U.S. Environmental Protection Agency


Date of this Version



July 2008


National Ambient Air Quality Standards (NAAQS) are promulgated by the U.S. Environmental Protection Agency (EPA) to meet requirements set forth in Sections 108 and 109 of the Clean Air Act (CAA). These sections require the EPA Administrator (1) to list widespread air pollutants that reasonably may be expected to endanger public health or welfare; (2) to issue air quality criteria that assess the latest available scientific information on the nature and effects of ambient exposure to the criteria pollutants; (3) to set “primary” NAAQS to protect human health with adequate margin of safety and to set “secondary” NAAQS to protect against welfare effects (e.g., effects on vegetation, ecosystems, visibility, climate, manmade materials, etc); and (4) to periodically review and revise, as appropriate, the criteria and NAAQS for a given listed pollutant or class of pollutants.
The purpose of this Integrated Science Assessment (ISA) for Oxides of Nitrogen (NOx) – Health Criteria is to critically evaluate and assess the latest scientific information published since the 1993 NOx Air Quality Criteria Document (AQCD), with the main focus on pertinent new information useful in evaluating health effects data associated with ambient air nitrogen oxides exposures. A First External Review Draft of this ISA (dated August 2007) was released for public comment and was reviewed by the Clean Air Scientific Advisory Committee (CASAC) in October 2007; a Second External Review Draft was made available to the public in March 2008. Public comments and CASAC recommendations have been taken into account in making revisions to the document for incorporation into this final ISA. This document will provide inputs to the risk and exposure analyses prepared by EPA’s Office of Air Quality Planning and Standards (OAQPS), which will lead to the proposal and, ultimately, promulgation of decisions on potential retention or revision, as appropriate, of the current Nitrogen Dioxide (NO2) NAAQS by the EPA Administrator.
Preparation of this document was coordinated by staff of EPA’s National Center for Environmental Assessment in Research Triangle Park (NCEA-RTP). NCEA-RTP scientific staff, together with experts from other EPA/Office of Research and Development (ORD) laboratories and academia, contributed to writing of document chapters. Earlier drafts of document materials were reviewed by non-EPA experts in peer consultation workshops held by EPA. This ISA describes the nature, sources, distribution, measurement, and concentrations of nitrogen oxides in outdoor (ambient) and indoor environments. It also evaluates the latest data on human exposures to ambient nitrogen oxides and consequent health effects in exposed human populations, to support decision making regarding the primary (health-based) NO2 NAAQS.
NCEA acknowledges the valuable contributions provided by authors, contributors, and reviewers and the diligence of its staff in the preparation of this document.