This document describes EPA's decision to revise the national ambient air quality standards (NAAQS) for particulate matter (PM) based on its review of the available scientific evidence linking exposures to ambient PM to adverse health and welfare effects at levels allowed by the current PM standards. The current primary PM standards are revised in several respects: Two new PM<INF>2.5</INF> standards are added, set at 15 <greek-m>g/m<SUP>3</SUP> , based on the 3-year average of annual arithmetic mean PM<INF>2.5</INF> concentrations from single or multiple community-oriented monitors, and 65 <greek-m>g/m<SUP> 3</SUP> , based on the 3-year average of the 98<SUP>th</SUP> percentile of 24-hour PM<INF>2.5</INF> concentrations at each population-oriented monitor within an area; and the current 24-hour PM<INF>10</INF> standard is revised to be based on the 99<SUP>th</SUP> percentile of 24-hour PM<INF>10</INF> concentrations at each monitor within an area. The new suite of primary standards will provide increased protection against a wide range of PM-related health effects, including premature mortality and increased hospital admissions and emergency room visits, primarily in the elderly and individuals with cardiopulmonary disease; increased respiratory symptoms and disease, in children and individuals with cardiopulmonary disease such as asthma; decreased lung function, particularly in children and individuals with asthma; and alterations in lung tissue and structure and in respiratory tract defense mechanisms. The current secondary standards are revised by making them identical in all respects to the new suite of primary standards. The new secondary standards, in conjunction with a regional haze program, will provide appropriate protection against PM-related public welfare effects including soiling, material damage, and visibility impairment. In conjunction with the new PM<INF>2.5</INF> standards, a new reference method has been specified for monitoring PM as PM<INF>2.5</