More than 30 percent of Californians living near oil and gas wells reside in areas already heavily polluted, the Natural Resources Defense Council said.

A Wednesday report from the NRDC looked at state records to determine what portion of the state's population are, or could be, impacted by hydraulic fracturing operations. It found about 14 percent of the state's population live within a mile of an oil or natural gas well and more than 30 percent of those live near industrial facilities or other environmental dangers.

The advocacy group found that percentage of the population may be at risk should the state's oil and natural gas sector expand. It said it found a link between hydraulic fracturing operations and respiratory and neurological problems.

"Fracking is moving next door to more and more California homes, schools and neighborhoods," Miriam Rotkin-Ellman, a researcher at NRDC said in a statement. "From Los Angeles to the state's farms and ranches, this industry must not be allowed to poison our people's health."

Environmental advocacy groups have backed ballot initiatives in California that would limit or ban hydraulic fracturing, known also as fracking. Gov. Jerry Brown last year signed legislation regulating the controversial drilling process.

California last year ranked third in the nation in terms of oil production.