Helena Joyce
The six criteria pollutants, as defined by the US Environmental Protection Agency (EPA), include particulate matter, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. These pollutants are harmful to human health and the environment, causing property damage and contributing to smog, acid rain, and other environmental issues. They are commonly found in the air all over the United States and are emitted from a range of sources, including homes, motor vehicles, industry, power plants, and more. The Clean Air Act, enacted in 1970, empowers the EPA to establish National Ambient Air Quality Standards (NAAQS) for these six criteria pollutants, setting limits to protect public health and welfare.
| Characteristics | Values |
|---|---|
| Sulfur dioxide | Burning fossil fuels (coal and oil) in power plants, industrial facilities, and diesel-run vehicles |
| Nitrogen dioxide | Burning fuel at high temperatures, including industrial, commercial, and residential combustion, motor vehicles, and electric utilities |
| Particulate matter | Residential combustion activities, vehicle exhaust, wind-blown soil, and airborne residue from business and industry |
| Carbon monoxide | Incomplete combustion of carbon in fuels such as gasoline, heating oil, natural gas, wood, and charcoal |
| Ozone | Chemical reaction between other directly emitted pollutants, including nitrogen oxides (NOx) and volatile organic compounds (VOCs) |
| Lead | Commercial and industrial uses, such as cables, pipes, paints, and pesticides |
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What You'll Learn
- Carbon monoxide: from incomplete fuel combustion, e.g. in motor vehicles
- Nitrogen dioxide: from burning fuel at high temperatures, e.g. in vehicles and power plants
- Sulfur dioxide: from burning fossil fuels, e.g. coal, oil, and diesel
- Lead: from commercial and industrial uses, e.g. in cables, pipes, and paints
- Ozone: formed in the atmosphere by a chemical reaction between other emitted pollutants
Carbon monoxide: from incomplete fuel combustion, e.g. in motor vehicles
Carbon monoxide is one of the six criteria air pollutants, which are common across the United States and much of the world. These pollutants are harmful to human health, the environment, and can cause property damage.
Carbon monoxide (CO) is produced by the incomplete combustion of various fuels, such as propane, hexane, or other hydrocarbons. In a complete combustion reaction, a fuel source and oxygen combine to produce carbon dioxide and water. However, in incomplete combustion, there is an insufficient amount of oxygen to completely oxidize the fuel, resulting in the production of carbon monoxide and soot. This can occur due to a shortage of oxygen, an incorrect temperature, or a quick burn rate.
Motor vehicles are a significant source of carbon monoxide emissions. Approximately 80% of CO emissions come from highway vehicles and non-road mobile sources. The proximity to motor vehicle exhaust directly impacts one's exposure to carbon monoxide. Levels of CO within 20 meters of an interstate highway can range from 0.25 to 3 ppm, with maximum concentrations exceeding 4.0 ppm. In-vehicle CO concentrations are typically two to five times higher than ambient concentrations. Improvements in emission technologies have helped reduce overall CO emissions, despite an increase in motor vehicle miles traveled.
Other sources of carbon monoxide emissions include the burning of fossil fuels, such as coal and oil, by power plants and industrial facilities. Additionally, indoor combustion sources, such as cigarette smoke, contribute to CO exposure.
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Nitrogen dioxide: from burning fuel at high temperatures, e.g. in vehicles and power plants
Nitrogen dioxide (NO2) is a gaseous air pollutant and a member of the nitrogen oxides (NOx) family. It is formed when fossil fuels, such as coal, oil, methane gas (natural gas), or diesel, are burned at high temperatures. This combustion process occurs in vehicles and power plants, contributing to air pollution.
Vehicles, including trucks, buses, and cars, are the largest sources of NO2 emissions. The combustion of fuel in car engines, especially at high temperatures, produces NOx gases. In areas with high motor vehicle traffic, such as large cities, nitrogen oxide emissions from vehicles can significantly impact air quality. Additionally, diesel-powered non-road equipment, such as industrial boilers and other movable engines, also contributes to NO2 emissions.
Power plants, particularly those burning fossil fuels like coal and oil, are another significant source of nitrogen dioxide emissions. The combustion of these fuels at high temperatures releases NOx gases, including nitrogen dioxide. Gas-fired power plants and facilities that burn natural gas (methane) during extraction, processing, or transportation also contribute to NO2 emissions.
Other industrial sources of nitrogen dioxide include boilers, cement kilns, and turbines. Agricultural fertilization, the use of nitrogen-fixing plants, and biogenic (soil) NOx also contribute to atmospheric NOx levels. NOx emissions from these sources have been estimated at 28.5 million tonnes.
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Sulfur dioxide: from burning fossil fuels, e.g. coal, oil, and diesel
Sulfur dioxide (SO2) is one of the six criteria air pollutants that are found all over the United States and much of the world. These pollutants are extremely harmful and can negatively affect human health, the environment, and cause property damage.
