that define allowable concentrations of these substances in ambient air. In 1997, EPA concluded that several of our current national air quality standards do not provide sufficient public health protection. New, more stringent air quality standards were adopted for ozone and particulate matter. Implementation of these standards has been slowed by legal challenges, but the U.S. Supreme Court upheld most of EPA's air quality rules in February 2001. The Court approved EPA's new standard for particulates, which expanded regulation of fine pollution particles down to 2.5 microns. The Court approved EPA's new ozone standard as well, but ordered EPA to develop a new plan that sets out a schedule for when state and local air districts must be in compliance