What is an "exceedance"?

Monitors are used to measure the amount of pollutants in the air. The federal government has developed standards for various air pollutants that indicate the level at which a particular pollutant can be in the air and still be safe. An exceedance has occurred when a monitor measures that a pollutant has gone over the standard.

What is a "violation"?

When a specified number of exceedances occurs within a certain period of time, then a violation has occurred. The number of exceedances and the period of time vary depending on the pollutant. Communities that violate a standard are designated "nonattainment areas" for that pollutant.

(To review a summary of the Valley's air pollution exceedances in 2001, see page 9 of Maricopa County's 2001 Network Review.)