Fine Particles and Mortality

Vehicular traffic effects on survival within the Washington University-EPRI veterans cohort: new estimates and sensitivity studies

We analyzed survival patterns among approximately 70,000 U.S. male military veterans relative to vehicular traffic density in their counties of residence, by mortality period and type of exposure model. Previous analyses show traffic density to be a better predictor than concentrations of criteria air pollutants. We considered all subjects and also the subset defined by availability of air quality monitoring data from the U.S. EPA PM(2.5) Speciation Trends Network (STN). Traffic density is a robust predictor of mortality in this cohort; statistically significant estimates of deaths associated with traffic range from 1.3% to 4.4%, depending on the method of analysis. This range of uncertainty is larger than the traditional 95% confidence intervals for each estimate (1-2%). Our best estimate of the relative risk for the entire follow-up period is 1.03. These deaths occurred mainly before 1997 in counties with STN air quality data, which tend to be more urban. We identified a threshold in mortality responses to traffic density, corresponding to county-average traffic flow rates of about 4000 vehicles/day. Relative risks were significantly higher in the more urban (STN) counties in the early subperiods, but this gradient appears to have diminished over time. We found larger risks by pooling results from separate portions of the overall follow-up period, relative to considering the entire period at once, which suggests temporal changes in confounding risk factors such as smoking cessation, for example. These results imply that the true uncertainties in cohort studies may exceed those indicated by the confidence intervals from a single modeling approach.

Evidence of Health Impacts of Sulfate-and Nitrate-Containing Particles in Ambient Air

Ambient particulate matter (PM) is a complex mixture of inorganic and organic compounds. The U.S. Environmental Protection Agency (EPA) regulates PM as a criteria pollutant and promulgates National Ambient Air Quality Standards for it. The PM indicator is based on mass concentration, unspecified as to chemical composition, for specific size fractions. The numerical standards are based on epidemiologic evidence of associations between the various size-related particle mass concentrations as indicators and excess mortality and cardiorespiratory health effects as endpoints. The U.S. National Research Council has stated that more research is needed to differentiate the apparent health effects associated with different particle chemical constituents. Sulfate and nitrate constitute a significant portion of the particle mass in the atmosphere, but are accompanied by similar amounts of carbonaceous material, along with low concentrations of various species, including bioactive organic compounds and redox cycling metals. Extensive animal and human toxicology data show no significant effects for particles consisting only of sulfate and nitrate compounds at levels in excess of ambient air concentrations. A few epidemiologic studies, including both short-term time-series studies and long-term cohort studies, have included the sulfate content of PM as a specific variable in health effect analyses. There are much less data for nitrate. The results from the epidemiologic studies with PM sulfate are inconsistent. A detailed analysis of the time-series epidemiological studies shows that PM sulfate has a weaker "risk factor" than PM2.5 for health effects. Since sulfate is correlated with PM2.5, this result is inconsistent with sulfate having a strong health influence. However, there are many limitations with these types of studies that warrant caution for any comparison between a chemical component and mass concentration. In total, the epidemiologic and toxicologic evidence provide little or no support for a causal association of PM sulfate and health risk at ambient concentrations. For nitrate-containing PM, virtually no epidemiological data exist. Limited toxicological evidence does not support a causal association between particulate nitrate compounds and excess health risks. There are some possible indirect processes through which sulfate and nitrate in PM may affect health-related endpoints, including interactions with certain metal species and a linkage with production of secondary organic matter. There is insufficient evidence to include or exclude these processes as being potentially important to PM-associated health risk.

The Big Ban on Bituminous Coal Sales Revisited: Serious Epidemics and Pronounced Trends Feign Excess Mortality Previously Attributed to Heavy Black-Smoke Exposure

The effect of banning bituminous coal sales on the black-smoke concentration and the mortality rates in Dublin, Ireland, has been analyzed recently. Based on the application of standard epidemiological procedures, the authors concluded that, as a result of the ban, the total nontrauma death rate was reduced strongly (-8.0% unadjusted, -5.7% adjusted). The purpose of this study was to reanalyze the original data with the aim of clarifying the three most important aspects of the study, (a) the effect of epidemics, (b) the trends in mortality rates due to advances in public health care, and (c) the correlation between mortality rates and black-smoke concentrations. Particular attention has been devoted to a detailed evaluation of the time dependence of mortality rates, stratified by season. Death rates were found to be strongly enhanced during three severe pre-ban winter-spring epidemics. The cardiovascular mortality rates exhibited a continuous decrease over the whole study period, in general accordance with trends in the rest of Ireland. These two effects can fully account for the previously identified apparent correlation between reduced mortality and the very pronounced ban-related lowering of the black-smoke concentration. The third important finding was that in nonepidemic pre-ban seasons even large changes in the concentration of black smoke had no detectable effect on mortality rates. The reanalysis suggests that epidemiological studies exploring the effect of ambient particulate matter on mortality require improved tools allowing proper adjustment for epidemics and trends. Aspects of harvesting and more recent results derived from a distributed lag model covering the effects of black smoke and temperature are also discussed.