Lead author Lisa McKenzie of Colorado School of Public Health said the findings, based on three years of monitoring, found a number of potentially toxic petroleum hydrocarbons -- including benzene, ethylbenzene, toluene, xylene, heptane, octane and diethylbenzene -- in the air near wells.
Benzene has been identified by the Environmental Protection Agency as a known carcinogen, McKenzie said.
"Our data show that it is important to include air pollution in the national dialogue on natural gas development that has focused largely on water exposures to hydraulic fracturing," McKenzie said in a statement. "Our results show that the non-cancer health impacts from air emissions due to natural gas development is greater for residents living closer to wells. The greatest health impact corresponds to the relatively short-term, but high emission, well completion period."
The researchers calculated there were elevated cancer risks for residents living nearer to the wells as compared to those residing further away, the report said. However, benzene was the major contributor to lifetime excess cancer risk from both scenarios.
Energy In Depth, the research arm of the Independent Petroleum Assn. of America, noted the Colorado researchers said they used air toxics data collected in Garfield County from January 2008 to November 2010 as part of a special study of short-term exposure as well as ongoing ambient air monitoring program data to estimate chronic exposures and health risk.
However, Colorado updated its regulatory requirements for oil and gas systems in February 2009, which means at least a portion of the data collected by the researchers were from an operating environment that, by law, no longer exists, Energy In Depth said in an issue alert.
The study used out-of-date emissions data and overestimated by a factor of 10 how long it takes to develop a new natural gas well, the industry group said, adding that the study failed to account for pollution from Interstate 70, a mile away from the gas wells.