Information about the Air Quality Index (AQI)

Current conditions

Condensed from Measuring Air Quality: The Pollutant Standards Index; Office of Air Quality Planning and Standards, US EPA; EPA 451/K-94-001; February 1994.

Current EPA Air Quality Index and Clean Air Campaign Health Advisory

AQI Range EPA Color Scale EPA Descriptor Clean Air Campaign Health Advisory
0 to 50 Green Good The air quality is good and you can engage in outdoor physical activity without health concerns.
51 to 100 Yellow Moderate At this level the air is probably safe for most people. However, some people are unusually sensitive and react to ozone in this range, especially at the higher levels (in the 80s and 90s). People with heart and lung diseases such as asthma, and children, are especially susceptible. People in these categories, or people who develop symptoms when they exercise at "yellow" ozone levels, should consider avoiding prolonged outdoor exertion during the late afternoon or early evening when the ozone is at its highest.
101 to 150 Orange Unhealthy for Sensitive Groups In this range the outdoor air is more likely to be unhealthy for more people. Children, people who are sensitive to ozone, and people with heart or lung disease should limit prolonged outdoor exertion during the afternoon or early evening when ozone levels are highest.
151 to 200 Red Unhealthy In this range even more people will be affected by ozone. Most people should restrict their outdoor exertion to morning or late evening hours when the ozone is low, to avoid high ozone exposures.
201 to 300 Purple Very Unhealthy Increasingly more people will be affected by ozone. Most people should restrict their outdoor exertion to morning or late evening hours when the ozone is low, to avoid high ozone exposures.
Over 300 Black

Everyone should avoid all outdoor exertion.

The Air Quality Index (AQI) has been developed by the Environmental Protection Agency (EPA) to provide accurate, timely, and easily understandable information about daily levels of air pollution. The Index provides EPA with a uniform system of measuring pollution levels for 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 for Sensitive Groups or worse. In addition, 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 whatever precautionary steps may need to be taken if air pollution levels rise into the unhealthy range.

The EPA uses the Air Quality Index to measure five major pollutants for which it has established National Ambient Air Quality Standards under the Clean Air Act. The pollutants are sulfur dioxide, carbon monoxide, nitrogen dioxide and ozone. (Note: Ozone at the ground level can be a health and environment problem, but ozone is beneficial in the stratosphere (6-30 miles above the Earth) where it shields the Earth from the sun's harmful ultraviolet radiation. EPA has programs to reduce chlorofluorocarbons and related substances to protect the stratospheric ozone layer. The AQI relates only to ground-level ozone, a major component of smog.)

For each of the five pollutants, 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 example, the standard for sulfur dioxide - that is, the allowable concentration of this pollutant in a community's air - is 0.14 parts per million measured over a 24-hour period. Air concentrations higher than 0.14 parts per million (ppm) exceed the national standard. For ozone, the 8-hour average concentration permitted under the standard is 0.085 parts per million (ppm).

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 that number corresponds to the standard established under the Clean Air Act. A 0.14 ppm reading for sulfur dioxide or a 0.085 ppm reading for ozone would translaate to an AQI level of 100. An AQI level in excess of 100 means that a pollutant is in the Unhealthy for Sensitive Groups range or worse on a given day; 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 150.................unhealthy for sensitive groups
    From 151 to 200.................unhealthy
    From 201 to 300.................very unhealthy
    From 300........................hazardous

The intervals on the AQI scale relate to the potential health effects of the daily concentrations of each of these five pollutants. Each value has built into it a margin of safety that, based on current knowledge, protects highly susceptible members of the public.

EPA determines the index number on a daily basis for each of the five pollutants; it then reports the highest of the five figures for each major metropolitan area, and identifies which pollutant corresponds to the figure that is reported. For example, if EPA reports an AQI level of 90 for ozone for a given metropolitan area, residents of the area would know that the ozone level for the region is at the high end of the moderate range; they would also know that ozone is the pollutant with the highest AQI reading for the day, and that all other pollutants are therefore in the good or moderate range. On days when two or more pollutants exceed the standard (that is, have AQI values greater than 100), the pollutant with the highest index level is reported, but information on any other pollutants above 100 may also be reported.

