Learn about the definition of air pollution, and see air quality categories. Discover the list of air pollutants and their sources, and know types of air pollution.
Updated: 02/15/2022
Air Pollution: Overview and types
Air Pollution Definition
Like with water and soil, the air is also susceptible to pollution. Air pollution is the presence of materials (like gases or particles) or forms of energy in the atmosphere that can pose a risk, damage, or nuisance of varying severity to living beings. Among the direct consequences of air pollution are the development of diseases and conditions in humans and biodiversity, as well as the loss of visibility or appearance of unpleasant odors in areas of large, concentrated populations.
The production, development, and use of new means of transportation since the Industrial Revolution have increased the levels of carbon dioxide and other polluting gases in the atmosphere. This has caused air pollution to become a global environmental problem. The gray fog that encompasses large cities is the most common way in which air pollution is shown. The increase in greenhouse gases is causing a general warming of the planet and the deterioration of the ozone layer. Both effects pose a danger to the health of human beings and to the ecological balance of ecosystems.
Human action is the main culprit of air pollution. The burning of fossil fuels, such as coal or oil, is one of the main causes of this environmental problem that endangers not only nature but also our health.
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Air Quality Categories
After various events in the US and in the world that warned of the danger of air pollution, Congress approved the Clean Air Act of 1970. This law was a comprehensive federal response to fight and reduce air pollution. The same year, Congress created the EPA (Environmental Protection Agency) and gave them the task of executing that law. Since that day, the EPA has been responsible for designing and executing programs that seek the reduction of air pollution.
In the Clean Air Act, the EPA establishes limits on certain air pollutants, including limits on how many pollutants can stay in the air anywhere in the US. This helps to guarantee the basic protection of health and the environment against air pollution. The Clean Air Act also gives EPA the authority to limit emissions of air pollutants from sources such as chemical plants, utilities, and steel mills. Each particular state or tribe may have more specific laws concerning air pollution, but may not have lower limits than those established by the EPA.
The EPA created the Air Quality Index (AQI) to provide information about local air quality, health problems for different levels of air pollution, and how people can protect their health when pollutants reach unhealthy levels. This is a scale that ranges from 0 to more than 300, where 0 is a healthy air quality and more than 300 means that there are hazardous air quality conditions. There are six air quality categories, all labeled by a different color to help people identify the air quality value in any place. AQI values at or below 100 are generally thought of as satisfactory. When AQI values are above 100, air quality is unhealthy. Certain sensitive groups of people are primarily at risk, but as AQI values get higher, air quality is dangerous for everybody.
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| Daily AQI Color | Levels of Concern | Values of Index | Description of Air Quality |
| Green | Good | 0 to 50 | Air quality is satisfactory, and air pollution poses little or no risk. |
| Yellow | Moderate | 51 to 100 | Air quality is acceptable. However, there may be a risk for some people, particularly those who are unusually sensitive to air pollution. |
| Orange | Unhealthy for Sensitive Groups | 101 to 150 | Members of sensitive groups may experience health effects. The general public is less likely to be affected. |
| Red | Unhealthy | 151 to 200 | Some members of the general public may experience health effects; members of sensitive groups may experience more serious health effects. |
| Purple | Very Unhealthy | 201 to 300 | Health alert: The risk of health effects is increased for everyone. |
| Maroon | Hazardous | 301 and higher | Health warning of emergency conditions: everyone is more likely to be affected. |
Air Pollutants and its Sources
Air pollutants are substances or compounds that are introduced to the atmosphere by different sources, with certain concentrations that can have a measurable effect on humans, animals, vegetation, or building materials. Air pollutants can be classified by their effects as follows:
- Primary pollutants: the pollutant substances or compounds in the atmosphere that cause a direct effect or harm to environmental health.
- Secondary pollutants: substances or compounds that are created from reactions between primary pollutants and components of the atmosphere.
The sources are activities or locations that introduce pollutants into the atmosphere. They can be classified according to their origin as:
- Human-made sources: derive from human activities and industry. Also include emissions from cars and other means of transport.
