Is fracking taking place too close to homes and schools? According to a new analysis by a group of scientists, the answer is yes.

The study, published in the journal Environmental Health Perspectives, found setbacks in Pennsylvania, Colorado and Texas to be too close to peoples’ homes. According to the study, the current setbacks in these states leave residents vulnerable to explosions from well blowouts and to air pollution generated at wells “above health-based risk levels.”

Pennsylvania’s setback is 500 feet from any occupied building. Texas’ is 200 feet; Colorado’s is 500 to 1,000 feet.

“Five hundred feet, we know, is not sufficient,” says Marsha Haley, assistant professor of radiation oncology at the University of Pittsburgh and the study’s lead author. “Unfortunately, there’s no defined safe setback distance.”

LISTEN: “Is Gas Drilling Happening Too Close to Homes?”

The University of Maryland’s School of Public Health recommended that state set a distance of 2,000 feet from any well. “I’d probably say that’s a good minimum distance,” Haley said at a press conference called by environmental groups to deliver the paper to the Pennsylvania Department of Environmental Protection’s Pittsburgh office. “We know there are studies that show increased hospitalization rates, decreased birth weights and increased cancer risks in those that live that close to a well.”

The state’s minimum setback was defended by one of the main architects of Act 13, the 2012 law that set those buffers. Drew Crompton, chief of staff for Pennsylvania Senate Pro Tem Joseph Scarnati, says the legislature created the setback distances in consultation with environmental organizations, who “endorsed the 500-foot buffer.”

“There was a full analysis done, including a review of other state buffers, prior to the determination of 500 feet,” Crompton said in an email.

Crompton says the study used outdated information and that the state’s environmental record was considerably better after the law took effect.

“Energy development inherently has risks. However the environmental requirements set forth in Act 13 are indeed lowering that risk to a very reasonable level,” Crompton says.

The researchers painted a different picture of how states create their own buffer zones around oil and gas activities.

In most cases, setback distances aren’t derived from “peer-reviewed data, data-driven analysis or historical events,” they wrote. “[They] are a compromise between governments, the regulated community, environmental and citizen interest groups, and landowners.”

“Five hundred feet, we know, is not sufficient. Unfortunately, there’s no defined safe setback distance.”

The study noted two areas of concern: explosions from well blowouts and pollution from wells, like hazardous sulfide gases and carcinogens such as benzene.

The authors cited air samples taken in 2013 by co-author Michael McCawley, a professor in West Virginia University’s School of Public Health. He sampled air near seven Marcellus shale gas wells in West Virginia; each sample was taken 625 feet from the well.

In about one third of the samples taken, levels of benzene exceeded minimum risk levels for acute exposure established by the CDC—including one case, which was nine times higher than the risk level set by the government.

McCawley says some of the hazards posed by gas development aren’t necessarily from the well itself. “If you’re ever around a pad, you notice there are operations going on for some distance away from the pad—in fact, for essentially miles if you consider the transportation sources,” McCawley says.

He says the diesel exhaust from trucks delivering water and other materials for hydraulic fracturing can also pose risks. Particulates from diesel exhaust are a known carcinogen.

As drilling has expanded in recent years in Pennsylvania, setbacks from schools and buildings have become a concern in some communities. A 2013 report from the environmental group Penn Environment found 26 wells permitted in the state within a half mile from a school.

DEP spokesman John Poister said he would deliver the paper to the agency’s oil and gas office but declined to comment on the study.