Clean Air Act Section 109(d) requires EPA to review national ambient air quality standards (NAAQS) at least every 5 years, and revise them if newly available information indicates that the existing standards are not adequate to protect public health and welfare. The last EPA review of the primary (health based) NAAQS for nitrogen oxides (NOx) was completed in 2010. In that review, the agency added a new one-hour standard at a level of 100 ppb, while retaining the prior annual standard of 53 ppb, set in 1971.
EPA is now nearing the end of the next review of the primary NOx standards, and recently released a proposal not to revise the standards adopted in 2010. (The NOx secondary standards are being reviewed separately.) With respect to the one-hour standard, the proposal finds that the strongest evidence continues to come from human exposure studies of respiratory effects in asthmatics following short-term exposures (typically minutes to hours). Most of these studies were available in the last review, and an updated meta-analysis continues to show effects in some asthmatics following resting exposures to NOx concentrations from 100 to 530 ppb. However, the current data remain insufficient to calculate a reliable dose-response relationship between exposure and effects, and there is uncertainty regarding the potential adversity of reported responses, particularly at the lower exposure levels. While supporting evidence from epidemiological studies reduces uncertainty from the last review, the newly available studies do not fundamentally alter EPA’s prior understanding.
The annual standard is based on a reported relationship between long-term NOx exposures and asthma development in children. The strongest evidence supporting this conclusion comes from recent epidemiologic studies. While these studies strengthen the evidence for effects from long-term exposures presented in the 2010 review, they are subject to uncertainties resulting from the methods used to calculate exposures, the high correlations between NOx and other traffic-related pollutants, and the lack of information regarding the extent to which reported effects are independently associated with NOx rather than the overall mixture of traffic-related pollutants. Additionally, while the current evidence reports adverse effects (emergency department visits, hospital admissions, asthma incidence) in locations likely to have violated the current standards, studies do not indicate such associations in locations that would clearly have met those standards.
EPA also considered quantitative analyses estimating the potential for adverse NOx exposures that could be allowed by the current standards. Overall, these analyses indicate that the current standards provide substantial protection against adverse effects even under worst-case conditions across a variety of study areas in the U.S.
On the basis of these findings, EPA proposes to retain the current standards. The proposal is consistent with the advice of EPA staff in the NOx Policy Assessment and with the advice of the agency’s Clean Air Scientific Advisory Committee. Comments on the proposal will be due 60 days after it is published in the Federal Register.
Beyond this NOx review, a couple of these findings may affect the pending reviews of the PM and other NAAQS. These include recognition of the uncertainties in the studies at lower exposure levels, and the inability to separate NOx effects from the rest of the traffic pollution mixture.