Air Quality Criteria: Pollutants And Their Selection Process

The Clean Air Act of 1970 required the US 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 particle pollution, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. The EPA chose these pollutants because they are common across the United States and harmful to human health and the environment. The EPA set standards for these pollutants to protect public health and welfare, and states are required to develop plans to attain and maintain these standards.

The Clean Air Act

When the EPA establishes or revises a NAAQS, it sets in motion a series of actions to ensure that air quality throughout the country meets those standards. The Clean Air Act requires states to develop a general plan to attain and maintain the standards and a specific plan to attain the standards for each area designated as nonattainment. These plans are known as State Implementation Plans or SIPs.

National Ambient Air Quality Standards (NAAQS)

The Clean Air Act requires the Environmental Protection Agency (EPA) to set National Ambient Air Quality Standards (NAAQS) for six principal pollutants, known as "criteria pollutants", that are common in outdoor air, harmful to public health and the environment, and come from diverse sources. These criteria air pollutants are found all over the United States and can cause property damage, environmental damage, and harm to human health.

The Clean Air Act identifies two types of NAAQS: primary standards and secondary standards. Primary standards are designed to protect public health with an adequate margin of safety, including the health of sensitive populations such as asthmatics, children, and the elderly. Secondary standards are designed to protect the public welfare from adverse effects, including damage to soils, water, crops, vegetation, man-made materials, animals, wildlife, weather, visibility, and climate, as well as transportation hazards, economic values, and personal comfort and well-being.

The six criteria air pollutants that NAAQS are set for are particulate matter, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. These pollutants are generally combustion products released by the burning of fossil fuels. Of these six pollutants, particle pollution and ground-level ozone are the most widespread health threats. The EPA sets permissible levels for these pollutants based on human health and environmental criteria. The limits based on human health are called primary standards, while the limits intended to prevent environmental and property damage are called secondary standards.

The EPA tracks two types of air pollution trends for each of the six criteria air pollutants: air concentrations based on actual measurements of pollutant concentrations in the ambient (outside) air at selected monitoring sites, and emissions based on engineering estimates of the total tons of pollutants released into the air each year. The Clean Air Act requires the EPA to review and revise each of the NAAQS at five-year intervals to ensure that they are based on the most recent scientific findings. The process of setting NAAQS involves a comprehensive review of the relevant scientific literature, risk and exposure assessments, and public comment periods.

Health and environmental damage

The Clean Air Act requires the Environmental Protection Agency (EPA) to set National Ambient Air Quality Standards (NAAQS) for six common air pollutants, also known as "criteria air 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 particle pollution, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead.

Particle pollution, or particulate matter, refers to small particles that are suspended in the air. These particles can be solid or liquid and are often produced by the burning of fossil fuels, such as coal, oil, and diesel fuel. Particle pollution can be inhaled, leading to a range of health issues. Larger particles, known as PM10, irritate the eyes, nose, and throat, while smaller particles, PM2.5, can penetrate deep into the lungs and even enter the bloodstream. People with respiratory conditions such as asthma are particularly vulnerable to the effects of particle pollution, which can worsen their symptoms.

Ground-level ozone is another major criteria air pollutant. Ozone occurs naturally in the upper atmosphere, where it shields us from the sun's harmful ultraviolet rays. However, at ground level, it becomes a harmful pollutant. Ground-level ozone is formed when sunlight reacts with nitrogen oxides, which are emitted by vehicles and power plants. Exposure to ground-level ozone can cause a range of health problems, including chest pain, coughing, throat irritation, and inflammation of the airways. It can also worsen existing respiratory conditions such as asthma, bronchitis, and emphysema. Prolonged exposure may lead to permanent lung damage.

Carbon monoxide (CO) is a toxic gas that is colorless and odorless. It is produced by the incomplete burning of various fuels, including coal, wood, charcoal, oil, and natural gas. Carbon monoxide is harmful to human health because it reduces the oxygen supply to the body's organs and tissues. Exposure to carbon monoxide can cause headaches, dizziness, weakness, nausea, and confusion. In high concentrations, it can lead to loss of consciousness and even death.

Sulfur dioxide (SO2) is a colorless gas with a pungent odor. It is primarily produced by power plants and industrial facilities that burn fossil fuels containing sulfur, such as coal and oil. Short-term exposure to sulfur dioxide can irritate the respiratory system, causing symptoms such as nose, throat, and airway irritation, coughing, and chest tightness. It poses a greater risk to individuals with asthma or other chronic respiratory diseases. Additionally, elevated concentrations of SO2 in the atmosphere can contribute to regional haze and reduce visibility.

