Mathematics plays an important part in the air quality index (AQI), a scale developed by the U.S. government to measure how much pollution is in the air. The AQI measures five specific pollutants: ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide. The levels range from 0 (good air quality) to 500 (hazardous air quality); the higher the index, the higher the level of pollutants and the greater the likelihood of detrimental health effects.

Most people think about the AQI in terms of being outdoors—and most weather broadcasts include air quality listings, especially in larger cities. When the readings are high, people are warned not to participate in strenuous activities like sports or hard work outside; people with asthma or other lung problems are urged to stay inside.