UNITED STATES ENVIRONMENTAL PROTECTIONAGENCY WASHINGTON, D.C. 20460



## SEP 2 5 2009 THE ADMINISTRATOR

Dr. Joseph D. Brain Chair, Carbon Monoxide Review Panel Dr. Jonathan M. Samet Chair, Clean Air Scientific Advisory Committee Science Advisory Board (1400F) U.S. Environmental Protection Agency 1200Pennsylvania Avenue, N.W. Washington, D.C. 20460 Subject: Clean Air ScientificAdvisory Committee's Peer Review ofthe U.S. Environmental Protection Agency's Integrated Science Assessment for CarbonMonoxide (First External Review Draft, March 2009)
Dear Drs. Brain and Sarnet: Thank you very much for your letter about the Clean Air Scientific Advisory Committee Carbon Monoxide Review Panel's May 12 and 13,2009,review of the EPA's Integrated Science Assessmentfor CarbonMonoxide (FirstExternal Review Draft). EPA greatly appreciates the CASAC Panel's time and thorough review ofthe draft ISA.
We are in the midst ofrevising the draft CO ISA and anticipate providing CASAC and the public with a second draft by late September 2009. Our revisions to the document will give full consideration to the CASAC's comments and the recommendations and responses to charge questions in your letter, as well as to the individual comments from CASAC panel members and the public. When we complete the second draft, we will provide 

### amore detailed summary of the revisions, similar to the summary of changes we recently provided to CASAC for the second draft Particulate Matter ISA. Some ofthe more substantial changes planned for the second External Review Draft of the CO ISA include: Expanded discussion and clarification of the criteria for assessing the scientific quality and relevance of epidemiologic, controlled human exposure and toxicological studies, and for the interpretation and integration of study results to draw conclusions on causality;

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Rlcycled/RacyclaMe PrintedythVegetable 011 Based Inkson 100% Poetconsumer,ProcessChlorine Free Recycled PaperIn the integrative discussion of CO-related health effects, further evaluation ofthe evidence and discussion of the challenges faced in distinguishing the independent effects of CO fiom those of other pollutantsin urban air, particularly in interpretation of epidemiologic study findings;
Revised and expanded characterizationofintra-urban spatial variability to focus more explicitlyon CO monitors located at various distances from major urban sources, particularly roadways; Description of the differing capabilities for detecting low-level CO concentrations with the newer trace-level CO monitors in comparison with the detection capabilities of the standard monitors that predominate in the network. Discussion will also be added on the effect of monitoring limitations on exposure measurement error and its relevance for interpretation of epidemiologic study results; Incorporation of additional details in the discussion of CO's atmospheric chemistry and physics relating to its role in climate forcing; Inclusion of a summary in the health effects chapter (Chapter 5) to encapsulate information on relevant CO health effects; and
Revised discussion of the evidence on potentially susceptible subpopulations,with emphasis on information regarding individuals with preexisting conditions, particularly cardiovascular disease. The ISA will more fully characterize potential modification of the consequences of ambient CO exposure by factors such as activity patterns and exposure to CO from nonambientor endogenous sources. Again, my thanks to you and the CASAC panel members for your review ofthe draft ISA. Your advice and detailed comments have been very valuable in guiding EPA's revisions to the draft ISA. The CASAC panel's work will help ensure that the best science is used in informing the regulatory process. Sincerely,

Lisa P. Jackson

cc: Holly Stallworth, DFO for chartered CASAC Ellen Rubin, DFO for CASAC CO Review Panel CASAC CO Review Panel