(CASAC) Peer Review of EPA's Integrated Science Assessment

Dr. George ThurstonComments on SOx 2nd Draft ISA1. The framework for causal determination and judging the overall weight of evidence, ispresented in Chapter 1. Is this the appropriate approach? Is it appropriately applied inthe case of SOx? How could the framework or its application be refined?RESPONSE: Yes, I feel the staff has done an excellent job of properly incorporatingand explaining both the weigh of evidence criteria and the causality inference framework.2. The discussion of the atmospheric chemistry of SOx has been expanded to provide abetter characterization of the spatial heterogeneity of urban SO2 concentrations andcorrelations of SO2 with other pollutants. We also included new sections describingthe regulatory network and siting criteria with maps of SO2 and other monitors. Abrief section describing the available 5-minute SO2 data was also included. Inaddition, the relationships between outdoor, indoor, and personal exposure to SO2were clarified with additional details on sources of exposure error. Have theserevisions to Chapter 2 improved its assessment of the currently available scientificknowledge on atmospheric sciences and exposure and its relevance to the evaluationof human health effects presented in later chapters?RESPONSE: Yes, this is much improved, but I would like to see a better representationof the trends in short-term SO 2 concentrations (i.e., 1-hr maximum). While there hasclearly been improvement in annual average SO 2 concentrations, as would be expectedgiven the advent of the acid rain control program, these efforts have focused on loweringoverall tons of SO 2 emissions, and not on lowering maximum impacts. So the questionremains, have we seen a reduction in peak SO 2 exposures near major sources? As such, Iwould ask that plots of the distribution of 1-hour maximum values (for which many moresites are available than for 5-minute peaks) over time to better elucidate how muchimprovement we are making in lowering peak SO 2 exposures.Another issue I have with Chapter 2 is the inadequate coverage of the PM-SO 2interaction, and the potential for the co-presence of PM to increase lung dosage andhealth effects of SO 2 over and above that indicated by controlled animal and humanpopulations using pure SO 2 gas, without associated PM (which would be the case in thereal world). While the document includes research by Amdur on page 2-60, it does notinclude her work very clearly showing the way that co-presence of PM along with SO 2increases the impact of acute SO 2 exposure. While these studies are discussed inChapter 3, they are probable even more important to mention here as well, as they areimportant to our understanding of SO 2 doses in the real world. Now, while SO 2 has arelatively low Henry’s law constant, and it might therefore be expected that little will bestored as sulfite in the aqueous phase in a naturally occurring aerosol, but the Amdurstudies conversely imply that this SO 2 -PM interaction is a strong one, so the document48