Big dust, little harm

Science News, April 10, 1999 by J. Raloff

Dust storms are blowing away the argument that eroded soil and other relatively large, airborne particles are as hazardous to health as the far smaller particles generated by combustion.

Over the past decade, a host of studies has linked the outdoor buildup of combustion particles to a rise in hospital admissions and death rates for respiratory illness (SN: 4/6/91, p. 212) and heart disease (SN: 7/1/95, p. 5). Such data convinced the Environmental Protection Agency to create new limits (SN: 7/5/97, p. 6)--not yet in effect--for particles that measure 2.5 micrometers ([micro]m) in diameter or smaller (termed PM-2.5). Federal rules already limit a broader class of particles, those with diameters of up to 10 [micro]m (PM-10).

Representatives of combustion-intensive industries say that errors in measuring large particles have made the relative health impacts of large and small particles hard to distinguish. Thus, they have argued against rules focusing on PM-2.5, notes Joel Schwartz of the Harvard School of Public Health in Boston.

"It's a big fight," he explains, and it has threatened to derail implementation of the PM-2.5 limits. Hoping to settle the controversy, Schwartz teamed up with researchers from two universities in Washington State to study Spokane death rates during 17 major dust storms over 6 years. The average PM-10 concentration on storm days was 263 micrograms per cubic meter ([micro]g/[m.sup.3]) of air--well above the current EPA limit. On the stormless days, PM-10 averaged only 42 [micro]g/[m.sup.3]. During dust storms, PM-10 consists primarily of particles larger than 2.5 [micro]m in diameter, but the amount of fine particles present changes relatively little with weather.

Nonaccidental death rates were slightly lower during the dust storms than on calm days of the same date during the study, the researchers report in the May ENVIRONMENTAL HEALTH PERSPECTIVES. These data, they contend, argue against the industry position and confirm results of other studies "that toxicity of coarse particles is substantially less than that of fine particles."