Contra Costa County is a region located in the U. S. state of California, in the east bay of the San Francisco Bay Area.
Contra Costa Countyis part of the Bay Area and is considered an East Bay region.
This area is home to oil refineries in Hercules, Antioquia and Martínez, which have recorded emissions of hazardous materials, but these cities have more prosperous populations and, therefore, lower crime rates than Richmond. State Route 24, which passes through the Caldecott Tunnel, is the main entrance to Contra Costa from San Francisco, Oakland and Berkeley. Interstate 580 connects Contra Costa to the center of Alameda County to the west and to Stockton and the Central Valley to the east. There are no major airports in Contra Costa County with regular passenger service, although there are several general aviation airports. Interstate 680 is the main route that connects Contra Costa to South Bay and Silicon Valley to the south, and to Sacramento to the north. The hills that keep much of Contra Costa culturally isolated also provide a climate-protected environment; the area is usually 10 to 20° F warmer than the bay.
With over one million residents, Contra Costa County was one of the original 27 counties established in California in 1850.