POLLUTION LOCATOR|Criteria Air Pollution - A National Overview

Criteria air pollutants include the six most common air pollutants in the U.S.: carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide. Congress has focused regulatory attention on these six pollutants because they endanger public health and the environment, are widespread throughout the U.S., and come from a variety of sources. Criteria air pollutants are responsible for many adverse effects on human health, causing thousands of cases of premature mortality and tens of thousands of emergency room visits annually. They also cause acid rain and can significantly harm ecosystems and the built environment.

Criteria pollutants are the only air pollutants with national air quality standards that define allowable concentrations of these substances in ambient air. In 1997, EPA concluded that several of our current national air quality standards do not provide sufficient public health protection. New, more stringent air quality standards were adopted for ozone and particulate matter. Implementation of these standards has been slowed by legal challenges, but the U.S. Supreme Court upheld most of EPA's air quality rules in February 2001. The Court approved EPA's new standard for particulates, which expanded regulation of fine pollution particles down to 2.5 microns. The Court approved EPA's new ozone standard as well, but ordered EPA to develop a new plan that sets out a schedule for when state and local air districts must be in compliance.

EPA's air quality monitoring network indicates that over 170 million people live in counties with unhealthy air due to one or more criteria air pollutants. In counties where pollutant concentrations are monitored, Scorecard presents a single uniform measure of air quality, called the Air Quality Index. The AQI indicates the percentage of days in a year when an area experiences good, moderate, or unhealthful air quality. The AQI also provides the basis for ranking areas by their air pollution levels.

An area that persistently fails to meet air quality standards can be designated as a nonattainment area for one or more pollutants. Presently, over 100 nonattainment areas have been designated by the U.S. Environmental Protection Agency. Most are located in metropolitan areas, where air quality problems are more severe. The Clean Air Act requires states that have nonattainment areas to submit a plan that will reduce pollutant emissions and concentrations and ensure compliance with national standards.

EPA's criteria air pollutant emissions inventory indicates that releases of all criteria air pollutants except nitrogen oxides have been in decline since the passage of the 1970 Clean Air Act. Overall air quality across the country has improved significantly since the 1980s. These improvements, however, have not eliminated air quality problems, and major efforts to control pollution sources are still required to ensure everyone breathes air that meets Clean Air Act standards.

Air pollution comes from a wide variety of sources that are usually classified into three groups: mobile, area, and point sources. Mobile sources (like cars and trucks) are responsible for about 75% of carbon monoxide pollution, and for more nitrogen oxide and volatile organic compound emissions than area or point sources. Area sources (like small businesses) are responsible for over 50% of particulate matter emissions and for nearly half of all volatile organic compound emissions. Point sources (like power plants and industrial factories) account for nearly 90% of sulfur dioxide emissions. See Scorecard's national summary of criteria air pollution emissions.

Scorecard's emissions and exposure information for criteria air pollutants is derived from two U.S. EPA sources: the
Air Quality System (AQS) database and the National Emissions Trend (NET) database.

Exposure data are available for 2003. Criteria air pollutant concentrations are derived from EPA's Air Quality System database. AQS compiles concentration measurements from over 4,000 monitoring stations located in over 1,000 counties across the U.S. State and local environmental agencies operate air monitoring sites, and submit data to EPA for inclusion in the AIRS database. EPA summarizes the air pollution measurements from each monitoring site, reporting the highest values reported, the average value, and the number of values above National Ambient Air Quality Standards. AQI values are also derived from AQS and are based on 2003 monitoring data.

Emissions data are available for 1999. The NET is an emissions database developed by EPA for its internal use. The NET is based partially on emission data obtained from state and local agencies, but it is not an official compilation of database of state emissions data. There are known inconsistencies between EPA's NET and official state emission inventories. Scorecard's emissions data were derived from a 2001 version of the EPA NET database, which is incrementally modified by EPA to generate the most current version of the NET database. This online database captures corrections to facility or area criteria emissions data made by companies or state agencies.

Regulatory information about nonattainment areas data is derived from the EPA Green Book and is current as of October 2004. To characterize the health risks posed by criteria air pollution, Scorecard calculates the number of person-days an area was in exceedance of air quality standards.

Note: Many U.S. counties do not monitor air pollution, so it is not possible to describe pollutant exposures or characterize potential health risks in these areas.