It's not just fog that hangs in the sky over San Francisco. Tiny specks of diesel soot, wood smoke, fly ash, and other substances are also suspended in the city's air, and in air throughout the region. Because they're small enough to be inhaled, these particles cause a range of serious health effects. NRDC researchers found that particle levels make Bay Area air unhealthful several days a year. Between 1992 and 1996 violations of the health standards governing particles dropped, but since that date there has been a steady and sharp increase in violations.

Findings
NRDC researchers examined data from the Bay Area Air Quality Management District on the number of days counties violated California's health standards for breathable particles during the years 1990 through 1999. The district includes all of Santa Clara, San Mateo, Alameda, Contra Costa, San Francisco, Marin, and Napa counties, and portions of Solano and Sonoma. According to the data, most Bay Area counties made progress in reducing these particles. The number of days when the standard was violated fell from 18 in 1992 to 10 in 1993, and continued to decline until 1996, when there were only three violation days. Since 1996, violations have again increased, with a sharp rise between 1998 and 1999, from five days to 12.