Maximum legal concentration limits of air pollutants averaged over specified time periods.
These are prescriptive. If the actual measured concentration averaged over time exceeds the legal threshold, then the event is called an exceedence. Regulations often allow a limited number of exceedences each year, with fines or penalties imposed for too many exceedences. Regions with too many exceedences are required to develop plans to improve the air quality, for example, by changing automobile fuels and gas-station equipment or increasing the use of mass transportation. In the United States, the standards are called National Ambient Air Quality Standards (NAAQS). Primary standards are designed to protect human health. Secondary standards are designed to protect crops, animals, structures, and commerce. In Canada, these legal thresholds are called National Air-Quality Objectives. Three levels of standards exist within these objectives. In ascending order of concentration limit they are Maximum Desirable Level, Maximum Acceptable Level, and Maximum Tolerable Level.