PM, also known as particle pollution, is a complex mixture of air-borne particles and liquid droplets composed of acids (such as nitrates and sulfates), ammonium, water, black (or "elemental") carbon, organic chemicals, metals, and soil (crustal) material.

EPA groups particle pollution into two categories:

• "Coarse particles" (PM_10-2.5) such as those found near roadways and dusty industries range in diameter from 2.5 to 10 micrometers (or microns). The existing "coarse" particle standard (known as PM₁₀) includes all particles less than 10 microns in size.
• "Fine particles" (or PM_2.5) such as those found in smoke and haze have diameters less than 2.5 microns.

PM_2.5 is referred to as "primary" if it is directly emitted into the air as solid or liquid particles, and is called "secondary" if it is formed by chemical reactions of gases in the atmosphere. Major sources of primary fine particles include cars and trucks (especially those with diesel engines); open burning; wildfires; fireplaces, woodstoves, and outdoor wood boilers (also called hydronic heaters); cooking; dust from roads and construction; agricultural operations; and coal and oil-burning boilers. Major sources of secondary fine particles are power plants and some industrial processes, including oil refining and pulp and paper production.