The Good Air Lady
The Good Air Lady
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About Air Quality Index

Common environmental studies questions are about Air Quality Index (AQI)and air pollution. Air quality and air pollution are related, so this article focuses on the AQI.

The Air Quality Index (AQI) is a standardized indicator of the air quality in a given location. It measures mainly ground-level ozone and particulate matter (excluding pollen), but may also include sulfur dioxide and nitrogen dioxide.

An AQI level of 100 generally corresponds to the actual measured level of an air pollutant that is considered the maximum safe exposure. For example, an AQI of 100 corresponds to ground-level ozone readings of 0.08 parts per million (ppm), the U.S. EPA's standard for safe exposure. The AQI level is a composite of such actual measurements for ozone, particulates, and other pollutants included in the AQI.

Note that safe measured levels of pollutants may vary by nation. Also, different nations have different classifications for their Air Quality Index. Here is a comparison of AQI levels for Canada and the U.S.


  • U.S.

  • 0-50 Good

  • 51-100 Moderate

  • 101-150 Unhealthy for sensitive

  • 151-200 Unhealthy for all

  • 201-300 Very Unhealthy for all

  • 301-500 Hazardous



  • Canada

  • 0-25 Good

  • 26-50 Fair

  • 51-100 Poor

  • 101+ Very Poor

These differences represent less stringent standards for air quality in the United States. The U.S. Environmental Protection Agency (EPA) is considering tightening up the AQI standards, based upon recommendations from its scientific committees that there is no valid basis for the current weaker standards.

The AQI of a given location may go up or down due to varying wind conditions. Stronger winds blow ozone away, but can also stir up dust and other particulate matter from the ground. Still air lets particulates settle to the earth, but ozone and other gaseous air pollutants may become trapped under a temperature inversion. An inversion typically occurs during winter months, when warm air near the surface that would typically rise becomes trapped beneath a layer of cold air.

On days of poor AQI readings, news media often issue "ozone alerts" warning people to stay indoors and avoid strenuous exercise or unnecessary driving. These warnings should be taken seriously, particularly by infants, young children, pregnant women, and the elderly.

In the U. S. AQI classification system, "unhealthy for sensitive groups" means that air pollution measured by the AQI is likely to have adverse effects on particularly vulnerable populations. These populations include the young, pregnant women, the elderly, and anyone suffering from heart or respiratory disease. The general public is unlikely to be adversely affected by AQI levels in this range.

The AirNow.gov Web site provides color-coded maps showing AQI levels for every part of the United States. Visitors to Enviroflash, www.enviroflash.info, can sign up for free email alerts when local air quality is a concern.

The Air Quality Index is an important tool in communicating real-time air quality conditions to the public. Its classification system provides a simple way to tell if you or your loved ones should take steps to limit exposure to adverse levels of ozone, particulates, and other air pollutants. What you need to know about the Air Quality Index is that it can help save lives and reduce health costs by keeping people out of hospitals.