Understanding Criteria Air Pollutants: What, Why, And How?

The Clean Air Act regulates six criteria air pollutants, which are common air pollutants that the Environmental Protection Agency (EPA) controls. These pollutants include sulfur dioxide, nitrogen dioxide, particulate matter, carbon monoxide, ozone, and lead. The EPA sets primary and secondary National Ambient Air Quality Standards (NAAQS) to protect public health and the environment. The primary standard focuses on safeguarding human health, especially sensitive individuals such as children and the elderly, while the secondary standard aims to protect public welfare, including visibility, crops, and wildlife. The EPA also imposes penalties on entities that violate the criteria pollutant standards, working with state and local agencies to improve air quality.

The Clean Air Act

One of the key goals of the Clean Air Act is to establish National Ambient Air Quality Standards (NAAQS) to protect public health and welfare by regulating emissions of hazardous air pollutants. These standards set limits for concentrations of specific pollutants in outdoor air, including ground-level ozone (O3), carbon monoxide (CO), particulate matter (PM10, PM2.5), lead (Pb), sulfur dioxide (SO2), and nitrogen dioxide (NO2).

The 1990 amendments to the Clean Air Act were particularly significant, addressing four major environmental and health threats: acid rain, urban air pollution, toxic air emissions, and stratospheric ozone depletion. These amendments also established a national operating permits program and strengthened enforcement mechanisms to ensure better compliance with the Act.

Under the Clean Air Act, there are two types of standards for criteria pollutants. The EPA sets "primary standards" based on studies that determine when a pollutant becomes harmful to human life, especially vulnerable individuals such as children, the elderly, or those with respiratory diseases. "Secondary standards" are set at levels where pollutants affect visibility or become harmful to the environment, also based on controlled studies. The EPA specifies the permissible frequency for exceeding these limits, such as annual or daily limits, and works with state and local agencies to develop plans to improve air quality by reducing criteria pollutant levels.

Primary and secondary standards

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:

• Particulate matter
• Ground-level ozone
• Carbon monoxide
• Sulfur dioxide
• Nitrogen dioxide
• Lead

The EPA sets NAAQS for these pollutants based on the latest scientific information regarding their effects on health and welfare. The Clean Air Act identifies two types of NAAQS: primary standards and secondary standards.

Primary standards aim to provide public health protection, including protecting the health of "sensitive" populations such as asthmatics, children, the elderly, and those with cardiovascular disease. For instance, carbon monoxide poses a serious health threat to those with cardiovascular disease, and at higher levels of exposure, it can also affect healthy individuals. Similarly, sulfur dioxide can cause breathing issues and respiratory illness symptoms, particularly in asthmatics and those with chronic lung disease or cardiovascular disease.

Secondary standards, on the other hand, focus on public welfare protection, including protection against decreased visibility and damage to animals, crops, vegetation, and buildings. For example, elevated concentrations of sulfur dioxide in the atmosphere can react with other compounds to form small particles that contribute to particulate matter pollution, reduce visibility, and cause regional haze. Sulfur dioxide can also harm trees and plants, and contribute to acid rain. Similarly, nitrogen dioxide is one of a group of highly reactive gases known as nitrogen oxides (NOx) that form ozone or "smog" when reacting with certain volatile organic compounds in the presence of sunlight. Nitrogen dioxide can irritate airways, aggravate asthma, and increase susceptibility to respiratory infections.

To achieve and maintain NAAQS, state, local, and tribal agencies develop emission reduction strategies, plans, and programs. The Menu of Control Measures (MCM) provides these agencies with existing emission reduction measures and relevant information on their efficiency and cost-effectiveness. When necessary, the EPA designates areas as meeting (attainment) or not meeting (nonattainment) the standards, which triggers a series of actions to ensure that air quality meets the set standards.

Sources of pollution

Criteria pollutants are six common air pollutants that are regulated by the US Environmental Protection Agency (EPA) under the Clean Air Act. These pollutants are harmful to human health, the environment, and can cause property damage. The EPA sets National Ambient Air Quality Standards (NAAQS) for these pollutants, which include particle pollution, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead.

The sources of criteria pollutants vary depending on the specific pollutant. Here are the common sources of each type of pollution:

Particle Pollution (Particulate Matter, PM)

• Particulate matter (PM) refers to tiny particles suspended in the air that vary in size, composition, and origin.
• Major sources of PM include construction sites, unpaved roads, fires, and vehicle exhaust.
• PM10 includes particles with a diameter of up to 10 micrometers, while PM2.5 includes particles with a diameter of up to 2.5 micrometers.

Ground-Level Ozone

• Ground-level ozone is not directly emitted into the air but is formed by the reaction of other pollutants, such as nitrogen oxides and reactive organic gases.
• Nitrogen oxides are emitted from motor vehicles, power plants, and industrial boilers.
• Reactive organic gases evaporate into the air and react with other air pollutants, contributing to the formation of ground-level ozone.

Carbon Monoxide

• Carbon monoxide is a colorless and odorless gas produced by the incomplete combustion of carbon-containing fuels.
• Sources of carbon monoxide include vehicle emissions, industrial processes involving combustion, and the burning of fossil fuels for energy generation.

