Helena Joyce
Air pollution is defined as the contamination of air by harmful gases, dust, and smoke, which affects plants, animals, and humans. It is caused by air pollutants, which are substances that cause pollution. These pollutants can be classified in various ways, including their physical composition, sources, health impacts, and regulations. The US EPA, for instance, categorizes pollutants into Common Air Pollutants and Hazardous Air Pollutants. Common Air Pollutants, also known as criteria pollutants, include particulate matter, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. These pollutants are widespread and pose risks to human health, the environment, and property. Hazardous Air Pollutants (HAPs), on the other hand, are known or suspected to cause serious health and environmental issues, including cancer and reproductive problems. This classification system helps in understanding and managing air quality, with the EPA setting standards and regulations to mitigate the impacts of these pollutants.
| Characteristics | Values |
|---|---|
| Definition | Air pollution is the contamination of air by harmful gases, dust, and smoke which affects plants, animals, and humans drastically. |
| Types | Primary, secondary, quantitative, particulate matter, gaseous, common, hazardous, and more. |
| Examples | Oxides of carbon, nitrogen, and sulphur; polychlorinated biphenyls (PCBs); phosgene (COCl2); methyl isocyanate; chlorofluorocarbons (CFCs); particulate matter; ground-level ozone; carbon monoxide; sulfur dioxide; nitrogen dioxide; lead; benzene; dioxins; asbestos; toluene; cadmium; mercury; chromium; and lead compounds. |
| Causes | Fossil fuels, combustion, industrial and domestic effluents, burning plastics, secondary reactions, global warming, and more. |
| Effects | Global warming, acid rain, ocean acidification, melting of ice caps and glaciers, extreme weather events, water scarcity, desertification, smog, ozone depletion, health issues, property damage, etc. |
| Agencies | US EPA, state, tribal, and local air agencies. |
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What You'll Learn
Particulate matter
The US Clean Air Act requires the EPA to set National Ambient Air Quality Standards for six common air pollutants, known as "criteria pollutants". These pollutants are found all over the United States and include particulate matter (often referred to as particle pollution), ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead.
PM2.5, or fine inhalable particles, have diameters that are generally 2.5 micrometers and smaller. To put this into perspective, the average human hair is about 70 micrometers in diameter, making it 30 times larger than the largest fine particle. These fine particles are the main cause of reduced visibility (haze) in parts of the United States, including many national parks and wilderness areas. They can also induce adverse health effects, with short-term exposures primarily linked to the worsening of respiratory diseases, including asthma and chronic obstructive pulmonary disease (COPD). Long-term exposure to PM2.5 has been associated with premature death, particularly in people with chronic heart or lung diseases, and reduced lung function growth in children.
PM10 refers to particles with a diameter of 10 microns or less, which are inhalable into the lungs and can also induce adverse health effects. While PM2.5 comprises a portion of PM10, they often derive from different emission sources and have distinct chemical compositions. The EPA's national and regional rules to reduce emissions of pollutants that form PM help state and local governments meet the Agency's national air quality standards. Additionally, the Air Quality Index (AQI) provided by AirNow helps individuals understand the level of air pollution on a given day and take appropriate actions to protect their health.
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Gaseous pollutants
Ozone (O3) is a key component of smog, which forms when nitrogen dioxide reacts with volatile organic compounds like gasoline vapors. Sulfur dioxide (SO2) is produced by the combustion of fossil fuels containing sulfur and contributes to acid rain when it reacts with water vapor in the atmosphere. Nitrogen dioxide (NO2) is another byproduct of fossil fuel combustion and is a major component of smog. Carbon monoxide (CO) is a toxic gas emitted directly from combustion sources, posing serious health risks by reducing the blood's ability to carry oxygen.
Other significant gaseous pollutants include hydrogen sulfide (H2S), hydrogen chloride (HCl), hydrogen fluoride (HF), chlorine (Cl2), ethylene (C2H4), peroxyacetyl nitrate (PAN), and ammonia (NH3). These gases can cause plant injury, especially near large cities, industrial facilities, and highways. Additionally, the increasing use of fossil fuels has led to elevated levels of carbon dioxide (CO2) and other greenhouse gases, contributing to global warming and climate change.
The measurement of gaseous pollutant concentrations is crucial for understanding their impact on the environment and human health. Passive samplers have been widely used, especially in forest areas, as they are cost-effective and do not require a power supply. More complex methods, such as reacting gases with colorimetric reagents, are also employed to determine the intensity of the reaction product color. These measurements help set standards and regulations, such as the National Ambient Air Quality Standards (NAAQS) in the U.S., to mitigate the harmful effects of gaseous pollutants on human health and the environment.
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Primary and secondary pollutants
Primary pollutants are those that are emitted directly from a source. They can be of natural origin, such as volcanic eruptions or fires, or anthropogenic, such as carbon emissions from the burning of fossil fuels. Some primary pollutants include:
- Particulates
- Carbon monoxide
- Nitrogen oxide
- Sulfur oxide
- DDT
- Plastic
- CO2
- Oxides of nitrogen and sulphur
Secondary pollutants, on the other hand, are formed in the lower atmosphere by chemical reactions between primary pollutants. They are harder to control because their formation is not fully understood. Some examples of secondary pollutants include:
- Ground-level ozone, formed by the interaction of volatile organic compounds (VOCs) and nitrous oxides (NOx) with sunlight and heat.
