Particulate matter is small solid particles or liquid droplets from smoke, dust, fly ash and condensing vapors. It can be suspended in the air for long periods of time. It is generated from paved and unpaved roads, woodsmoke, burning fuels, earth moving, mining, construction, vacant lots and agricultural activities.

These microscopic particles can affect breathing and respiration, cause lung damage and possibly cause premature death. Children, the elderly and people suffering from heart or lung disease are especially at risk.

Small particles can be inhaled. The larger particles are mostly deposited in the nasal passages, while the very small particles can penetrate and be deposited in the lung sacs and membranes. Particulate matter can alter the body’s defense systems and cause cancer. Additionally, these particulates can damage paint, enhance metal corrosion, and soil buildings and clothing and reduce visibility. 

Before 1988, particulate matter was measured as total suspended particulates. The revision of the U.S. Environmental Protection Agency's health standard for particulates changed the emphasis to PM10, which refers to particles with a diameter less than or equal to 10 micrometers. The standards are based on health and welfare, and recent research shows that the smaller particles pose a greater threat than the larger particles. 

In 1997, the primary particulate matter standards were further revised, changing the form of the PM10 standard and creating a new 24-hour and annual PM2.5 standard. This was done to protect the public against the health effects associated with fine particles. In May 1999, the U.S. Supreme Court of Appeals for the D.C. Circuit remanded the new standards back to the EPA for further consideration. However in February 2001, the U.S. Supreme Court upheld the EPA's revised PM10 and PM2.5 standards. The EPA revised the health standards for particulates in September 2006, modifying both the PM10 and PM2.5 standards.

The EPA sets federal standards to protect public health. The limits for particulate matter are set at the following levels:

• Particulate Matter Trends