Based on its review of the currently available scientific information and the current secondary NAAQS for NOx and SOx, EPA is retaining the existing NOx and SOx secondary standards to address the direct effects on vegetation of exposure to gaseous oxides of nitrogen and sulfur in the air (e.g., decreased growth and foliar injury). The existing secondary standards are:
For NO: 0.053 ppm (parts per million) averaged over a year; and
For SO: 0.5 ppm averaged over three hours, not to be exceeded more than once per year.
This final rule recognizes that the existing secondary NOx and SOx standards do not provide adequate protection from these harmful deposition-related effects. While there is strong scientific support for developing a multi-pollutant standard to address these deposition-related effects, EPA does not yet have enough information to set a multi-pollutant standard that would adequately protect the diverse ecosystems across the country.
The Clean Air Act requires EPA to set NAAQS for "criteria pollutants." Currently, six major pollutants are criteria pollutants. In addition to sulfur oxides and nitrogen oxides, the criteria pollutants include ozone, lead, carbon monoxide, and particulate matter. The law also requires EPA to review the standards periodically and revise them if appropriate to ensure that they provide the requisite amount of health and environmental protection and to update those standards as necessary. (EPA)
To download a copy of the final rules [NOx & SOx]