Air Quality.

Air Quality. Air Quality, an indication of the healthfulness of the air based on the quantity of polluting gasses and particulates (liquid droplets or tiny solid particles suspended in air) it contains. Air is considered safe when it contains no harmful chemicals and only low levels of other chemicals that become harmful in higher concentrations to humans, other animals, plants, or their ecosystems (see Air Pollution). Air is commonly monitored by the United States Environmental Protection Agency (EPA) and state and local environmental agencies for concentrations of six pollutants: carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, ozone, and particulates. Air samples are collected and analyzed several times daily in cities and other industrial areas. The samples are graded on a scale of 0 to 500, indicating how many parts per million (ppm) contain these pollutants. A sample of 0 to 50 ppm indicates good air quality; 50 to 100 ppm, moderate air quality; 100 to 200 ppm, unhealthy; 200 to 300 ppm, very unhealthy; and 300 to 500 ppm, hazardous. If the concentration of one or more pollutants reaches either the very unhealthy or hazardous categories, people with heart or respiratory problems are warned to stay indoors. EPA data show an increase in air quality in the United States between 1985 and 1994. During this period, concentrations of carbon monoxide decreased 28 percent; lead, 86 percent; nitrogen dioxide, 9 percent; sulfur dioxide, 25 percent; ozone, 12 percent; and particulates, 20 percent. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.