At least four workers have been found dead since 2010 at oil and natural gas well sites that use hydraulic fracturing methods, according to the National Institute for Occupational Safety & Health, a division of the Centers for Disease Control & Prevention. In a recent blog post, NIOSH announced it would begin examining the incidents, which occurred in the Williston Basin of North Dakota and Montana. Available information, the blog says, indicates that the workers’ deaths appear to be linked to exposure to hydrocarbons contained in “flowback.” This term refers to fracking fluids that are pumped belowground and then returned to the surface and collected in tanks or ponds. Flowback carries volatile hydrocarbons, derived from underground formations, which are separated from the fracking fluids. Many of these hydrocarbons are acutely toxic at high concentrations, NIOSH said. The agency offered preliminary recommendations to limit worker exposure to the hydrocarbons and to boost safety.