Secondary National Ambient Air Quality Standards (NAAQS) for Nitrogen Dioxide (NO2) and Sulfur Dioxide (SO2)
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In 2012, EPA decided to retain the current secondary NAAQS for NO2 and SO2. The secondary NO2 standard is an annual arithmetic mean of 53 ppb. The secondary SO2 standard is 500 ppb averaged over three hours, not to be exceeded more than once per year.