NO2 Primary National Ambient Air Quality Standard

National Ambient Air Quality Standard (hereafter referred to as the "REA") assesses the adequacy of the current nitrogen dioxide (NO2) primary National Ambient Air Quality Standard (NAAQS) of 0.053 ppm (annual average) and alternative primary NO2 standards (US EPA, 2008a). This assessment is based on the scientific evidence provided in the NO2 Integrated Science Assessment for Oxides of Nitrogen (hereafter referred to as the "ISA") and the exposure and risk characterization data presented in the REA (US EPA, 2008a,b). US EPA concluded that "the scientific evidence reasonably supports a range of standard levels from 50 ppb to 200 ppb, with strong support for a level at or below 100 ppb " based on fewer NO2-related emergency department (ED) visits, on average, than those associated with just meeting the current standard. US EPA based this conclusion on epidemiology studies focused on these concentrations, and noted that this is also supported by "1) evidence from controlled human exposure studies of airway hyper-responsiveness in asthmatics, 2) controlled human exposure and animal toxicological studies of impaired host-defense systems and increased risk of susceptibility to viral and bacterial infection, and 3) controlled human exposure and animal toxicological studies of airway inflammation " (US EPA, 2008a). The REA fails to consider several issues, discussed below, which suggest that clinical and epidemiology studies do not provide a sufficient scientific basis for establishing