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Air Pollutants and Standards

The Clean Air Act and its amendments signify concern with two broad categories of air pollutants:

  • Criteria air pollutants (CAPs) are prevalent throughout the country and cause substantial harm to human health and welfare and the environment;
  • Hazardous air pollutants (HAPs, or "toxics") are known or expected to cause serious harm to humans even in very small doses.

The District is taking steps to address additional air quality problems:

  • Regional haze impacts visibility at 156 national parks and wilderness areas (called Class I areas) throughout the country, which is protected under the Clean Air Act.
  • Global climate change is impacting the District’s resources, and so is being addressed at the local level.

The following provides an overview of the types of pollutants and existing standards and efforts to control them.

Criteria Air Pollutants (CAPs)

The Clean Air Act and its amendments authorize the EPA to establish National Ambient Air Quality Standards (NAAQS) for common “criteria” air pollutants (CAPs). There are two types of NAAQS for CAPs:

  • Primary standards are designed to protect public health, including sensitive populations such as asthmatics, children and the elderly.
  • Secondary standards are designed to protect public welfare and the environment by preventing visibility impairment or damage to crops, vegetation and buildings.

NAAQS are currently in effect for six air pollutants: ozone (O3), carbon monoxide (CO), sulfur dioxide (SO2), nitrogen dioxide (NO2), particulate matter less than 10 microns (PM10) and less than 2.5 microns (PM2.5), and lead (Pb).

  • Current NAAQS Standards – The NAAQS indicate the maximum amount of pollution of certain pollutants allowable by law. Levels change over time and are set based on the most up-to-date scientific research. They include an adequate margin of safety to protect public health and the environment.
  • Sources and Impacts of each CAP – Air pollutants are a concern because they cause a range of health and environmental impacts. It is important to understand where emissions of certain pollutants come from in order to control and minimize them.

When air quality does not meet the NAAQS, the area is said to be in “nonattainment” of the NAAQS and is required to develop plans for meeting the standards. Nonattainment areas are classified based on the severity of the pollution problem (marginal, moderate, serious, severe, and extreme).

The entire Washington D.C. metropolitan area, which includes the District and parts of Maryland and Virginia, is currently in moderate nonattainment of the federal NAAQS for ozone and PM2.5.

Hazardous Air Pollutants (HAPS)

Hazardous air pollutants (HAPs), also called toxic air pollutants or “toxics,” can be detrimental to human health even in very small amounts. Scientific evidence shows that HAPs either do or may cause cancer, reproduction and developmental problems, or even death. Effects can be either acute or persistent. EPA has identified 188 HAPs. HAPs are controlled using National Emission Standards for Hazardous Air Pollutants (NESHAPs).

The District operates the only HAPs monitoring station in EPA’s Region 3 as part of the National Air Toxics Assessment. Data is collected on compounds such as black carbon, heavy metals, and hexavalent chromium-VI.

Regional Haze

Visibility, or the ability to see views clearly without the presence of haze, at 156 national parks and wilderness areas (called Class I areas) throughout the country is protected under the Clean Air Act. The main pollutant blamed for haze in the eastern United States is SO2, a precursor to PM2.5. States with facilities that emit large amounts of SO2 are considered to be “contributors” to visibility impairment at Class I areas, and are thus required to participate in a SIP process to reduce emissions that contribute to regional haze. The goal is return Class I areas to pristine conditions.

Facilities in the District do not contribute significantly to haze at any Class I area, but the District is participating in the regional haze planning process in partnership with states throughout the Mid-Atlantic region.

Climate Change

The District is supportive of efforts to reduce greenhouse gas emissions and adapt to climate change, and has begun the process of developing a Climate Action Plan.


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