About the AQS Database

Air Quality System (AQS) Database

The Air Quality System (AQS) database contains measurements of air pollutant concentrations in the 50 United States, plus the District of Columbia, Puerto Rico, and the Virgin Islands. The measurements include both criteria air pollutants and hazardous air pollutants.

Criteria Air Pollutants

Under provisions of the Clean Air Act, which is intended to improve the quality of the air we breathe, EPA sets limits on how much of a pollutant can be in the air anywhere in the United States. This ensures that all Americans have the same basic health and environmental protections. The law allows individual states to have stronger pollution controls, but states are not allowed to have weaker pollution controls than those set for the whole country. EPA calls these pollutants "criteria air pollutants" because the agency has regulated them by first developing health-based criteria (science-based guidelines) as the basis for setting permissible levels. One set of limits (primary standard) protects health; another set of limits (secondary standard) is intended to prevent environmental and property damage. A geographic area that meets or does better than the primary standard is called an attainment area; areas that don't meet the primary standard are called nonattainment areas.

States are required to develop state implementation plans (SIPs) that explain how they will clean up polluted areas. EPA must approve each SIP, and if a SIP isn't acceptable, EPA can take over enforcing the Clean Air Act in that state.

The criteria air pollutants are:

• Carbon Monoxide (CO)
• Nitrogen Dioxide (NO2)
• Sulfur Dioxide (SO2)
• Ozone (O3)
• Particulate Matter (PM10 and PM2.5)
• Lead (Pb)

Further information about criteria air pollutants, including descriptions of their health effects and the national air quality standards, is available at http://www.epa.gov/air/urbanair/6poll.html.

Hazardous Air Pollutants

The Clean Air Act designates 188 substances, known to have harmful health effects, as hazardous air pollutants (HAPs). Most of the HAPs are organic compounds, such as benzene and chloroform. Some HAPs are toxic metals and their compounds, such as arsenic and mercury.

The concentration of metals and their compounds are determined from analysis of particulate matter. The AQS database has multiple designations for some of those HAPs, to indicate the type of particulate matter that was collected and analyzed to determine a metal's ambient concentration. For example, AQS defines three HAPs - cadmium (PM2.5), cadmium (PM10), cadmium (TSP) - for cadmium concentration determined from a sample collected as fine particulate matter (PM2.5), coarse particulate matter (PM10), or total suspended particulate matter (TSP). Due to this terminology, AQS has more than 188 hazardous air pollutants. The "AQS HAPs" list gives the name of each HAP and indicates whether air monitoring data are available for it.

AQS Data in AirData

AirData has annual summaries of the air pollution measurements for the current and ten previous years. (The individual hourly and daily measurements reported by air monitoring stations are not available in AirData.) The AQS database is updated nearly every day by states and local environmental agencies that operate the monitoring stations. The states provide this monitoring data as required by the Clean Air Act. AirData has a "snapshot" of the AQS annual summary monitoring data for criteria air pollutants, extracted each month, and for hazardous air pollutants, extracted each calendar quarter.

For more detailed information about the AQS database, see http://www.epa.gov/ttn/airs/airsaqs/index.htm.