Air quality affects how you live and breathe. And like the weather, it can change from hour to hour. The U.S. Environmental Protection Agency (EPA) and local officials use the Air Quality Index (AQI) to provide you with simple information on local air quality, the health concerns for different levels of air pollution, and how you can protect your health when pollutants reach unhealthy levels.

The AQI reports daily air quality. It tells you how clean or polluted your air is, and what associated health effects could be a concern for you. The AQI focuses on health effects you may experience within a few hours or days after breathing polluted air.

The AQI calculates five major pollutants regulated by the Clean Air Act:

Ground level ozone

Particle pollution (also called particulate matter)

Carbon monoxide

Sulfur dioxide

Nitrogen dioxide

The AQI runs from 0-500. The higher the AQI number, the greater the level of air pollution and the greater the health concern.

The EPA has set 100 as the AQI value that generally corresponds to the national air quality standard for the pollutant. AQI values below 100 are generally considered satisfactory. AQI values above 100 are unhealthy -- at first for sensitive groups of people, then everyone as AQI values get higher.

The purpose of the AQI is to help you understand what local air quality means to your health. To make it easier to understand, the AQI is divided into six categories:

0-50: Air quality conditions are good; the color defining this level of air quality is green

51-100: Air quality conditions are moderate; the color defining this level of air quality is yellow

101-150: Air quality conditions are unhealthy for sensitive groups; the color defining this level of air quality is orange

151-200: Air quality conditions are unhealthy; the color defining this level of air quality is red

201-300: Air quality conditions are very unhealthy; the color defining this level of air quality is purple

301-500: Air quality conditions are hazardous; the color defining this level of air quality is maroon

So the next time you hear that your city is having a 'code red air quality day' you'll know that corresponds to an air quality value of 151-200 and that it's considered an unhealthy air day.

For more information on the Air Quality Index, visit the government Web site AirNow at http://www.airnow.gov.