PHOENIX — Firefighters put everything on the line to protect us, and now more of them are developing cancer. 

Cancer is a leading cause of death for firefighters, according to the International Association of Firefighters

And a growing amount of research shows firefighters are more likely to get cancer diagnoses and to die from cancer than the general public. 

In 2019, we saw four Valley firefighters die after battles with cancer they were believed to have developed on the job. 

 Goodyear firefighter dies after 4-year battle with job-related cancer

 3rd-generation Phoenix firefighter dies of occupational cancer

 Firefighter dies after battle with breast cancer possibly contracted in the line of duty

 Phoenix firefighter dies of cancer contracted in the line of duty, firefighters association says

Those are just the deaths. More firefighters were diagnosed with cancer, including Phoenix Fire Chief Kara Kalkbrenner. 

RELATED: Phoenix Fire Department chief says she has breast cancer, will undergo double mastectomy

Dr. Pablo Prichard and Dr. Vershalee Shukla with Vincere Cancer Center said the clinic diagnosed 22 firefighters with cancer in 2019. 

"They look strong, they look invincible, and so people don't think of them being prone to cancer, but we need to change," Dr. Prichard said. 

They said they are seeing firefighters getting more aggressive cancers at younger ages. 

"A majority of firefighters will die of their cancer," Dr. Shukla said. 

She said many of the firefighters are getting the cancers before it is even recommended to start screening for them. 

The cause, firefighters and doctors say, is modern synthetic materials that create more smoke when they burn, releasing a number of carcinogens. 

But there are steps being taken to protect firefighters. 

Goodyear introduced a special firetruck built to prevent the spread of chemicals. 

And Phoenix is providing free screenings to first responders to help catch cancer before it's too late.

In the U.S. Senate, lawmakers are considering legislation that would help limit the amount of toxins firefighters are exposed to.

RELATED: An inside look at how Phoenix firefighters handle the risks of the job