Field studies conducted by the U.S. Government have revealed that hydrofracking fluids are responsible for the deaths of four field workers since 2010.

The report, recently released by the National Institute for Occupational Safety, suggests that workers could be exposed to hazardous levels of toxic volatile hydrocarbons found in waste fracking fluids.

“According to our information, at least four workers have died since 2010 from what appears to be acute chemical exposures during flow back operations at well sites in the Williston Basin (North Dakota and Montana),” government researchers wrote. “While not all of these investigations are complete, available information suggests that these cases involved workers who were gauging flow back or production tanks or involved in transferring flow back fluids at the well site.”

The institute is also assessing worker exposure to other chemicals mixed into fluids that are injected into the earth during fracking. Those findings will be detailed in later publications, according to Max Kiefer, a NIOSH spokesperson.

“Right now, the exposures of concern from a worker standpoint are from endogenous hydrocarbons that can be emitted from returned flow back fluids, not from other chemicals,” Kiefer told Bloomberg BNA.

The report highlights how little is known about the on-site health dangers associated with hydrofracking compared to well-researched knowledge about the worksite hazards of conventional oil and gas extraction.

The fracking boom has proceeded at such fast pace in the U.S. that regulators have struggled to keep up with monitoring all the potential hazards involved in the process. The EPA has not been able to adequately keep track of how and where the fossil-fuel industry is disposing of fracking waste water. As a consequence, countless gallons of partially treated waste water has ended up in the rivers and aquifers from which millions of Americans get their drinking water.

A vast volume of water is needed to extract that natural gas, and the large amounts of wastewater generated during the process is causing increasing concern among geochemists, biologists, engineers, and toxicologists. Initially, they focused their worries on leaks of drilling fluids and other contaminants from well casings, which could potentially pollute groundwater supplies. But with engineering improvements that have reinforced well casings and reduced pollution from that source, experts now say fracking’s real pollution danger comes from wastewater.

A Duke University study conducted last year showed that some Marcellus Shale waste water, tainted by high levels of radioactivity, flows downstream into water sources for Pittsburgh and other cities, with uncertain health consequences.