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The Clean Air Act regulates six common air pollutants: particle pollution (particulate matter), ground-level ozone, carbon monoxide, sulfur oxides, nitrogen oxides, and lead. These are called “criteria” air pollutants because the Environmental Protection Agency sets human health-based and environmentally-based criteria for setting limits on the amount of these pollutants that are permissible in the ambient air. These limits are called primary and secondary standards.
In 1997 particle pollution was divided into two divisions, Particulate Matter (PM) 10, particles equal to or smaller than 10 micrometers in diameter, and Particulate Matter (PM) 2.5, particles equal to or smaller than 2.5 micrometers in diameter. At one time the belief was that if you pulverized particles, the danger went away. Health care professionals learned that pulverizing particles actually increased the danger because microscopic particles get deep into the lungs.
Ozone and particle pollution are the most widespread health threats.