Air Quality Alert: Criteria Pollutants Explained

The Clean Air Act requires the US Environmental Protection Agency (EPA) to set National Ambient Air Quality Standards (NAAQS) for six common air pollutants that are harmful to human health and the environment. These pollutants are commonly found in the United States and are known as criteria air pollutants. They include particle pollution, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. Despite improvements in outdoor air quality since the 1990s, millions of people continue to live in areas with unhealthy levels of these pollutants.

Particle pollution

To address particle pollution, the Environmental Protection Agency (EPA) in the United States has designated it as one of six "criteria air pollutants" for which they set National Ambient Air Quality Standards (NAAQS). These standards aim to protect public health and welfare, including sensitive populations such as asthmatics, children, and the elderly. Despite efforts to improve air quality, millions of people still live in areas with unhealthy levels of particle pollution.

Ground-level ozone

Ozone is a gas composed of three atoms of oxygen. It can be “good” or “bad” depending on where it's found in the atmosphere. Stratospheric ozone is “good” because it protects living things from ultraviolet radiation from the sun. Ground-level ozone, on the other hand, is “bad” because it can trigger a variety of health problems, particularly for children, the elderly, and people of all ages who have lung diseases such as asthma.

Ground-level or tropospheric ozone is created by chemical reactions between NOx gases (oxides of nitrogen produced by combustion) and volatile organic compounds (VOCs). The combination of these chemicals in the presence of sunlight forms ozone. The major anthropogenic sources of these ozone precursors are motor vehicle exhaust, industrial emissions, and chemical solvents. Although the ozone precursors often originate in urban areas, winds can carry NOx hundreds of kilometres, causing ozone formation to occur in less populated regions as well.

Carbon monoxide

The presence of carbon monoxide in the air can have significant health impacts. When inhaled, it can limit the amount of oxygen that blood can carry to vital organs, such as the heart and brain. This can be particularly dangerous for individuals with heart disease, who may experience chest pains during exercise or periods of higher stress. Carbon monoxide exposure has also been linked to a range of symptoms, including fatigue, impaired vision, and nausea, depending on the concentration.

Sources of carbon monoxide emissions are varied. Vehicle exhaust, gas-powered furnaces, and portable generators are among the main contributors to outdoor carbon monoxide pollution. Indoors, gas stoves, leaking furnaces, car exhaust from attached garages, and space heaters can be significant sources of carbon monoxide. Incomplete combustion, such as that which occurs in small stoves or fireplaces burning low-grade solid fuel or biofuels, can generate high levels of carbon monoxide. Lethal levels of carbon monoxide may be produced unless the combustion gases are vented outdoors through a chimney.

To ensure the safety of the public, the EPA has established standards and guidelines to control and limit carbon monoxide pollution. These standards are based on the latest scientific information and are designed to protect public health and welfare. State and local agencies are responsible for implementing these standards and monitoring air quality to keep carbon monoxide levels within safe ranges.

Nitrogen oxides

There are seven oxides of nitrogen that may be found in the ambient air, including nitrous oxide (N2O), nitric oxide (NO), and nitrogen dioxide (NO2). Nitrous oxide is a greenhouse gas with significant anthropogenic sources contributing to its worldwide abundance (around 0.3 ppm). However, it is nitric oxide and nitrogen dioxide that are the principal nitrogen oxides associated with combustion sources. Up to 95% of nitrogen oxides are emitted as nitric oxide, while 5-10% are emitted as nitrogen dioxide.

Nitrogen dioxide is a highly reactive gas and a primary pollutant. It is emitted from cars, trucks, and other vehicles or machinery that burn fuel, as well as from power plants and other fuel combustion equipment. In ambient conditions, nitric oxide is rapidly oxidized to form nitrogen dioxide by available oxidants such as oxygen, ozone, and VOCs. Road traffic is the primary outdoor source of nitrogen dioxide, while indoor sources include tobacco smoke and gas-, wood-, oil-, kerosene- and coal-burning appliances.

Nitrogen dioxide, when prevalent in the air, appears as a reddish-brown haze. It has a foul smell and can irritate the airways in the human respiratory system. Short-term exposure to high concentrations of nitrogen dioxide can aggravate respiratory diseases, especially asthma, leading to symptoms such as coughing, wheezing, or difficulty breathing. Prolonged exposure may contribute to the development of asthma and increase susceptibility to respiratory infections, with potentially irreversible damage to the respiratory system.

Sulfur oxides

Sulfur dioxide (SO2) is a common air pollutant and is part of a larger group of compounds found in the atmosphere called "sulfur oxides" (SOx). SO2 is the predominant sulfur oxide in the atmosphere and is, therefore, the component of greatest concern. The Environmental Protection Agency (EPA) has established a National Ambient Air Quality Standard (NAAQS) for SO2. The regulation of SO2 is generally assumed to cover all other sulfur oxides as well.

The largest source of SO2 in the atmosphere is the burning of fossil fuels, such as coal and oil, by power plants and other industrial facilities. Other sources include industrial processes such as metal extraction (smelting), non-road vehicles like large ships and locomotives, and natural sources such as volcanic activity.

SO2 emissions that lead to high concentrations of SO2 in the air can also lead to the formation of other sulfur oxides (SOx). These sulfur oxides can react with other compounds in the atmosphere to form small particles, contributing to particulate matter (PM) pollution. These particles may penetrate deeply into the lungs and, in sufficient quantities, can cause health problems. Additionally, at high concentrations, gaseous SOx can harm trees and plants by damaging foliage and decreasing growth, potentially leading to ecological issues.

Short-term exposure to SO2 can directly harm the human respiratory system and contribute to the formation of small particulate matter (PM2.5). Elevated concentrations of SO2 in the atmosphere can react with other compounds to form particles that contribute to particulate matter pollution, reduce visibility, and cause regional haze.

The EPA's national and regional rules to reduce SO2 emissions and pollutants that form sulfur oxides (SOx) help state and local governments meet the Agency's national air quality standards. These regulations are designed to protect public health and welfare and are periodically reviewed and updated as necessary to ensure their effectiveness.

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