"Criteria Pollutants" are those pollutants that governments in some countries have identified as posing a significant threat to peoples' health. Therefore, monitoring of these pollutants is key to ensuring that their levels do not exceed acceptable limits. In the United States, the Environmental Protection Agency (EPA) is responsible for determining these levels for these six Criteria Pollutants.
The six pollutants under this umbrella term are: lead, particulate matter (PM10), carbon monoxide (CO), sulfur oxides (SOx), nitrogen oxides (NOx), and ozone. Canada is also a country that monitors the levels of these pollutants. In fact, the measurement of these takes place at over 350 monitoring stations in 220 Canadian communities.
The set of limits as pertains to Criteria Pollutants are divided into two categories. These are primary standards and secondary standards. The primary standards govern human health. The secondary standards govern environmental and property damage because of these pollutants.
The indication from the Environmental Protection Agency's Criteria Air Pollutant Emissions Inventory is that release of all these pollutants (except for the nitrogen oxides) have been falling over the years. They have been falling since the passage of the Clean Air Act in 1970. In 1977 and in 1990, major amendments were added to the Clean Air Act.