

Aside from taking steps to limit air pollution, you could also help mitigate its effects by planting trees, which sequester CO2 and also filter some other air pollutants with their leaves. Along with cleaner air, you’ll also get to enjoy the many other benefits trees can bring.

Originally written by
Larry West

Larry West is an award-winning environmental journalist and writer. He won the Edward J. Meeman Award for Environmental Reporting.

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