Meghan Hodges
The Clean Air Act, enacted in 1970, contains provisions to control common pollutants that formed dense, visible smog in many cities and industrial centres across the United States. The Act requires the Environmental Protection Agency (EPA) to establish National Ambient Air Quality Standards (NAAQS) for six common air pollutants, also known as criteria air pollutants. These criteria pollutants are common in the United States and much of the world and are generally combustion products released by the burning of fossil fuels.
| Characteristics | Values |
|---|---|
| Number of pollutants | Six |
| Names of pollutants | Particulate matter, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead |
| Particulate matter size | PM2.5 and PM10 |
| Particulate matter sources | Farming operations, combustion sources, construction, industrial processes, residential combustion activities, and vehicle exhaust |
| Ozone sources | Chemical reaction between oxides of nitrogen and volatile organic compounds, including motor vehicle exhaust, industrial emissions, gasoline vapors, and chemical solvents |
| Carbon monoxide sources | Vehicles, gasoline-burning equipment, industrial processes, ore and metal processing, waste incinerators, and lead-acid battery manufacturing |
| Sulfur dioxide sources | Electrical generation sources that burn sulfur-containing fuels, metal processing, and smelting facilities |
| Nitrogen dioxide sources | Fuel burned at high temperatures, including industrial, commercial, and residential combustion, motor vehicles, and electric utilities |
| Lead sources (historical) | Burning of leaded gasoline by vehicles |
| Health effects | Dizziness, chest pain, heart complications, lung inflammation, asthma attacks, reduced IQ in children, respiratory issues, cardiovascular issues, and premature death |
| Environmental effects | Damage to animals, crops, vegetation, buildings, and other property |
| Regulatory body | Environmental Protection Agency (EPA) |
| Regulatory standards | National Ambient Air Quality Standards (NAAQS) |
| Regulatory act | Clean Air Act |
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What You'll Learn
- Particulate matter (PM) is a broad set of pollutants, grouped by particle size
- Ozone is formed by a chemical reaction between other directly emitted pollutants
- Carbon monoxide (CO) is a harmful, odourless gas, primarily emitted by vehicles
- Sulfur dioxide (SO2) is emitted by electrical generation sources burning sulfur-containing fuels
- Nitrogen dioxide (NO2) is part of a group of highly reactive gases known as oxides of nitrogen
Particulate matter (PM) is a broad set of pollutants, grouped by particle size
Particulate matter (PM) is a broad set of pollutants that are grouped by particle size. It is a mixture of solid particles and liquid droplets found in the air. PM is not a single pollutant but a mixture of many chemical species. These particles vary widely in size, shape, and chemical composition and may contain inorganic ions, metallic compounds, elemental carbon, organic compounds, and compounds from the earth's crust. The particles are defined by their diameter for air quality regulatory purposes.
PM10 refers to particles with a diameter of 10 micrometers or less, which can be inhaled into the lungs and cause adverse health effects. Sources of PM10 include dust from construction sites, landfills, agriculture, wildfires, industrial sources, wind-blown dust from open lands, pollen, and fragments of bacteria.
PM2.5 refers to fine inhalable particles with diameters of 2.5 micrometers or less. These particles pose the greatest risk to health and are the main cause of reduced visibility (haze) in parts of the United States. PM2.5 can originate from both outdoor and indoor sources. Outdoor sources include the combustion of gasoline, oil, diesel fuel, or wood, while indoor sources include smoking tobacco, cooking, burning wood or incense, and household cleaning products.
Long-term exposure to PM2.5 has been linked to premature death, particularly in individuals with chronic heart or lung diseases, and reduced lung function growth in children. Scientific studies have also linked particle pollution exposure to various health problems, including increased respiratory symptoms such as irritation of the airways, coughing, or difficulty breathing.
