These factors make a difficult epidemiological challenge even tougher. Doctors and toxicologists say symptoms reported by people working or living near the gas fields are often transient and irregular. They say they need precise data on the prevalence and onset of medical conditions, as well as from air and water sampling, to properly assess the hazards of drilling.
"There are considerable issues about health effects," said John Deutch, former director of the CIA and a professor of chemistry at MIT, who heads a Department of Energy panel examining the environmental effects of shale gas drilling, with an emphasis on hydraulic fracturing. "Frankly, I'm not even sure ... what serious public health work has been done in making a connection."
The health questions are intensifying at a moment when communities and states are already weighing the benefits and costs of drilling for natural gas. Drilling has brought much-needed jobs and cash infusions to some of the nation's poorer regions; bullish estimates of U.S. gas reserves promise plenty of drilling development in the future. At the same time, fracking's lasting environmental toll—particularly the threat it may pose to water supplies—has become the subject of intense debate. Since 2008, ProPublica has reported about hundreds of cases of water contamination in more than six states where drilling and fracking are taking place as well as the difficulties of handling the vast quantities of waste the drilling processes produce.
Medical and government groups are beginning to sound alarms about drilling's potential to damage health.
In May, Sen. Robert Casey Jr., D-Pa., wrote to Environmental Protection Agency administrator Lisa Jackson, the Centers for Disease Control and Prevention and state officials, asking them to investigate illness clusters in Pennsylvania. "Despite being above the normal rate, these disease groupings are often dismissed as statistically insignificant," Casey wrote.
In July, when the EPA proposed new emissions rules for the drilling industry, it warned that without them there could be an unacceptably high risk of cancer for people living close to major facilities. In August, a national association of childrens' doctors published a fact sheet detailing concerns about fracking and warning that children are more susceptible to chemical exposure. The group called for more epidemiological research and disclosure of chemicals used in drilling.
The gas drilling industry says it supports such research and that health concerns should be taken seriously, but that the public should be careful of jumping to conclusions. "Sound science does exist on these issues," wrote Chris Tucker, a spokesman for the industry group Energy in Depth, in an email. Tucker pointed to a case in Pennsylvania where a woman alleged that drilling had contaminated her water and made her sick. A state investigation found that her water was indeed foul, but that it had been that way long before drilling began. "Eventually, pretty firm conclusions can be made with respect to potential causes and effects. Unfortunately, it takes time to do all that in a rigorous, data-driven way."
No such research is under way on a significant scale, however.
Portier, whose agency is a sister agency of the CDC and charged with determining the toxicity of industrial chemicals and preventing exposure to them, says the anecdotal evidence of environmental illness is sufficient to warrant a more serious and systematic approach to studying it. His agency, in conjunction with the EPA, is performing at least five health consultations for communities concerned about health impacts, including two in Pennsylvania. These smaller-scale studies assess health risks based on data already collected, giving a snapshot of a community at a particular moment. But what's needed is a nationwide study that tracks people living close to drilling over time, Portier said. That could cost upward of $100 million. "We can't do everything yet," Portier said. "We only have so much money available."