Air Pollution: Overview and types

Ingrid Yanet Sanchez Torres, Elizabeth Friedl
  • Author
    Ingrid Yanet Sanchez Torres

    Ingrid is an engineer in Renewable Energies from Mexico. She is currently studying his doctorate and has a masters degree in this area. She has taught for over 3 years topics like Math, Calculus, Statistics, Algebra, Physics and Biology in elementary school, high school and university.

  • Instructor
    Elizabeth Friedl

    Elizabeth, a Licensed Massage Therapist, has a Master's in Zoology from North Carolina State, one in GIS from Florida State University, and a Bachelor's in Biology from Eastern Michigan University. She has taught college level Physical Science and Biology.

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

Table of Contents


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.

Air pollution in cities causes a great damage to human and environmental health

Polluted cities with a gray smog cloud

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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  • 0:07 What Is Air Pollution?
  • 0:54 Sources of Air Pollution?
  • 2:02 Types of Air Pollutants
  • 5:24 Summary
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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.

Industry is one of the main air pollution sources

Industry emission

Types of Air Pollution

The EPA has identified six of the most common types of air pollutants:

  1. carbon monoxide
  2. sulfur dioxide
  3. nitrogen oxides
  4. ozone air
  5. particulate matter
  6. 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.

Fireplaces are household sources of carbon monoxide emissions

A fireplace in a house

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.

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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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