- Natural sources: include natural activities. For example, volcanic eruptions emit particles and gases to the atmosphere. Also, natural forest fires produce gases that pollute the atmosphere.
The way the pollution is emitted to the atmosphere is classified as:
- Point sources: when the pollution comes from a single point of origin, like a single coal plant.
- Non-Point sources: when the air pollutants enter the atmosphere from various sources, like all the vehicles that use gasoline in a country.
The emission factors are representative values that try to relate the quantity of a pollutant released to the atmosphere with an activity associated with the release of that pollutant. This is a concept that relates the air pollutants and their sources.
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Types of Air Pollution
The EPA has identified six of the most common types of air pollutants:
- carbon monoxide
- sulfur dioxide
- nitrogen oxides
- ozone air
- particulate matter
- lead
The EPA focuses on establishing standards and limits for these groups of pollutants, because these six pollutants can cause great damage to humans and environmental health. The organization is constantly monitoring the ambient air quality and the stationary source emissions to ensure that the limits established in laws and regulations are not exceeded.
Carbon Monoxide Pollution
Carbon monoxide (CO) is a colorless and odorless gas that is formed by the incomplete combustion of organic material in the presence of oxygen deficiency. It is considered one of the most abundant types of air pollution in the Earth's atmosphere, and one of the biggest environmental problems around the world.
The main sources of this contaminant are motor vehicles that use gasoline or diesel as fuel, industrial processes, forest and urban fires, and the incineration of organic matter. With the growing use of household appliances and gas appliances, in addition to other elements such as fireplaces and heating systems, homes have become an important risk factor for indoor carbon monoxide poisoning.
Depending on the degree and length of exposure, carbon monoxide poisoning can have many health effects. Among these effects are permanent brain damage, damage to the heart, which can lead to life-threatening heart complications, and fetal death or spontaneous abortion.
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Sulfur Dioxide Pollution
Sulfur dioxide (SO2) is one of the most common types of air pollutants. It is a colorless gas with a pungent odor. SO2 is part of a larger group of gaseous sulfur oxides (SOx). It is the most worrying of the SOx groups, as it is found in higher concentrations than other gaseous SOx, such as SO3. Exposure to sulfur dioxide can occur through inhalation, most commonly, or through contact with the skin or eyes. Once SO2 enters the lungs, it can enter the bloodstream. To leave the body, SO2 breaks down and leaves through urine. Some short-term effects of exposure to this compound are sneezing, coughing, tightness in the chest, and difficulty breathing. Long-term effects include lung damage, loss of smell, and bronchitis.
Sulfur dioxide is emitted both by natural and human sources. Some of the main sources are volcanic eruptions, the burning of fossil fuels by power plants and other industrial facilities, the manufacturing of sulfuric acid, paper, food preservatives, fertilizers, locomotives, ships, and other heavy vehicles and equipment that burn fuel with high sulfur content.
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Nitrogen Oxides Pollution
Nitrogen oxides are other hazardous types of air pollutants. The nitrogen oxides of greatest concern as contaminants in air quality are nitric oxide ({eq}NO {/eq}) and nitrogen dioxide ({eq}NO_2 {/eq}). {eq}NO_2 {/eq} is a reddish-brown, non-flammable gas with a suffocating and toxic odor. Its main source is combustion processes at high temperatures due to the combination of oxygen and nitrogen present in the air. This causes nitric oxide ({eq}NO {/eq}), due to the action of atmospheric oxidants such as ozone, to become {eq}NO_2 {/eq}. In cities, traffic is the main source of nitrogen oxides.
Among the possible damages to health from nitrogen oxides are bronchitis and pneumonia, and lower resistance to respiratory tract infections. Low levels of nitrogen oxides in the air can irritate the eyes, nose, throat, and lungs, causing coughing and a feeling of shortness of breath, tiredness, and nausea. Low-level exposure can also cause fluid buildup in the lungs one to two days after exposure. Breathing high levels of nitrogen oxides can rapidly cause burns, spasms, and dilation of tissues in the throat and upper airways, reducing oxygenation of body tissues, leading to fluid buildup in the lungs, and death.
