This first external review draft Integrated Science Assessment (ISA) is a concise review, synthesis, and evaluation of the most policy-relevant evidence, and communicates critical science judgments relevant to the National Ambient Air Quality Standards (NAAQS) review. As such, the ISA forms the scientific foundation for the review of the primary (health-based) and secondary (welfare-based) NAAQS for lead (Pb). The ISA accurately reflects 'the latest scientific knowledge useful in indicating the kind and extent of identifiable effects on public health which may be expected from the presence of (a) pollutant in ambient air' (42 U.S.C. 7408). Key information and judgments formerly contained in an Air Quality Criteria Document (AQCD) for Pb are incorporated in this assessment. This ISA thus serves to update and revise the evaluation of the scientific evidence available at the time of the previous review of the NAAQS for Pb that was concluded in 2008. The draft Integrated Review Plan for the National Ambient Air Quality Standards for Lead (U.S. EPA, 2011) identifies a series of policy-relevant questions that provide a framework for this assessment of the scientific evidence. These questions frame the entire review of the NAAQS for Pb, and thus are informed by both science and policy considerations. The ISA organizes, presents, and integrates the scientific evidence which is considered along with findings from any risk analyses and policy considerations to help the U.S. Environmental Protection Agency (EPA) address these questions during the NAAQS review for Pb.