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Ozone is formed by a chemical reaction between other directly emitted pollutants
The Clean Air Act, enacted in 1970, contains provisions to control common pollutants that formed dense, visible smog in many cities and industrial centers across the United States. The Act requires the Environmental Protection Agency (EPA) to establish National Ambient Air Quality Standards (NAAQS) for six common air pollutants, also known as "criteria" air pollutants. These pollutants include particulate matter, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. These criteria pollutants are found all over the United States and much of the world and can harm human health, the environment, and cause property damage.
Ground-level ozone, formed by chemical reactions between other directly emitted pollutants, is one of the most harmful criteria pollutants. Ozone (O3) is a highly reactive and unstable gas capable of damaging living cells, including those present in the linings of the human lungs. It is the main ingredient in smog and contributes to what we experience as haze. Ozone is not emitted directly into the air but is a byproduct of chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOCs). These reactions traditionally require heat and sunlight, leading to higher ozone concentrations in the summer months. However, high ozone levels have also been observed in cold months under specific conditions.
Tropospheric ozone is primarily formed from photochemical reactions between two major classes of air pollutants: volatile organic compounds and nitrogen oxides. VOCs are emitted from sources such as chemical plants, gasoline pumps, oil-based paints, auto body shops, and print shops. Nitrogen oxides, on the other hand, result from high-temperature combustion in power plants, industrial furnaces, boilers, and motor vehicles. When these pollutants are emitted, they react in the presence of sunlight to form ozone.
Ozone formation is not limited to large metropolitan areas but can occur in smaller cities and rural areas. Areas downwind of major sources of VOCs and NOx may experience ozone peaks in the afternoon and evening as wind carries the precursors many miles from their sources. This results in high ozone concentrations in remote areas and at various times of the day. Ozone levels are typically higher during the afternoon when sunlight is most intense. However, high ozone concentrations can occur throughout the year in some southern and mountain regions.
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Carbon monoxide (CO) is a harmful, odourless gas, primarily emitted by vehicles
Carbon monoxide (CO) is a harmful, odourless gas that is primarily emitted by vehicles and gasoline-burning equipment, such as generators. CO is also released during various industrial processes, including food, chemical, and metal manufacturing. The burning of fossil fuels in boilers and engines also produces CO, but in smaller amounts compared to vehicles.
Carbon monoxide is a significant health hazard as it reduces the amount of oxygen delivered to vital organs like the heart and brain. Exposure to high levels of CO, especially in enclosed spaces, can lead to dizziness, confusion, unconsciousness, and even death. People with heart disease are particularly vulnerable to the harmful effects of carbon monoxide.
As a criteria air pollutant, carbon monoxide is a key indicator of air quality and is regulated by environmental agencies. The Clean Air Act, established in 1970, mandates that the US Environmental Protection Agency (EPA) set National Ambient Air Quality Standards (NAAQS) for six common air pollutants, including carbon monoxide. These standards aim to protect public health and the environment from the harmful effects of air pollution.
Motor vehicles, especially in urban areas, are the primary source of carbon monoxide emissions. Poorly ventilated parking garages, tunnels, or areas with high traffic congestion can have higher concentrations of CO. Additionally, machinery that burns fossil fuels, such as boilers and industrial equipment, also contributes to CO emissions. However, the majority of carbon monoxide comes from mobile sources, making vehicles the primary culprits.
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Sulfur dioxide (SO2) is emitted by electrical generation sources burning sulfur-containing fuels
Sulfur dioxide (SO2) is one of the six common air pollutants, also known as criteria pollutants, outlined in the Clean Air Act. These criteria pollutants are subject to primary and secondary National Ambient Air Quality Standards (NAAQS) set by the EPA. The Clean Air Act was enacted in 1970 to control common pollutants that formed dense, visible smog in many cities and industrial centers across the nation.
SO2 is a colorless gas with a pungent smell, often described as the odor of burnt matches. It is a toxic gas that can be harmful to human health, particularly when inhaled in large quantities. SO2 is emitted by the burning of fossil fuels or other materials that contain sulfur. This includes the burning of sulfur-bearing fossil fuels and the combustion of elemental sulfur.
