Meghan Hodges
The Clean Air Act requires the US Environmental Protection Agency (EPA) to regulate six criteria air pollutants based on human health and environmental criteria. These criteria air pollutants are found all over the US and can harm human health, the environment, and cause property damage. The six criteria air pollutants are particle pollution, ground-level ozone, carbon monoxide, lead, nitrogen oxides, and sulfur oxides. These pollutants can be further classified into primary and secondary pollutants, with primary pollutants being directly emitted from a source and secondary pollutants formed in the atmosphere from precursor compounds.
| Characteristics | Values |
|---|---|
| Number of criteria air pollutants | 6 |
| Criteria pollutants | Particle pollution, ground-level ozone, carbon monoxide, lead, nitrogen oxides, sulfur oxides |
| Other names for criteria air pollutants | "Hazardous air pollutants" (HAPs) or toxic air pollutants |
| Examples of HAPs | Benzene, perchloroethylene, methylene chloride |
| Particulate matter (PM) composition | Sulphate, nitrates, ammonia, sodium chloride, black carbon, mineral dust, water |
| PM size | PM2.5, PM10, PM10-2.5 |
| PM sources | Combustion of fuels, industrial processes, combustion activities, power plants, wood burning, motor vehicles, certain chemical reactions between gases |
| Sources of coarse particles (PM10) | Crushing or grinding operations, dust from paved or unpaved roads |
| Sources of fine particles (PM2.5) | Combustion of fuels in power generation facilities, industries or vehicles, chemical reactions between gases |
| Health risks associated with PM | Cardiovascular disease, cerebrovascular disease, respiratory issues, adverse perinatal outcomes, lung cancer |
| Nitrogen dioxide (NO2) | Reddish-brown gas, soluble in water, strong oxidant |
| Ambient sources of NO2 | High-temperature combustion of fuels in heating, transportation, industry, power generation |
| Household sources of nitrogen oxides (NOx) | Furnaces, fireplaces, gas stoves, ovens |
| Effects of NO2 | Irritates airways, aggravates asthma, increases susceptibility to respiratory infections |
| Effects of NOx | Contributes to nutrient pollution in coastal waters, reduces visibility, causes regional haze |
| Ozone | Composed of three atoms of oxygen, occurs in the upper atmosphere and at ground level |
| Effects of ground-level ozone | Harmful to health, especially for people with asthma, children, older adults, and people who are active outdoors |
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Particle pollution
There are two types of particle pollution: primary and secondary. Primary particles are emitted directly from a source, such as construction sites, unpaved roads, smokestacks, or fires. Secondary particles form in complicated atmospheric reactions involving chemicals such as sulfur dioxides and nitrogen oxides, emitted from power plants, industries, and automobiles. Cooking, smoking, dusting, and vacuuming can also produce particle pollution, particularly indoors.
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Ground-level ozone
Tropospheric ozone is not emitted directly into the air but is created by chemical reactions between pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources. These pollutants react in the presence of sunlight to form ground-level ozone. The concentration of ground-level ozone is typically 20-30 parts per billion by volume (ppbv), but it can reach close to 100 ppbv in polluted areas. It is least concentrated in the ground layer (planetary boundary layer) of the troposphere.
Ozone levels are typically higher during the summer months due to increased heat and sunlight, which promote ozone formation. Ozone can also be transported long distances by wind, affecting rural areas as well as urban centres. To monitor ozone levels, various techniques are employed, such as LIDAR (a ground-based remote sensing technique using lasers) and ozonesondes (local ozone-measuring instruments attached to meteorological balloons). These measurements help governments and agencies implement regulations and plans to reduce ground-level ozone and improve air quality.
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Carbon monoxide
The health risks associated with carbon monoxide exposure have led to the implementation of various measures to control and monitor CO pollution. In the United States, the Environmental Protection Agency (EPA) plays a crucial role in setting standards and providing data to help state, tribal, and local agencies maintain safe levels of carbon monoxide. The Clean Air Act, which addresses National Ambient Air Quality Standards (NAAQS), also includes carbon monoxide as one of the six criteria air pollutants that are commonly found and can harm human health, the environment, and cause property damage.
Additionally, carbon monoxide is a significant contributor to outdoor air pollution, particularly in urban areas with high traffic congestion. Outdoor sources of carbon monoxide include motor vehicles, power plants, industrial activities, and waste burning. Indoor sources of carbon monoxide pollution can include poorly vented stoves or space heaters, fireplaces, and gas stoves. Incomplete combustion of fuels during household activities such as cooking, space heating, and lighting can also produce carbon monoxide.
To mitigate the risks associated with carbon monoxide, it is essential to have proper ventilation and regular maintenance of fuel-burning appliances. Carbon monoxide detectors are also recommended to monitor levels and alert individuals to potential dangers. By taking these precautions and staying informed about the sources and health effects of carbon monoxide, we can help reduce the impact of this harmful pollutant on our health and the environment.
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Nitrogen oxides
NOx is a significant component of photochemical smog, which is characterised by its yellowish-brown colour. Elevated levels of nitrogen dioxide, especially in poorly ventilated areas, can irritate and damage the human respiratory tract. Short-term exposures can aggravate respiratory conditions, particularly asthma, leading to coughing, wheezing, and difficulty breathing. Prolonged exposure to high concentrations of NO2 may contribute to the development of asthma and increase susceptibility to respiratory infections.
NOx also has adverse environmental impacts. When nitrogen oxides react with volatile organic compounds (VOCs) in the presence of sunlight, they form ground-level ozone, which acts as a pollutant that traps heat, decreases visibility, and affects breathing abilities. Additionally, nitrogen oxides contribute to the formation of acid rain and influence the Earth's temperature.
The measurement of nitrogen oxides is performed using techniques such as chemiluminescence and differential optical absorption spectroscopy (DOAS). Chemiluminescence involves the oxidation of nitric oxide (NO) to nitrogen dioxide (NO2) by ozone (O3), resulting in the emission of light that can be measured to determine NO concentration. DOAS instruments are also employed to measure NOx levels.
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Sulfur oxides
SO2 is a significant component of air pollution and is monitored by organizations like the US Environmental Protection Agency (EPA) and the World Health Organization (WHO). The EPA has established national ambient air quality standards for SO2 to protect human health and the environment from the harmful effects of SO2 and other sulfur oxides. These standards are designed to reduce people's exposure to SOx and mitigate the formation of particulate sulfur pollutants.
SO2 and other sulfur oxides can contribute to the formation of fine particles, reducing visibility and causing haze. They can also react with other compounds in the atmosphere to form small particles that can penetrate deeply into the lungs, potentially leading to health problems. Additionally, high concentrations of SOx can harm trees and plants, inhibit their growth, and damage sensitive ecosystems and waterways.
Diesel vehicles and equipment were once a major source of SO2 emissions, but federal regulations to reduce sulfur in diesel fuels have significantly lowered emissions from this source. However, power plants and other industrial facilities remain the largest sources of SO2 emissions.
SO2 is one of the six common air pollutants, known as criteria pollutants, which are subject to primary and secondary National Ambient Air Quality Standards under the federal Clean Air Act. These standards are designed to protect human health and the environment from the adverse effects of air pollution.
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