Levels above 100 may trigger preventive action by State or local officials, depending upon 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 upon 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 a "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 transporation. A 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 experience symptoms that would necessitate restrictions on normal activity. Before determining which stage is to be called, officials examine both current pollutant concentrations and prevailing and predicted meteorological conditions.

The table above identifies health effects associated with different levels of air pollution, along with the cautionary statments that would be appropriate if air pollution in a community were to fall into one of the "unhealthful" categories on the AQI scale.

In most communities in the United States, AQI levels generally fall between zero and 100; readings in excess of 100 are likely to occur only a few times a year, if at all. Only 1.4% of all readings in the U.S. exceeded 100 during calendar years 1990 and 1991. Several metropolitan areas in the U. S. have more severe air pollution problems, and may often experience AQI levels in excess of 100. However, even in these areas, AQI readings in excess of 200 are quite rare. During calendar years 1990 and 1991, for example, just one-tenth of one percent of the AQI readings exceeded 200, and only 0.003 of one percent exceeded 300. (Urban areas outside the U. S. with dense population centers and large numbers of uncontrolled pollution sources frequently report AQI levels in excess of 250.)

Significant seasonal variations can occur in AQI-reported values. In winter, carbon monoxide is likely to be the pollutant with the highest AQI levels, because cold weather makes it much more difficult for automotive emission control systems to operate effectively. In summer, the chief pollutant in many communities is likely to be ozone, since emissions of volatile organic compounds and nitrogen oxides form ozone much more rapidly in the presence of heat and sunlight.

The AQI places maximum emphasis on acute health effects occurring over very short time periods - 24 hours or less - rather than chronic effects occurring over months or years. By notifying the public when an AQI value exceeds 100, citizens are given an adequate opportunity to react and take whatever steps they can to avoid exposure. The approach EPA follows is conservative, because (1) each standard has built into it a margin of safety that is designed to protect highly susceptible people, and (2) the public notice is triggered as soon as a single sampling station in the community records an AQI level that exceeds 100.

Use of the AQI allows for flexible reporting. A typical television or radio announcement might read: "The pollution index reported at noon today is 150, and the air is considered unhealthful. The pollutant causing this problem is ozone, which, along with other components of smog, can cause eye, nose and throaat irritation, as well as chest pain. We expect the concentration of ozone to diminish this afternoon. People with respiratory ailments and heart disease should reduce physical exertion and outdoor activity at this time. The forecast for tomorrow calls for no change in the index." A more detailed account could be provided by recorded telephone reports or newspapers. For example, listeners can be informed that ozone normally peaks in he afternoon so that later AQI reports will show the index declining, unless there is a significant episode taking place that would cause ozone to continue to build throughout the day. Likewise, if carbon monoxide is the pollutant of concern, the AQI report could add that carbon monoxide is usually only a problem during morning or evening rush hours with acceptable air quality expected during the rest of the day.

What the AQI Cannot Do

Although it is uniform across the country, the AQI cannot be used as the sole method for ranking the relataive healthfulness of different cities - a variety of factors in addition to AQI levels would have to be considered. For example, the number of people actually exposed to air pollution, transportation patterns, industrial composition, and the representativeness of the monitoring sites would also need to be taken into account in developing an accruate ranking of metropolitan areas.

Moreover, the AQI does not specifically take into account the damage air pollutants can do to animals, vegetation, and certain materials, like building surfaces and statues. There is, however, likely to be a correlation between increased AQI levels and increased damage to the overall environment, and a local regulatory agency might choose to point out the impact that an elevated AQI value is likely to have on agriculture and property in the region.

Finally, the AQI does not take into account the possible adverse effects associated with combinations of pollutants (synergism). As more research is completed in the future, the AQI may be modified by EPA to include such effects.

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