One of the largest sources of SO2 emissions is the burning of fossil fuels by power plants and other industrial facilities. This includes the combustion of sulfur-containing fuels, such as liquified sulfur, which is sprayed through an atomizing nozzle to generate fine drops of sulfur with a large surface area. The combustion of hydrogen sulfide and organosulfur compounds is another source of SO2 emissions.
In addition to power plants, metals processing and smelting facilities, and vehicles, particularly diesel vehicles, are also significant sources of SO2 emissions. However, federal regulations to reduce the sulfur in diesel fuels have helped to lower emissions from this source.
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Nitrogen dioxide (NO2) is part of a group of highly reactive gases known as oxides of nitrogen
Nitrogen dioxide (NO2) is a highly reactive gas and a member of the oxides of nitrogen or nitrogen oxides (NOx) group. It is a strong oxidizer and can combust explosively in the presence of hydrocarbons. NO2 is a significant air pollutant and is commonly found in the air due to the burning of fuel, particularly from emissions produced by cars, trucks, and buses, power plants, and off-road equipment.
NO2 is formed by the oxidation of nitric oxide by oxygen in the air, often as a result of corona discharge. The higher the combustion temperature, the more nitric oxide is generated. This oxidation process occurs rapidly in outdoor air due to the presence of oxygen, ozone, and VOCs, but it is generally much slower indoors. Road traffic is the primary outdoor source of nitrogen dioxide, while tobacco smoke and gas-, wood-, oil-, kerosene-, and coal-burning appliances are the most significant indoor sources.
Nitrogen dioxide has a reddish-brown colour and a pungent odour. It is corrosive and poorly soluble in water but soluble in sulfuric and nitric acids. NO2 can irritate the airways in the human respiratory system, aggravating respiratory diseases, especially asthma. Prolonged exposure to elevated concentrations of NO2 may contribute to the development of asthma and potentially increase susceptibility to respiratory infections.
In addition to its direct health impacts, NO2 interacts with water, oxygen, and other chemicals in the atmosphere to form acid rain, which harms sensitive ecosystems such as lakes and forests. The nitrate particles resulting from NOx contribute to haze and reduced visibility, affecting the air quality in many national parks.
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Frequently asked questions
There are six criteria pollutants: particulate matter, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. These criteria are used to set standards for good air quality and protect human health and the environment from air pollution.
Particulate matter (PM) refers to a broad set of pollutants grouped based on particle size. These groups include particles smaller than 2.5 micrometers (PM2.5) and those between 2.5 and 10 micrometers (PM10). Sources include farming operations, combustion, construction, and industrial processes. PM exposure is linked to heart attacks, irregular heartbeats, respiratory issues, and asthma.
Ground-level ozone is formed in the atmosphere by a chemical reaction between other directly emitted pollutants, including oxides of nitrogen (NOx) and volatile organic compounds (VOCs). Major contributors to ozone formation include vehicle exhaust, industrial emissions, gasoline vapors, and chemical solvents. Ground-level ozone can inflame and damage lungs, increasing asthma attacks and susceptibility to infection.
Carbon monoxide is a colorless, odorless gas emitted primarily from combustion processes and mobile sources, especially motor vehicles and machinery burning fossil fuels. CO reduces the oxygen delivered to vital organs and can lead to dizziness, confusion, unconsciousness, and death. It is particularly harmful to individuals with heart conditions.
Sulfur dioxide is primarily emitted by electrical generation sources burning sulfur-containing fuels, as well as metal processing and smelting facilities. Exposure to SO2 can cause difficulty breathing, wheezing, and shortness of breath. Elevated concentrations of SO2 can react with other compounds to form small particles that contribute to particulate matter pollution and regional haze.
Nitrogen dioxide is a highly reactive gas and part of the oxides of nitrogen (NOx) group. NO2 is harmful to the respiratory system and contributes to the formation of other pollutants, including PM, ozone, acid rain, and regional haze. It is emitted when fuel is burned at high temperatures, with sources including industrial processes, motor vehicles, and electric utilities.
