The U.S. Environmental Protection Agency (U.S. EPA) developed the Air Quality Index (AQI) to provide accurate, timely and easily understandable information about daily levels of air pollution. The index provides a uniform system for measuring pollution levels of the major air pollutants regulated under the Clean Air Act.
Once these levels are measured, the AQI figures are reported in all metropolitan areas of the United States with populations exceeding 200,000. Index figures enable the public to determine whether air pollution levels in a particular location are good, moderate, unhealthy or very unhealthy.
U.S. EPA and local officials use the AQI as a public information tool to advise the public about the general health effects associated with different pollution levels, and to describe precautionary steps that may need to be taken if air pollution levels rise into the unhealthy range.
U.S. EPA has established air quality standards protecting against health effects that can occur within short periods of time (a few hours or a day) for each of the major air pollutants regulated under the Clean Air Act.
The AQI converts the measured pollutant concentration in a community's air to a number on a scale of 0 to 500. The most important number on this scale is 100, since this number corresponds to the standard established under the Clean Air Act.
For example, the eight-hour standard for ground-level ozone — that is,
the allowable concentration of this pollutant in a community's air — is
75 parts per billion (ppb). A 75 ppb eight-hour average for ozone would translate to an AQI level of 100. An AQI level higher than 100 means that a pollutant is in the unhealthy range; an AQI level at or below 100 means that a pollutant reading is in the satisfactory range. The intervals and the terms describing the AQI air quality levels are as follows:
From 0 to 50 .......................... Good
From 51 to 100 ....................... Moderate
From 101 to 200 ..................... Unhealthy for Sensitive Groups
From 151 to 200 ..................... Unhealthy
From 201 to 300 ..................... Very Unhealthy (Alert)
The intervals on the AQI scale relate to the potential health effects of the daily concentrations of each pollutant. Each value has built into it a margin of safety that, based on current knowledge, protects highly susceptible members of the public. For example, if U.S. EPA reports an AQI level of 90 for ozone for a given metropolitan area, residents of that area would know that the ozone level for the region is at the high end of the moderate range.
Before determining which stage is to be called, officials examine both current pollutant concentrations and prevailing and predicted meteorological conditions.
Levels higher than 100 may trigger preventive action by State or local officials, depending on the level of the pollution concentration. This could include health advisories for citizens or susceptible individuals to limit certain activities and potential restrictions on industrial activities.
The 200-level is likely to trigger an "Alert" stage. Activities that might be restricted by local governments, depending on the nature of the problem, include incinerator use and open burning of leaves or refuse.
A level of 300 on the AQI will probably trigger "Warning," which is likely to prohibit the use of incinerators, severely curtail power plant operations, cut back operations at specified manufacturing facilities, and require the public to limit driving by using car pools and public transportation.
An AQI level of 400 or above would constitute an "Emergency," and would require a cessation of most industrial and commercial activity, plus a prohibition of almost all private use of motor vehicles. If air pollution were to reach such extremely high levels, death could occur in some sick and elderly people, and even healthy people would likely necessitate restrictions on normal activity.