It's all in the mix: proving adverse effects of fine and ultrafine PM. (Science Selections).

The association between exposure to airborne particulate matter with a median diameter of 10 micrometers ([micro]m) or less and increase in respiratory disease has been well established, although the physical and chemical properties involved remain unknown. Suspicion has been growing that exposure to even smaller particles, in the fine (2.5-10 [micro]m) and ultrafine (less than 0.1 [micro]m) ranges, might be even more likely to cause adverse health effects. Until recently there had not been a reliable method available to recreate the particulate mix in ambient air--especially the ultrafine particles--in sufficient concentrations to allow analysis in a controlled setting. Now, however, using advanced technology to recreate such a particulate mix, Kevin R. Smith from the University of California, Davis, and colleagues have proven adverse respiratory effects of fine and ultrafine concentrated ambient particles (CAPs) on healthy adult rats [EHP 111:902-908].

The researchers used a newly developed device called a Versatile Aerosol Concentration Enrichment System (VACES). This system can enrich the concentration of ambient particles in selected size ranges of 0.01-10 [micro]m by up to a factor of 40, depending on the desired output flow rate, while preserving the characteristics of the component particles during the process.

Smith and his group exposed groups of 6 rats to either filtered ambient air (as control) or to ambient air with enriched particle concentrations created using the VACES. The procedures were conducted in Fresno, California, which lies in a region known to have some of the highest ambient particle concentrations in the United States, particularly in the P[M.sub.2.5] range. The enriched ambient fine and ultrafine particle concentrations were approximately 20-fold higher than outdoor concentrations at the Fresno test site.

Exposures were for 4 hours per day for 3 days, during 1 of 3 consecutive weeks of fall 2000 and 3 consecutive weeks of winter 2001. Immediately following the third day of exposure, the researchers conducted bronchoalveolar lavage (BAL) on the animals, washing their lungs with a neutral solution and then determining the nature and number of bronchial cells washed into the lavage fluid. This allowed them to determine whether exposure to the fine and ultrafine CAPs had a cytotoxic or proinflammatory effect upon the lungs.

For rats that were exposed during the first week of winter, exposure to CAPs was associated with a significant increase in the total number of cells recovered by lavage compared with the controls for that week. This same group also had a significantly higher number of BAL macrophages, as well as a significant increase in the number of neutrophils. The CAPs-exposed group for the first week of fall also had a significant increase in BAL neutrophils. Although a clear dose response could not be established, the highest levels of particulate matter mass, nitrate, and organic carbon were measured in the ambient air during the 2 weeks when the increases were greatest.

The CAPs-exposed animals exhibited increases in other markers of lung damage compared to the control groups, although none of these markers achieved statistical significance. The researchers also measured BAL cell permeability, an indicator of decreased membrane integrity, and determined that the proportion of nonviable BAL cells increased significantly in the CAPs-exposed groups compared to controls in 4 of the 6 test weeks.

The authors did not attempt to correlate particle components with health effects, but did uncover striking evidence of "consistent cytotoxic and inflammatory responses associated with exposure to CAPs." It is also noteworthy that this detectable exposure-related lung damage occurred in healthy adult rats, in contrast to some previous studies that utilized rat models with respiratory diseases. Perhaps most importantly, the authors achieved their findings through the use of a potentially powerful new tool in the analysis of the impact of ambient exposure to fine and ultrafine particulate matter.