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What is the AQI
The AQI was developed by the U.S. Environmental Protection Agency (EPA) to provide a simple, uniform way to report daily air quality conditions. The AQI translates each pollutant measurement to a common index, with an index of 100 set to reflect where health effects might be expected in sensitive populations. An AQI value of 100 generally corresponds to the National Ambient Air Quality Standard for the pollutant, which is the level EPA has set to protect public health. The pollutant with the highest index value is used to determine the overall AQI.

For Information on the Different Types of Pollutants, click on the links below

How Does the AQI work?
You can think of the AQI as a yardstick that runs from 0 to 500. The higher the AQI value, the greater the level of air pollution and the greater the health danger. For example, an AQI value of 50 represents good air quality and 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 ambient air quality standard for the pollutant, which is the level EPA has set to protect public health. So, 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 a certain sensitive group of people, then for everyone as AQI values get higher.

Understanding the AQI
The purpose of the AQI is to help you understand what local air quality means to your health. To make the AQI as easy to understand as possible, EPA has divided the AQI scale into six categories, shown below:

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. For example, when the AQI for a pollutant is between 51 and 100, the health concern is "Moderate." Here are the six levels of health concern and what they mean:

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

  • "Moderate" The AQI for Philadelphia 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 people. 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 greater risk from exposure to ozone, while people with either lung disease or heart disease are at 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.