The Environmental Protection Agency (EPA) is proposing to tighten the National Ambient Air Quality Standards (NAAQS) for ground-level ozone, aka "smog." Specifically, the EPA is proposing to lower the ozone NAAQS from 0.08ppm to somewhere between 0.070ppm and 0.075ppm, though it will also consider tightening the standard to 0.060ppm and leaving the standard as is. EPA has posted background material on the proposed rule change here.

There was an all out political and legal brawl the last time the EPA tightened the ozone standard. In 1997 the Clinton Administration dropped the standard from 0.12ppm (measured as a one-hour average) to the current standard of 0.08ppm (measured as an eight-hour average). The EPA tightened the NAAQS for fine particulate matter at the same time. Industry groups and a handful of states fought the standards to the Supreme Court and back, but were ultimately unsuccessful. As the Washington Post reports, this proposal could prompt an equally fierce fight.

Under the Clean Air Act, the EPA is required to periodically reevaluate existing standards in light of new scientific evidence, and propose changes if the agency concludes that existing standards are insufficient to protect public health with an adequate margin of safety. The EPA's decision to reevaluate the ozone NAAQS was prompted by a lawsuit filed by several environmental groups, alleging that the agency had failed to conduct the required periodic review. Interestingly enough, litigation over the 1997 standard has continued through to this year, and the EPA has yet to finalize all the regulations necessary to implement the 1997 rule.