Particulate matter refers to tiny particles of solid or semi-solid material suspended in the atmosphere. This includes particles between 0.1 micron and 50 microns in diameter. Particles larger than 50 microns tend to settle out quickly due to gravity. Particles smaller than 0.1 micron tend to act like molecules. Particulates are categorized as follows:

• TSP: Total Suspended Particulate refers to all particles between 0.1 - 50 microns. Up until 1987, the EPA standard for particulates was based on levels of TSP.
• PM10: Particulate matter of 10 microns or less (approximately 1/6 the diameter of a human hair). EPA has a standard for particles based on levels of PM10.
• PM2.5: Particulate matter of 2.5 microns or less (approximately 1/20 the diameter of a human hair), also called "fine" particles. In July 1997, the EPA approved a standard for PM2.5.

Sources of particles larger than PM10 include wind blown dust from roads and fields, sea salt, pollen spores, and emissions from forest fires and volcanos. Most manmade particles, however, are in the range of 1 to 10 microns in diameter. PM10 is generally created during burning processes and consists of fly ash from power plants, carbon black from diesel and gasoline engines, and soot from woodburning. Elemental and organic carbon make up a significant fraction of PM10 and PM2.5. The fine particles (PM2.5 and less) are typically secondary aerosols that form when chemical reactions occur between sulfate (from power plants) or nitrate (from industry and motor vehicles) and ammonia (from feedlots).

The health risk from particulates is a function of the size and concentration of the dose inhaled. PM10 can be breathed into the lungs, and therefore, its health effects are more severe than large particles. PM2.5 can be breathed even more deeply into the alveoli of the lungs, where they remain for a long time and can cause the greatest amount of damage. Particulate matter can reduce lung functioning and can cause or aggravate respiratory conditions, and increase the long term risk of lung cancer or other lung disease such as emphysema, bronchiectasis, pulmonary fibrosis, and cystic lungs.

Fine particles also cause visibility impairment, thus affecting human welfare. PM2.5 have the greatest impact on visibility reduction because of their ability to scatter light. Much of Denver's infamous "brown cloud" is caused by fine particles.

There is strong evidence that asthmatics are more sensitive to the effects of particluates than healthy people. Conversely, little scientific evidence exists that elderly people (>65 years) are particularly sensitive to particulates.

The primary National Ambient Air Quality Standard (NAAQS) for PM10 is 150 ug/m3 (micrograms/meter-cubed), averaged over a 24-hour period. The NAAQS for PM2.5 are 35 ug/m3 averaged over a 24-hour period, and a 12 ug/m3 annual average.

Particulate matter concentrations of PM10 and PM2.5 are reported in the Fort Collins Air Quality Trends Reports.