Original 1997 8-Hour
Ozone and PM-2.5 Standards
In September 1997, new 8-hour ozone and
PM-2.5 standards became effective in the United States.
The revisions to the ozone National Ambient
Air Quality Standards are as follows:
- A primary standard (to protect public
health) set at 0.08 part per million/ 8-hr average, 4th highest
daily maximum concentration, 3-yr average. The secondary standard
is identical to the primary standard.
- The concentration-based standard is determined
by identifying the 4th highest 8-hr daily maximum concentration
for each year and averaging it across a 3-year period. Violations
do not occur when the 3-year average of the 4th highest 8-hour
daily maximum concentration is less than 0.085 ppm.
For particulate matter, the agency implemented
- A annual spatially averaged PM-2.5 primary
standard set at 15 ug/m3.
- A 24-hour PM-2.5 primary standard set
at 65 ug/m3 at the 98th percentile.
- The standard for coarse particles (i.e.,
PM-10) remains essentially unchanged.
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On October 17, 2006, the EPA reduced the
24-hour primary PM-2.5 standard from 65 to 35 ug/m3 at the 98th
percentile. The annual PM-2.5 primary standard was retained at
15 ug/m3. On December 15, 2006, public health and environmental
groups filed suit against the U.S. EPA for refusing to strengthen
the PM-2.5 annual standard.
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