Researchers with the National Institute for Occupational Safety and Health (NIOSH) said nine oil and gas extraction workers were killed over a five-year period after inhaling volatile organic compounds (VOCs) while manually gauging or sampling production tanks at oil and gas well sites.

In a blog post on Friday, NIOSH, part of the Centers for Disease Control and Prevention (CDC), said the fatalities occurred between January 2010 and December 2014. Using photoionization detectors, the researchers found that peak short-term levels of total VOCs were above 2,000 parts per million (ppm) during tank gauging activities at separators and flowback tanks, and sustained levels were as high as 500 ppm. But they ruled out exposure to hydrogen sulfide (H2S), another toxic gas associated with oil and gas production.

NIOSH said researchers from the Occupational Safety and Health Administration also found concentrations of butane, propane and pentane at open hatches of production tanks.

“Data collected to date shows that concentrations of these hydrocarbons in excess of immediately dangerous to life or health levels can occur near open tank hatches and these concentrations can exceed the lower exposure limit [LEL] presenting risks for fires and explosions,” NIOSH said. “Many VOCs from hydrocarbon sources have occupational exposure limits much lower than their LEL.”

NIOSH added that data collected from a multi-gas monitor worn by one of the deceased workers found LEL concentrations greater than 100%, and simultaneous oxygen deficiency — meaning less than 10% — at around the time of the worker’s death. “These conditions could occur due to high concentrations of gases and vapors inside the tank which are released in a burst of pressure as the tank hatch is opened by the worker for manual gauging or sampling operations.”

The agency said it is continuing to work with industry partners to further investigate the magnitude of the risk to oil and gas workers. In the meantime, NIOSH recommended that the industry help limit the exposure to workers by implementing alternative tank gauging procedures, including electronic testing. They also recommended additional training for workers and having them consistently wear calibrated multi-gas monitors to measure LEL and oxygen concentration.