– The Obama administration is investigating the health risks of hydraulic fracturing after at least four deaths among oil field workers since 2010 in North Dakota and Montana.

The National Institute for Occupational Safety and Health said the workers were exposed to high levels of volatile hydrocarbons during the drilling process known as fracking.

“NIOSH is actively conducting research on exposures for workers,” Christina Spring, a spokeswoman with NIOSH’s parent agency, the Centers for Disease Control and Prevention, said Monday in an e-mail.

Fracking that has helped push U.S. oil and gas production to record levels also has spurred worries about tainted water as well as earthquakes triggered by pumping the wastewater underground.

In fracking, chemically treated water and sand are injected into shale rock to free trapped oil and gas. When the fluid returns to the surface as wastewater, it contains volatile hydrocarbons from the rock formation, according to the NIOSH. The fluid is temporarily stored in tanks or pits on the surface.

Hydrocarbons can affect the eyes, lungs and nervous system and at high levels also may lead to abnormal heartbeat, NIOSH said Monday in a blog post. Workers can be at risk when they measure the fluid in the tanks with handheld gauges using access hatches.

The deaths occurred at wells in the Williston Basin in North Dakota and Montana, the institute said, citing media reports and government agencies. Some of the deaths remain under investigation.

“Often these fatalities occurred when the workers were performing their duties alone,” the blog post said.

The institute is asking oil and gas drillers to help assess the risks of exposure to chemicals in used fracking fluids. It is recommending that companies develop different ways to measure the fluid stored in tanks and to provide hazard awareness training.