On Jan. 31, 2011 photo, razor wire sits atop a border fence as a building in the Mexican border city of Tijuana sits behind, as seen from San Diego. (AP Photo/Gregory Bull)
More than half a million undocumented immigrants live in the Bay Area according to a new study by the Public Policy Institute of California.
The study is the first of its kind to try and measure the number of undocumented immigrants living in specific counties in California.
The PPIC used tax returns to make many of their measurements, looking at how many people filed taxes using an Individual Taxpayer Identification Number, which is given to taxpayers who do not have a Social Security Number.
Napa County is one of the tops in the state when it comes to undocumented immigrants. The PPIC says they make up 12% of the county's population.
That's just behind Monterey and San Benito counties which combine for 13.5%.
In Imperial County, undocumented immigrants make up 12.8% of the population.
As for the other Bay Area counties:
For a look at the full breakdown, click here.
You can click here for the report issued by the PPIC.