U.S. Environmental Protection Agency


Date of this Version



December 2008


This Integrated Science Assessment (ISA) is a synthesis and evaluation of the most policy-relevant science that forms the scientific foundation for the review of the secondary (welfare-based) national ambient air quality standards (NAAQS) for oxides of nitrogen (NOx) and oxides of sulfur (SOx). The Clean Air Act definition of welfare effects includes, but is not limited to, effects on soils, water, wildlife, vegetation, visibility, weather, and climate, as well as effects on man-made materials, economic values, and personal comfort and well-being. The current secondary NAAQS for SOx, set in 1973, is a 3-hour average of 0.5 ppm1 sulfur dioxide (SO2), not to be exceeded more than once per year. The secondary NAAQS for NOx is identical to the primary standard set in 1971: an annual average not to exceed 0.053 ppm nitrogen dioxide (NO2). The current secondary NAAQS were set to protect against direct damage to vegetation by exposure to gas-phase SOx or NOx.