Air quality is usually expressed as Air Quality Index (AQI), the Air Pollution Index (API) or Pollutant Standard Index (PSI). These indexes represent the measurement of the concentration of certain contaminants (pollutants) in the air. When the air quality is bad, API increases. The poor air quality (pollution) can irritate one's eyes, nose, and throat, cause shortness of breath, aggravate asthma and other respiratory conditions, and even affect heart and cardiovascular system.
The common contaminants to be measured in the metropolitan area are ozone and particulate matter (PM). Cars and trucks and other mobile sources are the major sources of ozone in the summertime. In the winter, wood smoke from wood stoves and fireplaces is a major source of PM pollution. Many countries monitor ground-level ozone, particulates, sulfur dioxide, carbon monoxide and nitrogen dioxide and calculate air quality indexes for these pollutants.