Sulfur dioxide is produced from the burning of fossil fuels such as coal, oil, and diesel fuel. It is also produced during the smelting of mineral ores (aluminum, copper, zinc, lead, and iron) that contain sulfur. The largest sources of SO2 emissions are fossil fuel combustion at power plants and other industrial facilities. Locomotives, large ships, and some non-road diesel equipment currently burn high-sulfur fuel and release sulfur dioxide into the air.
SO2 has various detrimental effects on human health and the environment. Short-term exposure to high levels of SO2 can cause a burning sensation in the nose and throat and difficulty breathing. People with asthma may experience changes in respiratory effects due to SO2 exposure even at low concentrations. Long-term exposure to SO2 can cause changes in lung function and aggravate existing heart disease.
Once in the air, SO2 reacts with other molecules to form small particles called sulfates, which have a cooling effect on the planet. Unlike greenhouse gases, these sulfates scatter sunlight away from the Earth. Sulfur dioxide also contributes to acid rain, which can damage forests, crops, and make lakes and streams acidic and unsuitable for fish.
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Lead: from commercial and industrial uses, e.g. in cables, pipes, and paints
Lead is one of the six criteria air pollutants that are harmful to human health and the environment. It is a malleable, blue-grey, heavy metal that occurs naturally in the Earth's crust.
Lead has been widely used in commercial and industrial applications for centuries, including in cables, pipes, and paints. In the context of commercial and industrial uses, lead exposure is prevalent in various sectors, including construction, manufacturing, wholesale trade, transportation, and even recreation.
In construction, lead exposure occurs during the removal, renovation, or demolition of structures painted with lead-based paint. Lead-based paint was commonly used before 1978 due to its colour enhancement, surface coverage, and durability. However, flaking and peeling lead-based paint can mix with the soil near buildings, leading to potential lead exposure, especially for children who play in these areas. Lead pipes and plumbing materials are another source of exposure, as they can contaminate drinking water, particularly in older homes.
In manufacturing, lead is used in the production of various products, including batteries, electronics, and plastics. Lead exposure can occur during the manufacturing process, and workers may inadvertently bring lead dust home on their clothing, shoes, or skin, exposing their families to the toxic substance.
To mitigate the harmful effects of lead exposure, governments and organisations have implemented various measures. For instance, the Consumer Product Safety Commission (CPSC) banned the use of lead in paint for residential and public buildings in 1977, and the federal government banned consumer uses of lead-based paint in 1978. Additionally, the Occupational Safety and Health Administration (OSHA) has established lead standards and guidelines for general industry and construction to protect workers from lead exposure.
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Ozone: formed in the atmosphere by a chemical reaction between other emitted pollutants
Ozone is a gas composed of three oxygen atoms. It exists in the upper atmosphere (stratosphere) and lower atmosphere (troposphere), and its effects on life on Earth vary depending on its location in the atmosphere. While stratospheric ozone is beneficial, tropospheric or ground-level ozone is harmful.
Stratospheric ozone is formed naturally through the interaction of solar ultraviolet (UV) radiation with molecular oxygen (O2). The ozone layer, approximately 6 to 30 miles above the Earth's surface, acts as a protective shield, reducing the amount of harmful UV radiation reaching the Earth's surface. This beneficial ozone layer has been partially destroyed by man-made chemicals, creating a "hole in the ozone." However, it is important to note that this hole is gradually diminishing.
Tropospheric or ground-level ozone is not emitted directly into the air but is formed by chemical reactions between specific pollutants. Ground-level ozone is a by-product of photochemical reactions between volatile organic compounds (VOCs) and nitrogen oxides (NOx). VOCs are emitted by various sources, including chemical plants, gasoline pumps, oil-based paints, auto body shops, and print shops. Nitrogen oxides, on the other hand, result primarily from high-temperature combustion processes in power plants, industrial furnaces and boilers, and motor vehicles.
These precursors, VOCs and NOx, react in the presence of sunlight, leading to the formation of ground-level ozone. Peak concentrations of ozone typically occur during the afternoon when sunlight intensity is at its maximum. However, areas downwind of major sources of VOCs and NOx may experience ozone peaks in the afternoon or even during the early evening or night as wind carries these precursors over long distances. While ground-level ozone was once thought to be limited to large cities, it can occur anywhere, affecting both urban and rural areas.
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Frequently asked questions
Particulate matter comes from wind-blown soil and airborne residue from businesses and industries.
Carbon monoxide is a poisonous gas that forms when carbon in fuels like gasoline, heating oil, natural gas, wood, and charcoal does not burn completely. Motor vehicle exhaust contributes about 60% of carbon monoxide emissions nationwide.
Lead is a soft and highly toxic elemental metal found naturally in the environment. It is usually extracted from ore deposits along with copper, silver, and zinc. Most of the lead in the environment today is due to earlier commercial and industrial uses, such as in cables, pipes, paints, and pesticides.
Nitrogen dioxide is emitted from burning fuel at high temperatures, including industrial, commercial, and residential combustion, motor vehicles, and electric utilities.
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).
