The U.S. Environmental Protection Agency (EPA) is presently conducting a review of the carbon monoxide (CO) national ambient air quality standards (NAAQS). EPA's overall plan and schedule for this CO NAAQS review are presented in the Plan for Review of the Carbon Monoxide National Ambient Air Quality Standards (US EPA, 2008b), or Integrated Review Plan (IRP). This IRP (US EPA, 2008b) outlines the Clean Air Act (CAA) requirements related to the establishment and periodic review of the NAAQS and the process and schedule for conducting the current CO NAAQS review. It presents the key policy-relevant issues to be addressed in this review as a series of policy-relevant questions that will frame our approach to determining whether the current primary NAAQS for CO should be retained or revised. The IRP also discusses two key components in the NAAQS review process - an Integrated Science Assessment (ISA) and a Risk and Exposure Assessment (REA) - in addition to the policy assessment and rulemaking components that complete the review.