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ABOUT AIR QUALITY INDEX (AQI)

The AQI is an index for reporting daily air quality.  It tells you how clean or polluted the air is, and what associated health effects might be a concern.  The AQI focuses on health effects that may be experienced within a few hours or days after breathing polluted air.  The U.S. Environmental Protection Agency (EPA) calculates the AQI for five major air pollutants regulated by the federal Clean Air Act: ground-level ozone, particle pollution (also known as particulate matter), carbon monoxide, sulfur dioxide, and nitrogen dioxide.  For each of these pollutants, EPA has established national air quality standards to protect public health.  Ground-level ozone and airborne particles are the two pollutants that are generally considered to pose the greatest threat to human health in the United States.

How Does the AQI Work?

The AQI can be thought of as a yardstick that extends from 0 to 500.  The higher the AQI value, the greater the level of air pollution and the greater the health concern.  For example, an AQI value of 50 represents good air quality with little potential to affect public health, while an AQI value over 300 represents hazardous air quality.

An AQI value of 100 generally corresponds to the national air quality standard for the pollutant, which is the level EPA has set to protect public health.  AQI values below 100 are generally thought of as satisfactory.  When AQI values are above 100, air quality is considered to be unhealthy, at first for certain sensitive groups of people, then for everyone as the AQI values get higher.

An AQI value can be calculated for each of the air pollutants for which an air quality standard has been established.  An Overall AQI value is calculated based on the highest AQI for all pollutants.  This provides a single measure of the overall air quality at a given location and time.

Understanding the AQI

The purpose of the AQI is to help the general public understand local air quality measurements and what their significance is.  To make it easier to understand, the AQI is divided into six categories:

Air Quality Index (AQI) Values Levels of Health Concern Colors
When the AQI is in this range: ...air quality conditions are: ...as symbolized by this color:
0 to 50 Good Green
51 to 100 Moderate Yellow
101 to 150 Unhealthy for Sensitive Groups Orange
151 to 200 Unhealthy Red
201 to 300 Very Unhealthy Purple
301 to 500 Hazardous Maroon

Each category corresponds to a different level of health concern.  The six levels of health concern and what they mean are:

"Good"  The AQI value is between 0 and 50.  Air quality is considered satisfactory, and air pollution poses little or no risk.

"Moderate"  The AQI is between 51 and 100.  Air quality is acceptable; however, for some pollutants there may be a moderate health concern for a very small number of individuals.  For example, people who are unusually sensitive to ozone may experience respiratory symptoms.

"Unhealthy for Sensitive Groups"  When AQI values are between 101 and 150, members of sensitive groups may experience health effects.  This means they are likely to be affected at lower levels than the general public.  For example, people with lung disease are at a greater risk from exposure to ozone, while people with either lung disease or heart disease are at a greater risk from exposure to particle pollution.  The general public is not likely to be affected when the AQI is in this range.

"Unhealthy"  Everyone may begin to experience health effects when AQI values are between 151 and 200.  Members of sensitive groups may experience more serious health effects.

"Very Unhealthy"  AQI values between 201 and 300 trigger a health alert, meaning everyone may experience more serious health effects.

"Hazardous"  AQI values over 300 trigger health warnings of emergency conditions.  The entire population is more likely to be affected.

AQI Colors

EPA has assigned a specific color to each AQI category to make it easier for people to understand quickly whether air pollution is reaching unhealthy levels in their communities. For example, the color orange means that conditions are "unhealthy for sensitive groups", while red means that conditions may be "unhealthy for everyone", and so on.

Air Quality Index
Levels of Health Concern
Numerical Value Meaning
Good 0-50 Air quality is considered satisfactory, and air pollution poses little or no risk.
Moderate 51-100 Air quality is acceptable; however, for some pollutants there may be a moderate health concern for a very small number of people who are unusually sensitive to air pollution.
Unhealthy for Sensitive Groups 101-150 Members of sensitive groups may experience health effects.  The general public is not likely to be affected.
Unhealthy 151-200 Everyone may begin to experience health effects; members of sensitive groups may experience more serious health effects.
Very Unhealthy 201-300 Health alert; everyone may experience more serious health effects.
Hazardous >300 Health warnings of emergency conditions.  The entire population is more likely to be affected.

Source: U.S. Environmental Protection Agency (www.airnow.gov)

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