Particulate matter is airborne mists, fumes, soot, aerosols, ash or dusts. Smoke is the most obvious form of particulate matter. Smoke is visible when dense concentrations of microscopic particulates are present. After the smoke plume disperses and cannot be seen, the story is not over. The airborne particles can remain suspended for weeks. Particulate matter can also form when airborne chemicals react to change from gases into liquid aerosols or solids. Some particles carry attached molecules of toxic substances.

Particulates can harm lung tissue, cause eye and throat irritation, premature death and reduced visibility from haze. Very fine particulates can travel hundreds, even thousands of miles in the wind before settling out or falling to earth in rain, snow or fog.


Particle Sizes

There are several classes of particulates. Very large particles pose less health risks, as they are large enough for our respiratory defenses to remove. Grit-like dust is an example. Large particulates can, however, create a nuisance by soiling homes, property and temporarily reducing visibility. It may also cause eye, nose and throat irritation and damage vegetation by covering leaves and pores.

Two particle sizes, PM10 and PM2.5, are regulated pollutants for health and environmental concerns. "PM" stands for particulate matter. PM10 means each microscopic particle is ten microns or smaller in diameter. Some PM10 comes from moving dry, dusty materials such as sand, gravel, grain, coal, and other materials, although much of the dust may be larger in size than 10 microns. Cutting, grinding, crushing and other activity can release PM10. Vehicles release PM10 from tire wear, abrasion against road surfaces and exhaust.

The newest class of particulates is called PM2.5. They are 2.5 microns in diameter or smaller---about the thickness of a human red blood cell. They pose the greatest health risk as they bypass respiratory defenses to penetrate the deepest lung passages where they can become imbedded. Most PM2.5 comes from combustion of wood, leaves, coal, garbage, gasoline, fuel oil and other fuels.

These particles can also form in the air during chemical reactions. Airborne sulfates, nitrates and ammonia emitted as gases from tailpipes, smokestacks, and livestock and fertilizer use can react to form solid particles. This is known as "secondary" or "atmospheric" formation.

The state began measuring PM2.5 levels in 1999. Unhealthy levels have been found several days each year since monitoring began. Fortunately, this is limited to a few days, but continued work and action is needed by motorists, citizens and business to maintain clean air and avoid as many polluted days as possible.