Air pollution occurs in many forms but can generally be thought of as gaseous and particulate contaminants that are present in the earth’s atmosphere. Chemicals discharged into the air that have a direct impact on the environment are called primary pollutants. These primary pollutants sometimes react with other chemicals in the air to produce secondary pollutants.

Air pollution is typically separated into two categories: outdoor and indoor air pollution. Outdoor air pollution involves exposures that take place outside of the built environment. Examples include fine particles produced by coal burning, noxious gases such as sulfur dioxide, nitrogen oxides, and carbon monoxide, ground-level ozone, and tobacco smoke. Indoor air pollution involves exposure to particulates, carbon oxides, and other indoor air or dust pollutants. Examples include household products and chemicals, out-gassing of building materials, allergens (cockroach and mouse dropping, mold, pollen), and tobacco smoke.

Sources of Air Pollution

A stationary source of air pollution refers to an emission source that does not move, also known as a point source. Stationary sources include factories, power plants, and dry cleaners. The term area source describes many small sources of air pollution located together whose individual emissions may be below thresholds of concern but whose collective emissions can be significant. Residential wood burners are a good example of a small source, but when combined with many other small sources, they can contribute to local and regional air pollution levels. Area sources can also be considered non-point sources, such as the construction of housing developments, dry lake beds, and landfills.

A mobile source of air pollution refers to a source capable of moving under its own power. Mobile sources generally imply “on-road” transportation, including cars, sport utility vehicles, and buses. In addition, there is also a “non-road” or “off-road” category that includes gas-powered lawn tools and mowers, farm and construction equipment, recreational vehicles, boats, planes, and trains.

Agricultural sources arise from operations that raise animals and grow crops, which can generate emissions of gases and particulate matter. For example, animals confined to a barn or restricted area produce large amounts of manure. Manure emits various gases, particularly ammonia, into the air. This ammonia can be emitted from animal houses, manure storage areas, or land after the manure is applied. In crop production, the misapplication of fertilizers, herbicides, and pesticides can potentially result in aerial drift of these materials, and harm may be caused.

Unlike the above-mentioned sources of air pollution, air pollution caused by natural sources is not caused by people or their activities. An erupting volcano emits particulate matter and gases, forest, and prairie fires can emit large quantities of “pollutants,” dust storms can create large amounts of particulate matter, and plants and trees naturally emit volatile organic compounds which can form aerosols that can cause a natural blue haze. Wild animals in their natural habitat are also considered natural sources of “pollution.”

Six Common Air Pollutants

The most common air pollutants are particulate matter, ground-level ozone, carbon monoxide, sulfur oxides, nitrogen oxides, and lead. These pollutants can harm health and the environment and cause property damage. Of the six pollutants, particle pollution and ground-level ozone are the most widespread health threats. The U.S. Environmental Protection Agency (EPA) regulates them by developing criteria based on human and environmental health considerations.

Ground-level ozone is not emitted directly into the air but is created by chemical reactions between nitrogen oxides (NOx) and volatile organic compounds (VOC) in the presence of sunlight. Emissions from industrial facilities and electric utilities, motor vehicle exhaust, gasoline vapors, and chemical solvents are some of the major sources of NOx and VOC. Breathing ozone can trigger various health problems, particularly for children, the elderly, and people of all ages with lung diseases such as asthma. Ground-level ozone can also have harmful effects on sensitive vegetation and ecosystems. (Ground-level ozone should not be confused with the ozone layer, which is high in the atmosphere and protects Earth from ultraviolet light; ground-level ozone provides no such protection).

Particulate matter, also known as particle pollution, is a complex mixture of extremely small particles and liquid droplets. Particle pollution comprises several components, including acids (such as nitrates and sulfates), organic chemicals, metals, and soil or dust particles. The size of particles is directly linked to their potential to cause health problems. EPA is concerned about particles that are 10 micrometers in diameter or smaller because those are the particles that generally pass through the throat and nose and enter the lungs. Once inhaled, these particles can affect the heart and lungs and cause serious health effects.

Carbon monoxide (CO) is a colorless, odorless gas emitted from combustion processes.  Nationally and, particularly in urban areas, the majority of CO emissions to ambient air come from mobile sources.  CO can cause harmful health effects by reducing oxygen delivery to the body’s organs (like the heart and brain) and tissues. At extremely high levels, CO can cause death.

Nitrogen dioxide (NO₂) is one of a group of highly reactive gasses known as “oxides of nitrogen,” or nitrogen oxides (NOx). Other nitrogen oxides include nitrous acid and nitric acid. EPA’s National Ambient Air Quality Standard uses NO₂ as the indicator for the larger group of nitrogen oxides. NO2 forms quickly from emissions from cars, trucks, buses, power plants, and off-road equipment. In addition to contributing to the formation of ground-level ozone, and fine particle pollution, NO2 is linked with a number of adverse effects on the respiratory system.

Sulfur dioxide (SO₂) is one of a group of highly reactive gasses known as “oxides of sulfur.”  The largest sources of SO₂ emissions are from fossil fuel combustion at power plants (73%) and other industrial facilities (20%).  Smaller sources of SO₂ emissions include industrial processes such as extracting metal from ore and burning high sulfur-containing fuels by locomotives, large ships, and non-road equipment. SO₂ is linked with a number of adverse effects on the respiratory system.

Lead is a metal found naturally in the environment and manufactured products. The major sources of lead emissions have historically been fuels in on-road motor vehicles (such as cars and trucks) and industrial sources.  As a result of regulatory efforts in the U.S. to remove lead from on-road motor vehicle gasoline, emissions of lead from the transportation sector dramatically declined by 95 percent between 1980 and 1999, and levels of lead in the air decreased by 94 percent between 1980 and 1999. Today, the highest lead levels in the air are usually found near lead smelters. Today’s major sources of lead emissions to the air are ore and metals processing and piston-engine aircraft operating on leaded aviation gasoline.

Indoor Air Pollution 

Most people spend approximately 90 percent of their time indoors. However, the indoor air we breathe in homes and other buildings can be more polluted than outdoor air and can increase the risk of illness. There are many sources of indoor air pollution in homes. They include biological contaminants such as bacteria, molds, and pollen, burning fuels and environmental tobacco smoke, building materials and furnishings, household products, central heating and cooling systems, and outdoor sources. Outdoor air pollution can enter buildings and become a source of indoor air pollution.

Sick building syndrome is a term used to describe situations where building occupants have health symptoms associated only with spending time in that building. Causes of sick building syndrome include inadequate ventilation, indoor air pollution, and biological contaminants. Usually, indoor air quality problems only cause discomfort. Most people feel better as soon as they remove the source of the pollution. Ensuring that your building is well-ventilated and getting rid of pollutants can improve indoor air quality.