EPA Particulate Matter Standards
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.
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