Exhaustive variation: diesel studies should look at particle differences.

In the past 12-15 years, most experiments on the health effects of diesel exhaust particles (DEP) have employed samples from one of two types of engines. Automobile-generated DEP has been tested extensively for its effects on the heart and lungs, but not its mutagenic properties. Forklift-generated DEE on the other hand, has been tested primarily for genotoxicity but not for heart effects, and has been used in only limited studies of lung effects. In two companion papers published this month, Pramila Singh and David M. DeMarini of the U.S. Environmental Protection Agency and their colleagues report the findings of the first head-to-head testing of these two types of DEP samples [EHP 112:814-819, 820-825]. The results confirm that DEP can be highly variable and that future DEP health effects research would benefit from a fuller characterization of a wide variety of samples beyond the two standard samples used by researchers for more than a decade.

The researchers performed comparative physical and chemical analyses of a standard sample of automobile- and forklift-generated DEP and found that both samples had a pH of 3.3. But almost every other physical and chemical trait was markedly different. For instance, the automobile DEP had 10 times the organic carbon and about 13 times the extractable organic material of the forklift DEP, but less than one-sixth the elemental carbon. The automobile DEP had more polycyclic aromatic hydrocarbons per mass of particle than the forklift DEP, and a much higher proportion of aliphatic hydrocarbons and alkanes. The forklift DEP had a much greater range of particle sizes and a finer, more porous surface texture. This diversity in traits likely arises from differences in DEP collection methods and from the contrasting designs of the different engines.

The researchers also compared the pulmonary effects of the two DEP samples in mice, and their genotoxic effects in six strains of Salmonella. Both samples created similar levels of pulmonary injury but distinctly different cellular inflammatory responses; the authors suggest that the distinctive physical and chemical properties of the samples may have contributed to this difference in the mechanism of action. In the Salmonella experiments, the automobile DEP tended to be much more mutagenic than the forklift DEP in most, but not all, conditions.

The researchers did not draw any specific conclusions regarding human health impacts of either type of DEP, because the exposure techniques used do not reflect real-world experiences. Instead, the studies simply revealed differences and similarities between the two DEP samples.

The researchers conclude that an increased understanding of the health effects of DEP will require evaluation of a wide range of DEP samples produced by different engines and fuels under different operating parameters. As more studies find evidence of potential health risks from diesel exposures such as riding in diesel school buses and living near freeways, the authors write, DEP health effects research must become multidisciplinary, involving chemists, engineers, biologists, and modelers.