Airborne particulate matter originates from a wide variety of sources, including car and truck exhaust, tire wear, combustion sources such as wood stoves and fireplaces, dusty industrial and commercial processes, and wind-blown soils. Fine particulate matter is also formed in the atmosphere from chemical reactions with pollutant gases. Beginning in July 1987, particulate matter standards were changed from a "total suspended particulate" (TSP) standard to a standard that measures only those particles that are 10 microns or less (PM10). These particles were thought to be more damaging to human health and plant life. The 24-hour PM10 standard was set at 150 ug/m3 and the annual average standard set at 50 ug/m3.
    The U.S. EPA has promulgated a new standard for fine particulate matter, smaller than 2.5 microns in diameter (PM2.5). This new standard took effect on September 16, 1997. These fine airborne particles are the greatest threat to health and visibility. The new PM2.5 standard will be set at a 65 ug/m3 24-hour standard and a 15 ug/m3 annual standard. PSAPCA has been conducting PM2.5 monitoring since 1987. EPA also intends to retain the current PM10 24-hour and annual standards.

    There are 15 PM10 monitoring sites within our jurisdiction; four industrial sites and 11 residential or commercial. At two of the industrial sites, PM2.5 monitoring has also been conducted since 1987, and another site was added in 1989.
    Trends. PM10 levels are showing steady declines over the past 10 years as measured by annual averages, maximum daily values, and the number of days above the Washington State Impaired Air Quality trigger level of 75 ug/m3. These trends apply to industrial as well as residential/commercial monitoring sites. Effective industrial control strategies, cleaner burning residential wood stoves, along with changing heating habits in favor of clean-burning natural gas, are the chief factors behind the declines. There have been no exceedances of PM10 standards at any of the 15 monitoring sites since 1990. It is anticipated that EPA will formally redesignate all three central Puget Sound PM10 nonattainment areas in 1998.
    24-hour PM2.5 levels show similar decreases. Annual average PM2.5 data also shows improvement (averages for 1990 through 1993 were not calculated due to lack of data).