HARRISBURG — A group of environmental organizations, joined by celebrity activists Mark Ruffalo and Shailene Woodley, are urging Gov. Tom Wolf to halt new permits for fracking until the state investigates a news report that more than two dozen children in southwestern Pennsylvania, a hotbed for fracking activity, have developed cancer.

More than 100 groups and more than 800 individuals from Pennsylvania signed a letter asking for an official investigation into reporting by the Pittsburgh Post-Gazette of rare cancers in counties heavily impacted by shale gas development over the last decade.

The letter also calls for the governor to suspend all gas drilling permits until the investigation shows that fracking is not the cause of this emerging public health crisis.

“We have known for years that fracking presents clear threats to our drinking water, our air quality, and to the health and safety of residents in these sacrifice zones,” said Emily Wurth, Organizing Co-Director of Food & Water Watch. “But this investigative reporting points to an even more serious and heartbreaking toll on Pennsylvanians. Governor Wolf must take immediate action to protect the health and safety of his state.”

The Post-Gazette's investigative report documented at least 46 children in four counties in southwest Pennsylvania who have suffered from rare forms of cancer since 2008 — including at least 27 cases of Ewing sarcoma, a form of bone cancer that only affects between 200 and 250 people across the entire country each year.

The environmentalists plan to hold a rally Wednesday at the Capitol to call for action on the concerns.

"We have just received the letter and will review it. The Department of Health takes this issue seriously and wants to know about health concerns related to oil and natural gas production," said Wolf's spokesman J.J. Abbott. "We encourage any citizens with specific health complaints to send them to the Department of Health. The department investigates all complaints."

The reporters identified the cancer cases through interviews with family members, obituaries and social media postings, according to The Post-Gazette.

If there is a link between fracking activity and childhood cancer, it’s not clear how widespread it is because the state’s never fully researched the question and the reporters only looked at Washington, Greene, Fayette and Westmoreland counties, said Karen Feridun, co-founder of the Better Path Coalition, one of the co-authors of the letter.

“It’s never been studied,” Feridun said Monday. “It took the reporters to do it.”

The Department of Health did conduct a review based on earlier reporting by the Pittsburgh TV station WPXI focusing on cancer cases in Washington County. In a report released in April, the Department of Health announced there was no evidence that the cancer rates were any higher there than anywhere else in the state.

In that report, the agency said Health Department staff will “continue to closely monitor” for Ewing sarcoma and other types of pediatric cancer.”

In a story published Monday, the Post-Gazette reported that the Department of Health’s review didn’t include all of the cases they’d identified.

Feridun said that research in other parts of the country have also provided cause to be concerned about the health impacts of fracking. A Yale study found that at least 55 chemicals used in the fracking process are known, probable or possible carcinogens, she said. Another study in Colorado found that children who lived within 500 feet of gas wells had higher rates of leukemia.

“So, it’s not a brand new thing” to suggest that fracking has negative health effects, Feridun said.

In response to the environmentalists’ letter, the Marcellus Shale Coalition, which represents the natural gas industry, pointed to the Department of Health’s study, along with information from the American Cancer Society about Ewing tumors.

“There are no known lifestyle-related or environmental causes of Ewing tumors,” according to the cancer society.

In a separate letter to Wolf, David Spigelmyer, the president of the Marcellus Shale Coalition said the environmentalists’ letter included claims that “are an affront” to health researchers, those in the gas industry and state regulators.

“These claims ignore the very science they purport should guide our collective decision-making, while casually dismissing the significant work of industry, elected officials and environmental regulators to raise substantially the environmental and public health protections and standards that accompany modern shale-gas development in Pennsylvania,” Spigelmyer wrote.