The Air Quality Index, or AQI, is an index for reporting daily air quality. The color-coded index tells you how clean or polluted your air is, and what health concerns may be associated with exposure. The AQI focuses on health effects that can happen within a few hours or days after breathing polluted air. EPA uses the AQI for five major air pollutants regulated by the Clean Air Act: ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide. For each of these pollutants, EPA has established national air quality standards to protect against harmful health effects.

The AQI is scaled from 0 to 500, where 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 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 certain sensitive groups of people, then for everyone as AQI values get higher.

Across the country each day, air quality is measured through a network of monitoring stations in more than a thousand locations. The raw measurements are converted into AQI values using standard formulas developed by EPA. An AQI value is calculated for each of the individual pollutants in an area (ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide). Finally, the highest of the AQI values for the individual pollutants becomes the AQI value for that day. For example, if on July 12 a certain area had AQI values of 90 for ozone and 88 for sulfur dioxide, the AQI value would be 90.

In many U.S. communities, AQI values are mostly below 100, with values greater than 100 occurring several times a year. Several metropolitan areas in the United States have more severe air pollution problems, and the AQI in these areas may often exceed 100. AQI values higher than 200 are very infrequent, and AQI values above 300 are extremely rare.

AQI values can vary significantly from one season to another. In summer, ozone is the most significant air pollutant in many communities, since it forms in the presence of heat and sunlight.

AQI values also can vary depending on the time of day. For example, ozone levels often peak in the afternoon, while carbon monoxide is usually a problem during morning or evening rush hours.

In this lesson, students will use the AQI equation to calculate the AQI value for various air pollutants. for the pollutant ozone on that day.