Air Quality - Lesson 5: How Can We Tell What the Quality of the Air Is Today?

Background Information

Air pollution can be reported in several ways: concentration (amount of material in a volume of air in ppm or ppb), mass (weight of material in air in micrograms per cubic meter), or an Air Quality Index (AQI). The AQI is an index from 0 to 500 for reporting daily air quality based on human health considerations. 

The U.S. EPA calculates the AQI for five major air pollutants regulated by the 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 Ambient Air Quality Standards (NAAQS) to protect public health (see teacher resource on National Ambient Air Quality Standards.) The AQI is not a “standard,” but it is related to air quality standards. Note that the AQI is calculated hourly and is derived from data that have not been quality assured.

 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 AQI values get higher.

To make it easier to understand, the AQI is divided into six categories and a specific color is assigned to each AQI category. 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. There are different health messages for each of the AQI pollutants.

The six levels of health concern and what they mean are:

• "Good" (Green): AGI between 0 and 50. Air quality is considered satisfactory, and air pollution poses little or no risk.

• "Moderate" (Yellow): AQI 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 (extremely sensitive people).

• "Unhealthy for Sensitive Groups" (Orange): AQI between 101 and 150. These groups include people with asthma, children and teens, elderly, and people who work or exercise outside strenuously. 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" (Red): AQI between 151 and 200. The air is unhealthy for everyone and members of sensitive groups may experience more serious health effects. 

• "Very Unhealthy" (Purple): AQI between 201 and 300. Everyone should reduce and sensitive groups should avoid prolonged or heavy outdoor exertion.

• "Hazardous" (Maroon): AQI over 300. Everyone should avoid all outdoor exertion. 

In Michigan, ozone and fine particles are usually the pollutants that cause the AQI to be elevated. Newspapers, radio, television, and web sites report the daily AQI levels. The daily AQI for Michigan is posted on the MDEQ web site at http://www. That site has specific information about the actual values of pollutants at air monitoring stations and details of the air quality forecast. Maps from the U.S. EPA’s AIRNow site are particularly useful for identifying ozone patterns. These maps and AQIs for the whole country can be found at 

MDEQ meteorologists predict when conditions are favorable for excessive fine particle or ozone formation. Weather conditions conducive to ozone formation are high temperatures, high pressure, clear skies, and no wind. Low-pressure systems tend to dissipate the ozone. Inversions (atmospheric temperature increase with height) can trap pollutants, compounding the problem. High pollutant concentrations upwind can be transported into a different area and can cause substantial increases in air pollution that would not have otherwise occurred. An Action Day is called when weather conditions are likely to combine with pollution to create elevated amounts of ground-level ozone. 

The Southeast Michigan Council of Governments and the Clean Air Coalition of Southeast Michigan ( as well as the West Michigan Clean Air Coalition (http://www.wmcac. org/) operate local action programs to disseminate the air quality forecast message to communities. They encourage individuals and companies to make clean air choice on Action Days. Ozone Action and Clean Air Action are voluntary emissions reduction initiatives. These organizations provide teacher resources and tip cards for appropriate actions when DEQ meteorologists announce Action Days. 

The UV Index is a next-day forecast of the amount of skin-damaging UV radiation expected to reach the Earth’s surface at the time when the sun is highest in the sky (solar noon). The amount of UV radiation reaching the surface is primarily related to the elevation of the sun in the sky, the amount of ozone in the stratosphere, and the amount of clouds present. The UV index is a good way to link stratospheric ozone depletion with human health effects. The Index predicts UV intensity levels on a scale of 1 to 11+, where low indicates a minimal risk of overexposure and 11+ means an extreme risk. Calculated on a next-day basis for every zip code across the United States, the UV Index takes into account clouds and other local conditions that affect the amount of UV radiation reaching the ground in different parts of the country. 

The UV index can be found as a link from

sources: Michigan Department of Environmental Quality. Air Quality Division. Michigan's Air. Retrieved April 1, 2005, from http://www.michigan.gove/deqair/

U.S. Environmental Protection Agency. AIRNow. Retrieved April 1, 2005 from http://www.epq.gove/airnow

U.S. Environmental Protection Agency. What Is the UV Index? Retrieved April 1, 2005, from http://www.epa.gove/sunwise/uvwhat.html

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