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DENVER (KDVR) — Firefighters around the country are just starting the tedious process of removing firefighting foam that contains “forever chemicals.”

This toxic foam contains PFAS, or poly-fluoroalkyl substances, which can remain in the environment and humans. This foam may cause health problems, including cancer. Firefighters have used this foam for more than 20 years to help them put out difficult fires, not knowing it could actually be hurting them.

“We assumed this chemical was something that was not only effective on the fire ground but also safe to us,” Denver Fire Capt. Greg Pixley said.

In a methodical, step-by-step process, the Denver Fire Department is removing the foam known as AFFF, or aqueous film-forming foam. The fire engines can carry 25-50 gallons of the concentrate. They’ve called in a company that specializes in hazardous materials removal. They drain and flush all of the fire engines.

“We also have to do our best to remove all remnants of it in our materials, hoses, piping, tanks, our pumps. All the those things we use it for as firefighters,” Pixley said.

Which brings to mind the health consequences for firefighters who have been regularly exposed to those dangerous chemicals for years.

“It is overwhelmingly concerning. For a firefighter like myself who has spent a large part of their career in or around using this type of foam, it really becomes a passionate issue, because we didn’t know better. Nobody told us we should reduce our exposure to this. We trusted this chemical was not only benefit us on the fire ground, but wouldn’t kill us 10-15 years after we left this job,” Pixley said.

They are switching to a cleaner type of foam that is supposed to be just as effective.

Each fire engine takes about four hours to clean out. The Denver Fire Department started this process in late February and they expect it will last through the end of this month.

This week Colorado Attorney General Phil Weiser announced a lawsuit against 15 companies over firefighting foam that contains harmful chemicals detected in the water supply. He says the companies knew the dangers but failed to warn anyone.