WASHINGTON — Former Vice President Joe Biden believes his son’s fatal brain cancer may have been caused by exposure to military burn pits while serving in Iraq and Kosovo.

In an interview with PBS NewsHour posted Wednesday, Biden conceded that he does not have any direct evidence linking Beau Biden’s death in 2015 to the toxic fires. But he said “there is a lot of work to be done” investigating the issue given the high rates of illnesses seen in troops who worked near the waste pits.

"Science has recognized there are certain carcinogens that when people are exposed to them, depending on the quantities and the amount in the water and the air, can have a carcinogenic impact on the body,” he said in the interview.

Beau Biden was 46 when he died. He was a major in the Delaware Army National Guard who deployed to Iraq in 2009. In addition, as a civilian lawyer, he worked with local prosecutors and judges in Kosovo after the 1999 war there.

The former vice president said he was unaware of how closely his son worked to burn pits until last year, when former Army Staff Sergeant Joseph Hickman released the book “The Burn Pits: The Poisoning of America’s Soldiers.” It includes a chapter on the younger Biden and his battle with cancer.

The author found that in both the Iraq and Kosovo visits, Beau Biden was working closely to burn pits.

“That stunned me,” his father said in the PBS interview. “I didn’t know that.”

Burn pits were commonplace through the Iraq and Afghanistan wars to get rid of a wide variety of items, including fuel, human waste, batteries and plastics. At smaller bases the fires handled disposable refuse for a handful of troops. At larger ones, some of the pits burned toxic materials around-the-clock in massive fires.

Defense Department and Veterans Affairs officials have frequently cited the difficulty of linking troops’ illnesses to burn pits in the war zones, given the undocumented nature of exactly what was burned and just how much exposure individual veterans had.

But more than 126,000 veterans and current service members have enrolled in a VA burn pit registry in recent years, documenting a wide range of illnesses, including rare cancers and neurological disorders.

Near the end of his presidency, Barack Obama named then Vice President Biden to lead his “cancer moonshot” initiative, with the stated goal of doubling the rate of progress in cancer prevention. The effort continues with the newly formed Biden Cancer Initiative.

Joe Biden has said that his son’s death was the primary reason he did not enter the 2016 presidential campaign.