EPA has signed a proposed rule updating national ambient air quality standards
(NAAQS) for nitrogen oxides (NO2) and sulfur oxides (SO2). EPA proposes to retain
the existing secondary standards for NO2 and SO2 and establish an additional set
of secondary standards identical to health-based primary standards set by the
agency in 2010. The existing secondary standards are 0.053 ppm (NO2) averaged
annually and 0.5 ppm (SO2) averaged over three hours and not to be exceeded
more than once annually. The health-based standards are 100 ppb (NO2)
averaged hourly and 75 ppb (SO2) averaged hourly. EPA will accept comments on
the proposed rule for 60 days after it is published in the Federal Register.