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New Ozone and Particulate Matter Standards
Note: EPA no longer updates this information, but it may be useful as a reference or resource.
Please see www.epa.gov/airtrends for the latest information on Air Quality Trends.
In July 1997, EPA revised the ozone and particulate matter standards following a lengthy scientific review process. Prior to this time, the PM standard applied to particles whose aerodynamic size is less than or equal to 10 micrometers, or PM10. The standard's revision strengthened protection against particles in the smaller part of that range by adding an indicator for PM2.5 (those whose aerodynamic size is less than or equal to 2.5 micrometers). The combination of the PM10 and PM2.5 indicators will provide protection against a wide array of particles. Since this brochure deals with data for and prior to 1996, the trend data for ozone and PM10 are compared to the preexisting standards.