Also see: Legal: International

Federal PM2.5 NAAQS: US Environmental Protection Agency (EPA) established the PM2.5 NAAQS, October 2004 to be protective of human health with an adequate margin of safety. as the relevant health benchmark. The values are:

* Annual 15 µg/m3
* 24 Hour 65 µg/m3

Particulate Matter

EPA published final revisions to its particulate matter standards in July of 1997. (The standard had last been revised in 1987.) As noted above, the revisions included the addition of a PM2.5 standard. EPA developed the revised standards in response to research indicating that health effects were occurring even when levels of particulate matter in the air were in compliance with the PM10 standard, indicating that fine particles (PM2.5) are largely responsible for the health effects of greatest concern. In addition, EPA indicated that the existing PM10 standard did not adequately protect visibility. EPA retained the original annual PM10 standard, revised the 24-hour PM10 standard by changing its form to be more protective against short-term exposure to coarse particles and added the following PM2.5 standards:

65 mg/m3 PM2.5 24 hr (based on 98th percentile of data collected and averaged over three years)
15 mg/m3 PM2.5 annual (averaged over three years)
The new EPA standards were challenged in court, and in May of 1999 a three-judge panel of the U.S. Court of Appeals for the District of Columbia Circuit issued an opinion regarding, in part, EPA's new PM2.5 standard, explaining that the EPA must have a clearly articulated rationale, or that Congress must provide a rationale, for selecting a standard that could affect the national economy. The Court did not question the science and process used by EPA to set the more protective standards but questioned the criterion used to determine where the standard was set. On June 18, 1999 the Court ruled that the PM2.5 standard should remain in place, but that parties will be allowed to apply for the standard to be vacated if "the presence of this standard threatens a more imminent harm." On June 28, 1999 EPA and the Department of Justice filed a petition asking the full DC Circuit to reverse the decision.