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Ozone Air Pollution
Ozone (O3) is another type of air pollution. Ozone is a pale blue gas that gives off a strong odor. This gas is naturally present in the atmosphere at very low concentrations. In cities, it can form in large concentrations due to the chemical reaction between nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. The NOx and VOCs come from traffic and the emissions of combustion.
Ozone is considered an environmental pollutant because at high concentrations it can cause health damage such as irritating the respiratory system, aggravating asthma and chronic lung diseases, reducing lung function, and shortening life expectancy.
Particulate Matter
Particulate Matter (PM) are solid and liquid particles that are suspended in the air, most of which pose a hazard. This complex mixture contains dust, pollen, soot, smoke, and small droplets, among other things. These particles come in a wide variety of sizes, from coarse to fine to ultrafine. They also present disparate compositions and origins. Particles can be emitted directly into the air when they come from sources such as combustion processes or windblown dust, or they may be formed in the atmosphere by the transformation of gases emitted such as SO2. Sulfates and organic matter are the main components of the set of suspended particles that pollute the air. Mineral dust, nitrates, and soot can also become major components under certain conditions.
Long-term exposure to the concentrations of suspended particles found in the environment can affect the lungs of both children and adults, and can also cause a shorter life expectancy, especially in people with cardiac and pulmonary conditions. When suspended particles appear together with other air pollutants, the individual effects of each pollutant accumulate.
Lead Pollution
Lead (Pb) is another air pollutant. Gasoline-powered vehicles have been one of the main sources of lead emissions in the atmosphere for years, as alkyl derivatives of lead (tetraethyl and tetramethyl lead) are used as anti-knock additives in gasoline. The current environmental regulations prohibit and limit its use, considerably reducing these emissions with the widespread use of unleaded gasoline. Other human sources of lead include chemical industries like paint, mining industries, metal smelting and recovery processes, waste incineration, sewage sludge, coal combustion, and battery manufacturing.
Lead emissions into the atmosphere are in the form of particles and gaseous compounds. Gaseous emissions are mainly generated by the combustion of gasoline additives, while particles come from all types of emission sources. Most of the lead in the atmosphere is in the form of fine particles less than 1 micrometer in diameter. Lead is considered very dangerous for the environment because it does not undergo chemical or biological degradation, which causes it to remain highly toxic in the environment for long periods of time. This seriously affects food chains, due to the accumulation of lead in living organisms (known as bioaccumulation).
Lesson Summary
The air pollution definition is the presence of materials (like gases or particles) or forms of energy in the atmosphere that can pose a risk, damage, or nuisance of varying severity to living beings. These pollutants come from various natural or human sources. These can be single (point) or multiple (non-point) sources. The air pollutants that are emitted into the atmosphere can be primary if they cause direct damage to human and environmental health or secondary if they are formed from reactions with other components of the atmosphere.
The US Congress formulated the Clean Air Act of 1970, which established actions to detect, decrease and regulate air pollution. They also created the EPA, an organization that monitors and enforces these regulations. The EPA identifies and establishes limits for six of the most hazardous types of air pollutants to human health. These include carbon monoxide (CO), sulfur dioxide (SO2), nitrogen oxides (NOx), ozone (O3), particulate matter (PM), and lead. Also, the EPA established an Air Quality Index (AQI) so people can know current air quality in the place they live.
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Frequently Asked Questions
What is the most common type of air pollution?
The most common types of air pollution are sulfur dioxides and nitrogen oxides. This is because these compounds are frequently emitted by industry and by the combustion from vehicles.
What is air pollution?
Air pollution is the presence of materials (like gases or particles) or forms of energy in the atmosphere that can pose a risk, damage, or nuisance of varying severity to living beings. These materials come from different natural and human sources.
What are the 6 types of air pollution?
The EPA identified six groups of compounds that pose a great risk for human health and for the environment. These compounds are carbon monoxide, nitrogen dioxide, sulfur dioxide, ozone, particulate matter, and lead.
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