DOE's Division of Planning and Environmental Analysis, within its Office of Fossil Energy, Office of Coal and Power Systems, has been actively assessing the scientific underpinnings of EPA's National Ambient Air Quality Standards (NAAQS) for particulate matter (PM). The primary effort in this assessment has been the development of extensive comments on each version of "Air Quality Criteria for Particulate Matter," commonly referred to as the "PM Criteria Document", published by the United States Environmental Protection Agency (EPA) to serve as the basis for reevaluating the National Ambient Air Quality Standards (NAAQS) for particulate matter. The most important point made in the Office of Fossil Energy's comments was the need for EPA to fully evaluate, and incorporate into the PM Criteria Document, a large body of scientific information which examines large variations in local pollution concentration and composition in populated areas, and which finds large variations in health impacts (both illness and premature mortality) that coincide with these variations in pollution levels. Many of these studies, showing very large "exposure gradients," are relatively new. Taken as a whole, this body of work, in combination with new epidemiological and toxicological studies, clearly suggests particulate matter (and perhaps strongly correlated gases as well) from some local source categories, notably motor vehicles, produces health hazards and risks that would appear to be significantly greater than the hazards and risks posed by PM associated with coal plant emissions.

The Office of Fossil Energy has also recommended that EPA adopt several novel research approaches, including the use of "standardized regulatory toxicology," as a way to determine which types of PM, or sources of PM, are likely to have slight to negligible health impacts at ambient levels, and which types are likely to have more important health impacts. "Standardized regulatory toxicology" would test each pollutant type, or mix, of interest. Tests would use the same multiples of current ambient levels for each pollutant type or mix: e.g., a rural sulfate/nitrate/organic mix; roadside PM mixtures; residual oil fly ash; clean burning diesel and gasoline vehicular emissions; "dirty" diesel and gasoline emissions; various industrial emissions; and fractions of each (e.g., ultrafine PAHs and SVOCs, CO, roadside tire and brake component PM, etc.). Each of these, and many more PM types, would be tested against each health endpoint deemed relevant, at ~ 10, 40, 150, 500, and 1000 times ambient levels in animal models – for example, heart rate variability, blood viscosity, specific types of inflammation - once toxicologists have decided which of these tests seem more relevant to human health endpoints of concern. Some of these tests might produce meaningful effects at 10 times mean ambient levels; some might not produce effects until 1000 times ambient levels. In such a case, regulation of the first PM type or mixture would be more likely to protect public health than regulation of the second.