Residents of two northeastern Pennsylvania counties with a high density of active shale gas wells were more likely to be admitted to the hospital for heart, nervous system and other medical conditions than residents in neighboring areas with no drilling, researchers at the University of Pennsylvania and Columbia University reported in a study published Wednesday in the peer-reviewed multidisciplinary science journal PLOS One.

The researchers analyzed state drilling databases and more than 95,000 inpatient records to look for links between natural gas production activities and health care use from 2007 to 2011. They looked at hospitalization rates for residents of Bradford and Susquehanna counties, where shale gas drilling grew dramatically during the study period, and neighboring Wayne County, where the demographics are similar, but drilling has been prohibited because the county lies within the Delaware River watershed.

The researchers do not say that drilling activities caused the residents’ health problems, but co-author Reynold Panettieri, a pulmonologist and the deputy director of the University of Pennsylvania’s Center of Excellence in Environmental Toxicology, said they suspect that collective exposures from increased drilling — which can include “noise, the trucks, the drilling, the flaring, the anxiety,” he said — “all can impart an aberrant stress response on the body that could make people more susceptible.”

They found a significant association between hospitalization rates for cardiological or neurologic conditions and areas with either a higher number or higher density of wells, while hospitalizations for other medical categories, including skin and urological conditions and cancer, had a weaker association with drilling activity. Inpatient prevalence rates for most of the 25 medical categories the researchers studied remained relatively stable over the five-year period, they found.

Dr. Panettieri said the most profound finding was the rate of cardiovascular hospitalizations: ZIP codes that went from having no wells to the highest density of wells in the study area would be expected to have a 27 percent increase in cardiology hospitalization rates.

“With an inpatient stay costing on average [$30,000], this poses a significant economic health burden to the [commonwealth],” the researchers wrote.

Some doctors and researchers were skeptical about the paper’s conclusions.

Theodore Them, the chief physician in the Occupational and Environmental Medicine section at the Guthrie Medical Group, based in Bradford County, said changes in hospitalization rates in the region may be explainable by other aspects of the drilling boom, such as a population influx that was not fully captured by the 2010 census or much greater employment rates that gave more residents of the rural counties access to health insurance or more money to afford health care.

Furthermore, the primary regional hospital system is “a mecca for cardiology,” he noted.

“It’s just too confounded to draw any conclusions along these lines,” he said. “They are just looking at data and trying to draw inferences, and I think it’s weak.”

Bernard Goldstein, an environmental toxicologist and a former dean of the University of Pittsburgh Graduate School of Public Health, said the new paper is a solid example of an indirect study using existing data that helps highlight the need for more direct investigations into drilling-related health issues.

“They are careful in how they present this,” said Dr. Goldstein, who was not involved in the research. “They don’t present this as definitive evidence. It isn’t. But it is certainly something to make one even more upset about the fact that until now it has not been possible in Pennsylvania to do the kind of studies that would give a definitive answer.”

The paper’s authors said their research will help hone future studies that include individual monitoring or investigate connections to cardiovascular issues.

“This was a hypothesis-generating study,” said Trevor Penning, director of the University of Pennsylvania’s Center of Excellence in Environmental Toxicology, and a co-author of the paper. “This gives us a window into being able to design more focused studies moving forward having now seen this signal.”