This amended Quantitative Risk and Exposure Assessment for Carbon Monoxide (CO REA) contains revised results reflecting corrections to one of the input variables used in the dose model to generate estimates of carboxyhemoglobin (COHb). Modeling results presented in the May 2010 CO REA were generated with erroneous values for altitude for both the Los Angeles and Denver study areas. This error has been corrected and the model simulations repeated resulting in increases to the COHb estimates in both study areas. These increases, in terms of the ambient contribution to COHb levels, were small for the Denver study area and negligible for the Los Angeles study area, with somewhat larger increases to the total COHb estimates. Because altitude was not used in calculating exposure estimates, none of the exposure results previously presented were affected by this error. The sections of the REA revised to reflect the corrected dose estimates include several tables and associated text in chapters 5, 6, 7 and 8 and Appendix B. The specific areas of the report containing revisions to address this error include section 5.10.4 (Table 5-27), section 6.2 (Tables 6-15 through 6-23, Figures 6-5 and 6-6), section 6.3 (Tables 6-24 and 6-25), section 184.108.40.206 (Table 7-4), section 7.2.2 (Tables 7-5 through 7-8, 7-10, 7-11, and 7-14), chapter 8 and Appendix B.6 (Tables B-4 through B-6, Figures B-2 through B-5). In addition, the presentation of exposure estimates has been modified to reflect a correction to the rounding convention for percentages of the population less than 0.1%. Previously, values between 0.05 and 0.09% were rounded upwards to 0.1%. This correction is reflected in the presentation of exposure estimates in Tables 6-4, 6-7, 6-10, 6-13, and 6-14.