Heat-blocking gear worn by firefighters has been found to contain chemicals that can cause liver, kidney, and thyroid disease, the International Association of Fire Fighters said Tuesday.

As a result, the organization is recommending that firefighters limit their use of protective gear to situations where it is absolutely necessary, NBC Nightly News reported.

"It is essential that unnecessary carcinogens are eliminated or mitigated until we can find gear that poses no health risks to our firefighters," said the union’s medical officer, Dr. Danny Whu.

Polyfluoroalkyl substances, also known as PFAs, are nearly impossible to destroy so that they can withstand water, heat, and oil. The compounds are commonly found in raincoats and firefighters’ gear. Foam used to extinguish flames also contains PFAs.

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"We can’t control most of the toxins we encounter on the fire ground, but we can and must control the unnecessary exposures to chemicals like PFAS in our personal protective equipment," Whu said.

Toxicology professor Jamie DeWitt from East Carolina University said not much is known about how PFAs penetrate the skin.

"I wouldn't want firefighters to get the message that they shouldn't be fully protected when they need to be fully protected by that turnout gear," she said. "The solution is finding alternative moisture barriers to protect firefighters that don't contain PFAS."

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The association said cancer caused 61% of the deaths among career firefighters spanning from January 2002 to December 2016. These totals also include exposure to smoke and chemicals.

Firefighters have a 9% greater chance of developing cancer and 14% greater chance of dying from it than the general U.S. population, the Centers for Disease Control and Prevention reported.