Particulate matter (PM), Particulates, or Particle Pollution are general terms for solid or liquid particles found in the atmosphere. Particle size is expressed as a measurement of diameter in micrometers (or microns). "Coarse" particles are larger than 2.5 microns in diameter and "fine" particles are those less than 2.5 microns in diameter. For comparison, a human hair is approximately 70 microns in diameter. Secondary particles formed in the atmosphere from gases such as sulfur dioxide, nitrogen oxides, and volatile organic compounds are predominantly fine particles.

As with sulfur dioxide, the trends in coarse particulate matter concentrations in New Hampshire show a clear reduction in pollutant levels at all monitoring sites. No violations have occurred since 1982, except for one day in Berlin in 1989, when the PM standard was exceeded.

Fine particle air pollution causes regional haze in the eastern United States. This pollution comes from fuel combusion at power plants, industrial boilers and processes, and from cars and trucks, and it is often transported across state borders and over long distances. Fine particle air pollution can affect the health of people with heart or lung diseases and respiratory conditions, as well as older adults and children. By 2008 states are required to assess key contributors to regional haze formation, develop plans to reduce sources of haze-forming pollutants, and submit these plans to the U.S. Environmental Protection Agency.