Clean Air Act

Before the 1990 Clean Air Act went into effect, EPA set limits on airborne particles smaller than 10 micrometers in diameter called PM10. These are tiny particles (seven of these particles lined up next to each other would cover a distance no wider than a human hair). Research has shown that even smaller particles (1/4 the size of a PM10 particle) are more likely to harm our health. So in 1997, EPA published limits for fine particles, called PM2.5. To reduce particle levels, additional controls are being required on a variety of sources including power plants and diesel trucks.