What is PM₁₀?

PM₁₀ is the scientific term for particulate matter (soot, dust, ash, and fumes) with a diameter of less than ten microns in size (approximately one-seventh the diameter of a human hair). That's too small to be filtered by the nose and lungs and can reduce lung function, aggravate respiratory conditions, and may increase the long-term risk of cancer or respiratory diseases. In addition to affecting human health, PM₁₀ plays a major role (not yet fully understood) in causing the "brown cloud" that reduces visibility in Denver during the winter season.

Why is it Important?

PM ₁₀ is one of the six so-called "criteria" pollutants for which there is a National Ambient Air Quality Standard (NAAQS) as set in the federal Clean Air Act. The five other pollutants that fall under the NAAQS are: Ozone (O₃, or commonly referred to as smog), carbon monoxide (CO), sulfur dioxide (SO₂), Lead (Pb), and nitrogen dioxide (NO₂).

If a metropolitan area is in violation of the NAAQS, they must submit a plan (known as a State Implementation Plan) that describes how the region intends to comply with the standard in the near future. Those regions in violation of the standard, considered "nonattainment areas," are subject to withholding of federal transportation funds. As of June 1997, there are 80 nonattainment areas for PM₁₀ nationwide and close to 30 million people exposed to unhealthy levels of PM₁₀. The last violation of the PM₁₀ standard in the Denver Region was in 1992.