A huge breakthrough in modern fracking came in 1997, courtesy of a stubborn, tenacious Texan named George Mitchell. After 17 years of trial and error, he and his team stirred up a secret recipe of water, chemicals and sand, shot it down a well, and lots of natural gas came back up. Mitchell has since passed away but his colleague Kent Bowker remembers the moment they cracked the code.

“Mr. Mitchell got this big smile on his face. And he leans forward and he looks at all of us: ‘This is huge, this is the biggest thing that’s ever happened to this company. It’s also the biggest secret. No one can talk about this!’” Bowker said.

Secrecy lies at the heart of invention and capitalism to reward innovators with exclusive rights to their creations. But that secrecy can also come at a cost. With drilling and fracking, the ingredients that make up the chemicals used to obtain oil and gas are legally allowed to be kept confidential. According to newly released documents from the Environmental Protection Agency, that secrecy starts as soon as chemical manufacturers apply for government approval of their products.

Hundreds of agency papers were released under the Freedom of Information Act to the environment group Partnership for Policy Integrity. They show that from 2003 to 2014 chemical makers routinely withheld all kinds of information in their applications, including the molecular structure of chemicals, product names, commercial uses of chemicals, even the manufacturers’ names.

EPA scientists are privy to this chemical information, but cannot make it public, said Dusty Horwitt, an attorney with the Partnership for Policy Integrity.

The new documents show that the EPA listed varying health risks about most of the chemicals it approved, including the risk of poisoning to the brain, lungs and liver.

“If EPA’s own regulators are finding that there are health concerns about these chemicals, and then they allow them to be used in oil and gas drilling, the public has a right to know,” Horwitt said.

He and more than 100 toxicologists and advocates are sending a letter to the EPA, asking it to release the details on more than 40 drilling and fracking chemicals — ones the agency described as risky to human health. The EPA did not respond to a request for comment.

Click on the dots in the map below to see information about individual wells that used chemicals that the EPA knew posed potential health risks.

Source: EPA documents released to the Partnership for Policy Integrity

Many living near drilling sites experience those risks firsthand. Rebecca Bowen of Clarington, Ohio recalls an oil well site catching fire in 2014. It happened one ridge over from Bowen’s house, engulfing 20 trucks and triggering 30 explosions, according to a federal investigation. Black smoke filled the air.

“Our throats by then burnt so bad. They told me my daughter’s esophagus was melted. My husband, after this happened, he was diagnosed with six spots on his lungs. About six months later he had more spots in his lungs,” Bowen said.

Thousands of gallons of chemicals escaped into a local waterway, Opossum Creek, that dumps into the Ohio River, according to a federal investigation.

“That’s where all the fish died. They said there were 70,000 fish or something like that that died. There’s still people up there swimming. And who knows how them chemicals went into the earth?” Bowen said about the creek. EPA investigators determined water readings returned to normal three weeks after the incident, though critics remain worried about ecological damage.

The oil field services company operating the well, Halliburton, later provided a partial list of chemicals used. But the company deemed several chemicals proprietary and did not disclose those, which is allowed under state law.

So doctors and scientists assessing patients like Bowen and her family don’t always know what local residents have been exposed to. Some fracking chemicals are known, like ethylene glycol, which can affect the kidneys and lungs. Others are surprises, said David Brown, a toxicologist with the Southwest Pennsylvania Environmental Health Project.

“We’ve learned that there are chemicals there that we never expected them to be there. Had we known that, it would have been possible to organize a more systematic approach to protecting and determining the safety of people living near the gas drilling,” Brown said.

In the case of potential exposure, he often doesn’t know what to look for, or what to test for. “My alternative has been — think of everything. It’s very expensive to ask questions that didn’t need to be asked,” Brown said, like running a host of laboratory tests blindly.

The industry maintains the risk of drilling and fracking pollution is negligible. That’s been proven by multiple studies, Jack Gerard, president and CEO of the American Petroleum Institute, said. “Yes, the science should be settled on that. But there are some who aren’t content to have the science settled on it, because they have agendas and they are advocating for their particular position,” Gerard said in speech to the American Petroleum Institute last year.

Oil and gas groups note that chemicals do get reported to a national database known as FracFocus, though with exceptions for trade secrets. In emergencies, they said, all the chemical secrets are available to doctors and first responders.

But in the 2014 Ohio fire, responders weren’t given chemical identities for days, said Youngstown Fire Department Battalion Chief Silverio Caggiano. He is also a member of the regional hazardous materials response team.

“Firefighters, you know, we have this tradition of running in where people are running out. And without the knowledge of what’s in there, running in there may be turning us into victims as well,” Caggiano said.

Any sort of delay on the part of first responders can be catastrophic if a chemical gets out.

“So I have no way of containing it. I don’t know how to contain it,” Caggiano said. “That’s why that information is so critical to first responders within the first 30 minutes of us being able to formulate a game plan. Because after 30 minutes, game’s over. It’s gonna do what it’s going to do,” Caggiano explained.

Oil and gas chemical secrecy is unusual, said Caggiano. In every other sector, truck drivers who transport chemicals carry papers, called Material Safety Data Sheets, listing the contents.

“The driver’s MSDS sheets, even if he is not alive, I know where his MSDS sheets are. They’re within a foot of his right hand,” Caggiano said. “But with the fracking industry, whatever they’re hauling there’s not going to be one.” He has signed onto the letter asking the EPA for more disclosure on oil and gas chemicals.

Another signatory, toxicologist John Stolz at Duquesne University, said this is important now, because the North American fracking revolution is just starting to scale up.

“We’re flying blindly because we don’t have the facts. We don’t have all the information that we need. And there’s fracking going on in 34 states,” Stolz said.

For more information about the potential links between fracking and health risks, click here.

Correction (Nov. 16, 2017): A previous version of this story identified documents carried by truckers as “Medical Safety Data Sheets.” The correct term is “Material Safety Data Sheets.”