This document critically assesses the latest scientific information concerning health and welfare effects associated with the presence of various concentrations of lead (Pb) in ambient air, as pertinent to providing updated scientific bases for EPA's periodic review of the National Ambient Air Quality Standards for Lead (Pb NAAQS). As such, this document builds upon previous assessments published by the U.S. Environmental Protection Agency (EPA), including: (a) the 1977 EPA document, Air Quality Criteria for Lead; (b) an updated revision of that Lead Air Quality Criteria Document and an accompanying Addendum published in 1986 (1986 Lead AQCD/Addendum); and (c) an associated 1990 Supplement to that 1986 AQCD/Addendum. This document focuses on evaluation and integration of information relevant to Pb NAAQS criteria development that has become available mainly since that covered by the 1986 and 1990 criteria assessments.