Abstract:Carbon monoxide (CO) emission, concentration, and exposure trends in California and the United States were analyzed. The study also included data analyses for northern and southern California CO monitoring sites. Results demonstrate that CO concentrations are decreasing at both the neighborhood and micro scale levels. The micro scale concentration decreases are correlated with decreases in regional emissions and are projected to continue into the future. Due to declining CO concentrations and the relationship between neighborhood and microscale conditions, the study recommends that the U.S. Environmental Protection Agency reevaluate the conformity requirements to conduct transportation project-level CO analyses. The findings from this report suggest that conformity CO hot spot analysis requirements could be appropriately limited to unusual circumstances identified through interagency consultation.