The California Environmental Protection Agency has released a statewide list of census tracts most burdened by pollution. Many of the worst pollution pockets identified are in the San Joaquin Valley, Los Angeles County and the Inland Empire.
The screening tool, called CalEnviroScreen, was developed by the Office of Environmental Health Hazard Assessment, a branch of Cal/EPA, to pinpoint the communities with the highest exposure and vulnerability to multiple environmental hazards, including polluted air and water, waste facilities and contaminated soil. The rankings, however, are not based only on measures of environmental exposure. They also take into account socioeconomic characteristics and health data on residents to assess the overall vulnerability of communities. Those factors include poverty, education, unemployment, rates of asthma and low-birth-weight infants.
You can read more about it in this LA Times article.
As a side note, I find the CalEPA app a little slow in Internet Explorer. Google Chrome was a little faster. They are using ArcGIS Online technology for their mapping. If you do a search for “calenviroscreen” on ArcGIS.com, you will see a listing of their maps and apps for versions 1.1 and 2.0.