This external review draft is an updated revision of the Air Quality Criteria for Particulate Matter, published by the United States EPA (EPA) in 1996, and it will serve as the basis for reevaluating the current National Ambient Air Quality Standards (NAAQS) for particulate matter (PM) set in 1997. The present external review draft critically assessed the latest scientific information relative to determining the health and welfare effects associated with exposure to various concentrations of PM in ambient air. The document is not intended as a complete and detailed literature review, but it does thoroughly evaluate information relevant to PM NAAQS criteria development based on pertinent literature available through mid 1999. Two sections of the Clean Air Act (CAA), (Sections 108 and 109, U.S. Code 1991) govern the establishment, review, and revision of NAAQS. Section 108 directs the Administrator of the EPA to list pollutants that may reasonably be anticipated to endanger public health or welfare, and to issue air quality criteria for them. The air quality criteria are to reflect the latest scientific information useful in indicating the kind and extent of all exposure-related effects on public health and welfare that may be expected from the presence of the pollutant in ambient air.