A natural gas well site near a residential neighborhood, run by Encana Oil & Gas, in Erie, Colo., in September 2013. Brennan Linsley/AP

Living near hydraulic fracturing — or fracking — sites may increase the risk of some birth defects by as much as 30 percent, a new study suggests. In the U.S., more than 15 million people now live within a mile of a well.

The use of fracking, a gas-extraction process through which sand, water and chemicals are pumped into the ground to release trapped fuel deposits, has increased significantly in the U.S. over the past decade. Five years ago, the U.S. produced 5 million barrels of oil per day; today, it's 7.4 million, thanks largely to fracking.

Supporters of the industry say it creates jobs and spurs the economy, while critics say its development is largely unregulated and that too little is known about pollution and health risks.

The report by the Colorado School of Public Health, released Jan. 28, gathered evidence from heavily drilled rural Colorado, which has among the highest densities of oil and gas wells in the U.S.

“What we found was that the risk of congenital heart defects (CHD) increased with greater density of gas wells — with mothers living in the highest-density areas at greatest risk,” Lisa McKenzie, a research associate at the Colorado School of Public Health and the lead author of the study, told Al Jazeera.

The study examined links between the mother’s residential proximity to natural gas wells and birth defects in a study of more than 124,842 births from 1996 to 2009 in rural Colorado.

The study found that “births to mothers in the most exposed (areas with over 125 wells per mile) had a 30 percent greater prevalence of CHDs than births to mothers with no wells in a 10-mile radius of their residence.”

Many pollutants that are suspected of increasing the risk of birth defects are emitted into the air during development and production of natural gas, the report said.

McKenzie added that the study is not conclusive but found an “association.” But critics of the oil and gas industry were not so cautious about drawing conclusions from the evidence.

“This study suggests that if you want to have a healthy baby and you live near a fracking site, move,” Gary Wockner of Colorado’s Clean Water Action, said.

Colorado has more than 50,000 active oil and gas wells — including more than 20,000 in northern Weld County. Wockner told Al Jazeera the industry predicts another 50,000 wells will be added over the next 15 to 20 years in the state, “so the public health impact is of extreme concern.”

“The shocking story here is that fracking has moved forward with virtually no regulation and no study of public health impacts.”

The U.S. Environmental Protection Agency (EPA) is conducting an in-depth study on the potential impact of fracking on water resources, the findings of which are expected to be released late this year.