Sulfur Dioxide and Nitrogen Dioxide

• Sulfur dioxide and nitrogen dioxide are primarily emitted from fossil fuel-fired power plants, industrial facilities, and motor vehicles.
• The burning of coal, oil, and gas, which are rich in sulfur and nitrogen content, contributes significantly to the emission of these pollutants.

Lead

• Lead is released into the air through industrial processes, such as metal smelting and manufacturing facilities that use lead or lead-based products.
• Historical use of leaded gasoline in vehicles also contributed significantly to lead pollution, although this has been largely phased out.

It is important to note that these sources may vary based on geographical location and specific industrial activities within a region. Additionally, natural sources, such as volcanic eruptions and wildfires, can also contribute to criteria pollutants, especially particulate matter and carbon monoxide.

Health and environmental effects

Criteria air pollutants are common in the United States and much of the world. They are generally combustion products released by the burning of fossil fuels. These pollutants can harm human health, the environment, and cause property damage. Here are some of the health and environmental effects of the six criteria air pollutants:

Particulate Matter

Particulate matter pollution is caused by small particles suspended in the air. These particles can be made up of various substances, including dust, dirt, soot, and liquid droplets. Inhaling these particles can have adverse effects on human health, as they can enter the lungs and bloodstream, leading to respiratory and cardiovascular problems. Fine particulate matter (PM2.5) is of particular concern as these particles are small enough to penetrate deep into the lungs and even enter the bloodstream.

Ground-Level Ozone

Ozone is a gas composed of three oxygen atoms. While ozone in the upper atmosphere protects us from the sun's ultraviolet rays, ground-level ozone is a harmful air pollutant and a major component of smog. 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 irritate the lungs, causing coughing, throat irritation, and congestion. It can also worsen existing lung conditions such as asthma, bronchitis, and emphysema.

Carbon Monoxide

Carbon monoxide (CO) is a colorless, odorless, and toxic gas produced by the incomplete combustion of fossil fuels. It is harmful to human health as it reduces the blood's ability to carry oxygen, leading to symptoms such as headaches and fatigue in healthy individuals and potentially more severe consequences in those with heart disease. Carbon monoxide is found in higher concentrations in areas with poor ventilation, such as parking garages and areas with heavy traffic congestion.

Sulfur Dioxide

Sulfur dioxide (SO2) is a gaseous air pollutant released primarily during the combustion of fossil fuels containing sulfur, such as coal and oil. Elevated concentrations of SO2 can contribute to particulate matter pollution, reduce visibility, and cause regional haze. It can also react with other compounds to form small particles that can harm trees and plants, damaging foliage and decreasing growth rates.

Nitrogen Dioxide

Nitrogen dioxide (NO2) is a harmful gas emitted primarily by vehicles and power plants. Breathing in high levels of nitrogen dioxide can irritate the lungs and lead to respiratory problems. It can also impair the body's ability to fight off pulmonary infections, making individuals more susceptible to developing respiratory illnesses.

Lead

Lead is a heavy metal that can cause adverse health effects when inhaled or ingested. Sources of lead emissions include contaminated soil, dust, paint, fuels, coal combustion, and manufacturing processes. Lead exposure can affect multiple systems in the body, including the nervous system, kidney function, immune system, reproductive and developmental systems, and the cardiovascular system.

Enforcement and penalties

The Environmental Protection Agency (EPA) has identified six common air contaminants as "criteria pollutants". These pollutants are particulate matter, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. They are given this name because the EPA sets National Ambient Air Quality Standards (NAAQS) for them based on specific criteria. The Clean Air Act requires the EPA to establish these standards to protect public health and welfare.

The EPA has the authority to enforce the Clean Air Act and can monitor compliance with the law's requirements, seek penalties for violations, and compel regulated entities to comply. The EPA's enforcement efforts include tracking two kinds of air pollution trends: air concentrations based on actual measurements of pollutant concentrations in the ambient air, and emissions based on engineering estimates of the total tons of pollutants released into the air each year. The EPA also develops extensive administrative regulations and associated regulatory programs to implement the Clean Air Act's mandates.

The Clean Air Act also requires states to develop State Implementation Plans (SIPs) for how they will meet the NAAQS. These plans must be approved by the EPA, and if they are not acceptable, the EPA can retain CAA enforcement in that state. For example, California was unable to meet the new standards set by the 1970 amendments, which led to a federal state implementation plan for the state. The law recognizes that states should lead in carrying out the Clean Air Act because pollution control problems often require a special understanding of local conditions such as geography, industrial activity, transportation, and housing patterns. However, states are not allowed to have weaker controls than the national minimum criteria set by the EPA.

In addition to the EPA's enforcement efforts, the California Air Resources Board (CARB) also has regulatory and enforcement authority for airborne toxic control measures, stationary and area-wide sources of pollution, mobile sources (including motor vehicles) and fuels, and sources of greenhouse gas emissions. CARB's enforcement efforts include responding to concerns from the public, providing compliance assistance to regulated entities, conducting inspections and investigations, and taking enforcement action when necessary to resolve noncompliance. CARB's enforcement policy allows for the violator to achieve and maintain compliance with air quality laws and regulations and to pay a monetary civil penalty. In some cases, CARB may allow the responsible party to offset a portion of their civil penalty by performing or funding a Supplemental Environmental Project (SEP) that benefits air quality.

Frequently asked questions