Exposure to more intense shale gas development correlates with a higher risk of asthma attacks among asthma patients, according to a new study of Pennsylvania's Marcellus Shale, one of the nation's largest and most active fracking regions.

The paper, published Monday in JAMA Internal Medicine, a publication of the American Medical Association, didn't examine the exact cause of the trend. But lead author Sara Rasmussen, a Ph.D. candidate in environmental health sciences at Johns Hopkins Bloomberg School of Public Health, said air pollution and stress are both plausible explanations.

Natural gas development releases various air pollutants including particulate matter, volatile organic compounds and sulfur dioxide. The equipment also produces loud noises and bright lights, which can increase anxiety and sleeplessness. Years of research show that all these factors can exacerbate asthma.

The paper adds to the growing research linking the natural gas industry to various health impacts including birth defects, respiratory problems and skin rashes. Rasmussen and her six co-authors launched the study in 2012, about four years after the Marcellus Shale boom took off in Pennsylvania.

Nicole Deziel, an assistant professor of epidemiology at Yale School of Public Health who was not involved in the paper, praised the scientists' rigorous research methods. It "represents an important advancement," she said in an email. "More health studies like this are needed."

Previous studies relied heavily on less objective data, such as self-reported symptoms, Rasmussen said. But her team had access to detailed medical records from the Geisinger Health System. Geisinger provides care for more than 400,000 Pennsylvania residents, including many who live near shale wells.

Rasmussen said the paper is the first to examine unconventional natural gas development and "objective respiratory impacts," which refer to incidents reported to health providers in the form of hospital visits or prescriptions.

The researchers identified 35,508 patients ages 5 to 90 with a history of asthma between 2005 and 2012. They then located the patients who reported asthma attacks to Geisinger, and analyzed the intensity of shale gas development near their homes in the days leading up to the attacks. They also calculated the level of shale activity near the homes of asthma patients who did not report asthma attacks.

Intensity levels were calculated from several factors, such as the density of wells near the home and the home's proximity to those wells; the well's depth and gas production volume; and the well's stage of development (pad preparation, drilling, hydraulic fracturing or production).

Wells with heavy production activity, referring to those that produced the most gas, had the greatest effect. The researchers ranked the patients' homes based on the surrounding shale activity, and found that residents who lived in homes ranked among the top 25 percent for production activity were four times more likely to have a mild asthma attack, and 1.7 times more likely to suffer a severe asthma attack, than those in the bottom 25 percent.

The scientists controlled for other factors that could have influenced the results, such as the patient's smoking history, socioeconomic status and the home's proximity to the nearest road, which could affect air pollution levels.

Deziel, who has studied the potential health impacts of hydraulic fracturing fluids, said the study was "well-designed," with "numerous strengths, including the large study population, the use of medical records to determine asthma exacerbations, and the inclusion of several other factors that could have explained the observed associations."

Rasmussen said their biggest challenge was obtaining accurate data on the wells' phases of development. State drilling records were often spotty or inconsistent, she said, so they drew on data from several regulatory agencies as well as SkyTruth, a website that crowdsources aerial photos of well pad locations, to fill in the gaps.

Deziel said the researchers' analysis of the activity occurring at each well " is an important step towards a better understanding of whether a certain aspect of the unconventional development process may be more hazardous."

Bernard Goldstein, an emeritus professor at the University of Pittsburgh School of Public Health, said the study's findings are important but should come as no surprise, given what's known about links between air pollution and asthma. "There are regional ozone problems that could easily be exacerbated by fracking operations."

Funding for the study came from the National Institute of Environmental Health Sciences, the National Science Foundation, the Degenstein Foundation and the Robert Wood Johnson Foundation. One of the authors, Brian Schwartz, disclosed that he is an unpaid, informal adviser to the Post Carbon Institute, a California sustainability think tank. He said the relationship did not pose a conflict of interest because the institute played no role in this study. Schwartz is an environmental health sciences professor at Johns Hopkins and Rasmussen's faculty adviser.

The next step in this type of public health research is to study the mechanisms that lead to increased asthma attacks, and determine whether air pollution, psychological stress or another factor is the real culprit, Rasmussen said.

That would involve installing air pollution and noise monitors in communities, and using personal monitors to track residents' pollution exposure.

"For the most part, epidemiologists will never say one study proves cause," she said, so it could take many years to reach definitive conclusions.