Nitrogen dioxide (NO2) is a highly reactive gas that is reddish-brown in color and has a pungent, irritating odor. It is produced by vehicles, power plants, and other industrial sources. Nitrogen dioxide irritates the lining of the lungs and can cause coughing, wheezing, and impaired pulmonary function. Similar to sulfur dioxide, NO2 poses a greater risk to individuals with pre-existing respiratory conditions.

Lead is a toxic metal that can be found naturally in the environment, but human activities, such as burning fossil fuels and industrial processes, have increased its presence in the air. Inhalation of lead particles can lead to a range of health issues, including damage to the brain and nervous system, kidney problems, and increased blood pressure. Children are particularly vulnerable to the harmful effects of lead exposure, which can impact their cognitive development.

Primary and secondary standards

The Clean Air Act requires the US Environmental Protection Agency (EPA) to set National Ambient Air Quality Standards (NAAQS) for six criteria air pollutants: particle pollution, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. These pollutants are chosen because they are common in outdoor air and are harmful to public health and the environment.

The EPA tracks two types of air pollution trends for each of the six criteria air pollutants: air concentrations based on measurements of pollutant concentrations in the ambient (outside) air, and emissions based on engineering estimates of the total tons of pollutants released into the atmosphere each year. The EPA also works with state, local, and tribal agencies to develop emission reduction strategies to attain and maintain the NAAQS.

The NAAQS are periodically reviewed and revised. For example, in 2015, the ozone standard was lowered from 75 ppb to 70 ppb, and the annual PM2.5 standard was lowered from 15 µg/m3 to 12 µg/m3 in 2012. These revisions are important to ensure that the standards remain up-to-date and effective in protecting public health and the environment.

Air pollution trends

The selection of criteria air pollutants is a careful process, and these pollutants are considered the most common and harmful to human health and the environment. The Clean Air Act of 1970 in the United States first identified six criteria air pollutants, and these were chosen due to their prevalence and the understanding of their adverse effects at the time. The six criteria pollutants are carbon monoxide, lead, ground-level ozone, nitrogen dioxide, particle pollution, and sulfur dioxide.

Historical Context

The identification and regulation of air pollutants have evolved over time, driven by scientific advancements and a growing awareness of the impacts of air pollution on human health and the environment. The Clean Air Act of 1970 was a pivotal moment in the United States, as it provided a comprehensive framework for addressing air quality issues. The Act established National Ambient Air Quality Standards (NAAQS) for the six criteria air pollutants, setting allowable limits in the atmosphere to protect public health and welfare. This marked a shift towards a more systematic approach to air pollution control and laid the foundation for ongoing efforts to improve air quality.

Carbon Monoxide (CO)

Carbon monoxide is a colorless, odorless, and toxic gas produced by the incomplete combustion of fossil fuels. Its presence in the air can be attributed to vehicle emissions, industrial processes, and the use of gasoline-powered engines. High levels of carbon monoxide in the atmosphere can be detrimental to human health, as it interferes with the blood's ability to carry oxygen, leading to headaches, dizziness, and even death in extreme cases. Monitoring and controlling carbon monoxide emissions have been a key focus of air pollution regulations, especially in urban areas with high traffic volumes.

Lead (Pb)

Lead is a heavy metal that has historically been a significant concern due to its widespread use in industries such as paint, plumbing, and gasoline. Lead emissions can persist in the environment for extended periods and have severe health impacts, particularly on children, including neurological damage and impaired cognitive development. The phase-out of leaded gasoline and the implementation of regulations to reduce lead emissions from industrial sources have led to significant improvements in air quality regarding lead pollution.

Ground-level Ozone (O3)

Ground-level ozone, or tropospheric ozone, is not directly emitted but formed through complex chemical reactions involving volatile organic compounds (VOCs) and nitrogen oxides (NOx) in the presence of sunlight. Ozone is a primary component of smog, and elevated levels are often observed in urban areas with high traffic and industrial activity. Ozone can irritate the respiratory system, aggravate asthma, and reduce lung function. Efforts to reduce ozone pollution have focused on controlling emissions of its precursor pollutants, such as implementing stricter vehicle emission standards and regulating industrial processes that release large amounts of VOCs and NOx.

Nitrogen Dioxide (NO2) and Particle Pollution

Nitrogen dioxide is a highly reactive gas formed during the combustion of fossil fuels, and it contributes to the formation of ground-level ozone and fine particulate matter. Exposure to nitrogen dioxide can irritate the respiratory system and exacerbate respiratory conditions. Particle pollution, or particulate matter (PM), includes a mixture of solid particles and liquid droplets suspended in the air. These particles can be directly emitted or formed through chemical reactions in the atmosphere. Fine particulate matter, with diameters of 2.5 micrometers or smaller (PM2.5), is of particular concern due to its ability to penetrate deep into the respiratory system, causing cardiovascular and respiratory issues. Regulations have targeted the reduction of particle pollution from sources such as power plants, industrial facilities, and vehicle emissions.

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