Ground-level ozone is the main ingredient of smog and a major contributor to respiratory illness. Vehicle tailpipe emissions are the single largest source of the chemicals that create ozone pollution. These emissions are projected to increase, along with the region's growing population, and they will, in turn, increase levels of ozone -- unless the Bay Area cuts down on its use of diesel- and gasoline-powered engines.

Findings
NRDC analysts examined the number of days each year between 1990 and 1999 on which state ozone standards were exceeded in the Bay Area, using data from the Bay Area Air Quality Management District. The district includes all of Santa Clara, San Mateo, Alameda, Contra Costa, San Francisco, Marin, and Napa counties, and portions of Solano and Sonoma. The number of days the district as a whole exceeded the state ozone standard fluctuated. The worst year was 1996, with 34 days. But by 1999, the last year reported, violations had dropped to 20.

Violations varied by county, with geography playing a major role. Coastal areas recorded fewer violations than inland areas, for instance. Livermore, in Alameda County, showed the worst ozone pollution; violations peaked in 1996, with 22 days above the standard. The southern portion of Santa Clara County also exceeded the standard on many days: San Jose surpassed ozone standards on 14 days in 1995, Los Gatos on 13. San Francisco, Oakland, Mountain View, Napa, and Santa Rosa consistently had the fewest violations.