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Air Quality Index

Even though air quality is improving throughout the region, the standard for measuring ozone has become more stringent. On March 12, 2008, EPA revised its Air Quality Index (AQI) for ozone to reflect these changes. As a result, District residents may notice more Code Red and Code Orange days than in previous years.

The AQI revisions address the ranges of ozone that are represented by the AQI categories (see chart). EPA adjusted the upper end of the "moderate" range to 0.075 parts per million (ppm) to be equal to the new primary 8-hour ozone standard (i.e. ground level ozone concentrations averaged over an 8-hour period), and has made proportional changes to other categories. Under the revised AQI, ozone levels above 0.075 ppm would be considered in the "unhealthy for sensitive groups" category, and would result in a Code Orange day. When ozone is in this category, EPA recommends that certain groups adjust their activity levels to reduce ozone exposure.

The Clean Air Act requires the EPA to periodically re-evaluate the national ambient air quality standards for certain pollutants based on the most up-to-date scientific research. The 1997 standard for 8-hour ground-level ozone of 0.084 parts per million (ppm) was changed to account for findings from over 1,700 scientific studies that identified harmful health effects at the level of the old standard or below. EPA estimates the revised standards will yield health benefits valued between $2 billion and $17 billion.

The region's air quality forecasters anticipate at least twice as many exceedances during the 2008 ozone season as a result of the new standard. Ozone continues to be the District's most widespread air quality problem. "Ozone season" lasts from May to September.

Air Quality Action Guide

This printable brochure explains the Air Quality Rating and offers tips for improving air quality.

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