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 the following:

 

• A annual spatially averaged PM-2.5 primary standard set at 15 ug/m3. In 2012, the annual average was lowed to 12 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.

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. The annual standard was lowered to 12 ug/m3 in 2012.

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