Based on its review of scientific evidence, the US Environmental Protection Agency (EPA) proposes retaining the current national ambient air quality standards (NAAQS) for oxides of nitrogen (NOx). EPA proposes that the current NAAQS don’t need to be changed because they provide the appropriate public health protection, with an adequate margin of safety, including for older adults, children and people with asthma.
There are currently two primary standards for NOx. NO2 is the component of oxides of nitrogen of greatest concern for health and is the indicator for the primary NAAQS. The two primary NO2 standards are: a 1-hour standard established in 2010 at a level of 100 parts per billion (ppb) and based on the 98th percentile of the annual distribution of daily maximum 1-hour NO2 concentrations, averaged over 3 years; and an annual standard, originally set in 1971, at a level of 53 ppb and based on annual average NO2 concentrations.
The Clean Air Act (CAA) requires the EPA to review periodically the air quality criteria and standards. This latest review of the primary (health-based) NO2 NAAQS is being conducted pursuant to these statutory requirements and in adherence to a schedule established by a federal court order, which requires signature of a proposed determination by 14 July 2017, and a final determination by 6 April 2018.
The last review of the primary NO2 NAAQS was in 2010. In that review, the EPA supplemented the existing primary annual NO2 standard by establishing a new short-term standard with a level of 100 ppb. Revisions to the NAAQS were accompanied by revisions to the data handling procedures and the ambient air monitoring and reporting requirements, including the establishment of requirements for states to locate monitors near heavily trafficked roadways in large urban areas and in other locations where maximum NO2 concentrations can occur.Like the 2010 review, the current review is focused on the health effects associated with gaseous oxides of nitrogen—including no2 and nitric oxide (NO), as well as their gaseous reaction products—and on the protection afforded by the primary NO2 standards. Total oxides of nitrogen include these gaseous species as well as particulate species (e.g., nitrates).
The EPA is separately considering the health and non-ecological welfare effects of particulate species in the review of the NAAQS for particulate matter. In addition, the EPA is separately reviewing the ecological welfare effects associated with oxides of nitrogen, oxides of sulfur, and PM, and the protection provided by the secondary NO2, SO2 and PM standards.
Among EPA’s findings from the current review:
Although studies suggest a potential for nO2-induced increases in airway responsiveness (AR) (which is a hallmark of asthma) in individuals with asthma, there is not an apparent dose-response relationship between NO2 exposure and increased AR and there is uncertainty regarding the potential adversity of reported responses. In addition, these studies are largely focused on adults with mild asthma, rather than adults or children with more severe cases of the disease.
New epidemiologic evidence in the current review is consistent with evidence from the last review and does not fundamentally alter the understanding of respiratory effects related to short-term NO2 exposures. While the fundamental understanding of such effects has not changed, recent epidemiologic studies do reduce some uncertainty from the last review by reporting health effect associations with short-term NO2 exposures in copollutant models and by their use of improved exposure metrics.
There is likely to be a causal relationship between long-term NO2 exposures and respiratory effects, based on the evidence for asthma development in children. While studies strengthen the evidence for effects of long-term exposures, compared to the last review, they are subject to uncertainties resulting from the methods used to assign NO2 exposures, the high correlations between NO2 and other traffic-related pollutants, and the lack of information regarding the extent to which reported effects are independently associated with NO2 rather than the overall mixture of traffic-related pollutants.
While the evidence supports the occurrence of adverse NO2-related respiratory effects at ambient NO2 concentrations likely to have been above those allowed by the current primary NO2 NAAQS, available studies do not call into question the adequacy of the public health protection provided by the current standards. In particular, compared to the last review when the 1-hour standard was set, evidence from controlled human exposure studies has not altered our understanding of the NO2 exposure concentrations that cause increased AR. In addition, while epidemiologic studies report relatively precise associations with serious NO2-related health outcomes (i.e., emergency department visits, hospital admissions, asthma incidence) in locations likely to have violated the current 1-hour and/or annual standards during portions of study periods, studies do not indicate such associations in locations with NO2 concentrations that would have clearly met those standards.
… the Administrator reaches the proposed conclusion that the current body of scientific evidence and the results of quantitative analyses support the degree of public health protection provided by the current 1-hour and annual primary NO2 standards and do not call into question any of the elements of those standards.—“Review of the Primary National Ambient Air Quality Standards for Oxides of Nitrogen”