- Peroxyacetyl nitrate (PAN), formed by the interaction of nitrogen oxides and hydrocarbons.
- Photochemical smog, formed from the interaction of particulates, nitrogen oxides, ozone, and other air pollutants, especially in areas with a lot of industry and traffic.
- Acid rain, formed when sulphuric and nitric acids created by the combustion of fossil fuels drop down with rain, making the rain acidic.
Both primary and secondary pollutants can have detrimental effects on human health and the environment. They can cause visibility issues, health problems, environmental damage, and property damage.
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Common and hazardous pollutants
The US Clean Air Act requires the Environmental Protection Agency (EPA) to set National Ambient Air Quality Standards (NAAQS) for six common air pollutants, known as criteria pollutants. These pollutants are found all over the United States and can harm human health, the environment, and cause property damage. The six criteria air pollutants are:
- Particulate matter (often referred to as particle pollution): This includes fine metal dust, fly ash, soot, cotton dust, and radioactive substances.
- Ground-level ozone: Ozone at ground level is a health threat and a criteria air pollutant.
- Carbon monoxide: A common air pollutant and criteria pollutant.
- Sulfur dioxide: The burning of fossil fuels releases sulphur into the atmosphere, which reacts with water vapour to form sulphuric acid, contributing to acid rain.
- Nitrogen dioxide: Nitrogen oxides are released during the combustion of fossil fuels and are a primary pollutant.
- Lead: A criteria air pollutant that is harmful to human health and the environment.
In addition to the six criteria pollutants, there are also toxic air pollutants, known as hazardous air pollutants (HAPs). These are pollutants that are known or suspected to cause cancer, reproductive issues, birth defects, or other serious health issues. Examples of HAPs include polychlorinated biphenyls (PCBs), which are released during the burning of plastics, and phosgene (COCl2), accidentally released during industrial catastrophes.
Other types of pollutants include chlorofluorocarbons (CFCs), which are used in refrigeration, air conditioning, and aerosol sprays. CFCs damage the ozone layer. The combustion of fossil fuels also increases the number of suspended particles in the air, contributing to smog and reduced visibility. Additionally, the increasing use of fossil fuels is a leading cause of rising levels of greenhouse gases, contributing to global warming and climate change.
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National Ambient Air Quality Standards
The United States Environmental Protection Agency (EPA) established the National Ambient Air Quality Standards (NAAQS) under the Clean Air Act (42 U.S.C. 7401 et seq.). The NAAQS sets limits on the atmospheric concentration of six principal pollutants, also known as "criteria pollutants", that cause smog, acid rain, and other health hazards. These pollutants are commonly found in outdoor air throughout the country and are harmful to public health and the environment.
The six criteria air pollutants (CAP) for which limits are set in the NAAQS are:
- Ozone (O3), also referred to as ground-level ozone or tropospheric ozone.
- Atmospheric particulate matter (PM2.5/PM10), also known as particle pollution.
- Lead (Pb).
- Carbon monoxide (CO).
- Sulfur oxides (SOx), previously known as sulfur dioxide (SO2).
- Nitrogen oxides (NOx), previously known as nitrogen dioxide (NO2).
The EPA establishes two types of NAAQS standards: primary and secondary. The primary standards are designed to protect the health of 'sensitive' populations, including asthmatics, children, and the elderly, by providing an adequate margin of safety. The secondary standards focus on protecting the environment and public welfare by addressing visibility, damage to crops, vegetation, buildings, animals, and other effects on soils, water, climate, man-made materials, etc.
The NAAQS are periodically reviewed and revised as necessary. For instance, in 2010, the EPA replaced the dual primary standards for SO2 with a new 1-hour standard set at 75 ppb. The EPA also undertakes regulatory and remedial actions to minimise exposure to pollutants from non-air sources and reduce air emissions.
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Frequently asked questions
Air pollution is defined as the introduction of pollutants, organic molecules, or other unsafe materials into Earth’s atmosphere. This can be in the form of excessive gases like carbon dioxide and other vapors that cannot be effectively removed through natural cycles.
Pollutants can be classified in a variety of ways, including their physical composition, their sources, their health impacts, and how they are regulated. Examples of air pollutants include particulate matter, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead.
The USEPA, or FEPA, classifies pollutants into two categories: Common Air Pollutants and Hazardous Air Pollutants (HAPs). Common Air Pollutants include the six pollutants regulated by the National Ambient Air Quality Standards (NAAQS): particulate matter, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. HAPs, also known as toxic air pollutants, are those that are known or suspected to cause serious health and environmental effects.
Air pollutants can come from anthropogenic processes such as vehicles, industries, and construction sites, or natural sources like pollen, volcanic eruptions, and natural gaseous precursors.
The US Clean Air Act requires FEPA to set NAAQS for six common air pollutants, also known as "criteria pollutants." FEPA works with state, tribal, and local air agencies to attain these standards throughout the country. FEPA also publishes emissions factor information for over 200 air pollution source categories.
