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Primary Standard The CASAC Ozone Review Panel agreed with the choice of indicator, statistical form and averaging time for the primary Ozone NAAQS suggested by Agency staff. The Final Ozone Staff Paper recommended that "consideration be given to a standard level within the range of somewhat below 0.080 ppm to 0.060 ppm," adding that "[s]tandard levels within this range that were considered in staff analyses of air quality, exposure, and risk include 0.074, 0.070, and 0.064 ppm, representative of levels within the upper, middle, and lower parts of this range, respectively." Reiterating what was stated in the CASAC's previous letter to you on this review (EPA-CASAC-07-001), Ozone Panel members were unanimous in recommending that the level of the current primary ozone standard should be lowered from 0.08 ppm to no greater than 0.070 ppm. The above-referenced CASAC letter (from October 24, 2006), in addition to EPA's own findings in the Final Ozone Air Quality Criteria Document (AQCD) and the Final Ozone Staff Paper, provide overwhelming scientific evidence for this recommendation. Furthermore, the Ozone Panel recommends that the NAAQS should be specified to the third decimal place of the ppm scale to avoid any rounding issues - as indicated by the standard levels that the Agency itself considered in the Final Ozone Staff Paper. Pursuant to the Clean Air Act, the primary NAAQS for criteria air pollutants must be set to protect the public health with an adequate margin of safety. Significantly, the Final Ozone Staff Paper does not address the issue of a margin of safety. (On page 6-86, the authors conclude that the proposed standard would "...provide an appropriate degree of public health protection...;" however, there is no explicit mention of a margin of safety, per se.) Such a discussion should be added to the document and taken into consideration in setting the primary ozone standard. • There is an underestimation of the affected population when one considers only twelve urban "Metropolitan Statistical Areas" (MBAs). The CASAC acknowledges that EPA may have intended to illustrate a range of impacts rather than be comprehensive in their analyses. However, it must be recognized that ozone is a regional pollutant that will affect people living outside these 12 MSAs, as well as inside and outside other urban areas. There is an urgent need to fund more research on the effects on sensitive subpopulations of low levels of the photochemical oxidant mixture for which ozone is used as a surro- gate. In addition to the three field studies pointing to higher responses to the oxidant mixtures than to pure ozone that the Agency has already referenced in the Final Ozone AQCD (1-3), three other such studies are referenced below (4-6). More information on the effects of low levels of oxidant mixtures on public health is essential to inform the future decision-making process. • Finally, with respect to policy-relevant background (PRB), the Ozone Panel wishes to point out that the Final Ozone Staff Paper does not provide a sufficient base of evidence from the peer-reviewed literature to suggest that the current approach to determining a PRB is the best method to make this estimation. One reason is that part of the PRB is not - ~~`