Review of the Air Quality Criteria Document for Particulate Matter

understand. Further, PM is qualitatively no different than the gaseous pollutants in this regard.Therefore, it still makes sense to consider the gaseous pollutants as potential confounders of PM.Similarly, attempts to stop considering some of the co-pollutants as confounders, arguing, as isdone in the CD, that they are merely steps in the mechanistic causal chain (see below), are notvalid.4. Regional heterogeneityThe emphasis in this CD on NMMAPS II is justified. The heterogeneity in the estimatesof PM effect across US cities is obvious (28 of 88 cities having non-positive estimates of effect).This is the first good impression available of the degreee of heterogeneity that is present. Theheterogeneity might be due to random variation in estimates of effect (presumably supported bythe CD in developing the argument that in general the absence of effect is observed for citieswith fewer observations, i.e., less power), or it may represent true regional differences. It shouldbe noted that the heterogeneity observed in the NMMAPS II study of 88 cities is present forsingle-pollutant models. PM effects from two-pollutant, or multipollutant models, would beexpected to have shown even more heterogeneity among the cities. Further, interpreting themeaning of an estimate of overall effect (0.5% increase for a 10 mcg/m 3 increase at lag 1)assumes that the effects across city come from a single distribution of effects, which might notbe the case if heterogeneity of effects is real and due to some as yet to be identified factor(s) thatdistinguishes cities in which effects are detected from those in which they are not. The cause(s)of this apparent heterogeneity (random variation or “real”) clearly has implications for settingUS-wide, health-based standards.An attempt is made to explain part of the observed heterogeneity of effect by noting thatnegative or absent effects were more likely to be seen in cities with the lower concentrations ofPM (6-263, line 30-). This is unjustified given that the NMMAPS investigators explicitly testedthat hypothesis and found no support for it.5. Chronic effectsAlthough the cohort studies are invariably referred to as studies evaluating the effects of“long-term”, or “chronic”, exposure, this is an assumption. The title of section 6.2.3 (“MortalityEffects of Long-Term Exposure to Ambient Particulate Matter”) already makes this assumption.Merely because exposure in these studies is specified in terms of long-term averages, this doesnot imply that the observed associations are in fact due to these long-term averages. Analternative is that these effects are simply a cumulation of acute effects. It is argued that simpleaccumulation of acute effects cannot account for the size of effects estimated in the cohortstudies. However, these estimates are somewhat sensitive to covariates and analytic approach(e.g., adjustment for population mobility, spatial correlation and control for SO 2 as demonstratedin the ACS Reanalysis Study). Therefore, confidence in the size of these reported effectestimates is not great. Reflecting even more confusion is the statement in the CD (p.6-80, line31) that chronic effects must be present since effect estimates for chronic PM exposure are muchhigher than those for the time series studies; this point is irrelevant and in no way argues that achronic effect above that observed in the time series studies must be present (this is an “applesand oranges” comparison). The comparison of the spatial features of effects from NMMAPS IIand the Cohort Reanalysis Project (6-265, line 24-31) does not necessarily enhance the argumentfor consistency, given the above.Effects of acute exposures can theoretically be approximated by calculating a cumulationof acute effects, something which has been attempted previously. I recommend revisiting theissue of cumulating time series effects (incorporating the impact of multiple days) to compare tothe range of estimates of PM effect from the cohort studies (esp. the ranges of effects estimatedin the ACS reanalyses based on different models). If this argument is convincing in showingthat acute effects could not conceivably reproduce findings from the cohort studies, then theabove points become moot. The lung cancer findings, if valid, would provide a strong argumentfor chronic effects, but this discussion is largely lacking from this version of the CD.In my opinion, given the above, the conclusion regarding “long-term exposure to PM”(6-94) needs to be qualified.A - 13