US President Joe Biden on Wednesday signed into law a bill boosting benefits for veterans exposed to toxic fumes, a cause close to his own heart after his son died of brain cancer.

Burn pits, lit up with jet fuel, were commonly used to dispose of waste in military camps during the years of the US wars in Afghanistan and Iraq.

They exposed large numbers of servicemen and women to potentially harmful toxins, although there is not always a proven link between the exposure and later illnesses.

The PACT Act, passed by the US Senate earlier this month after fierce lobbying by veterans and celebrity comedian Jon Stewart, will formalize new rules for ensuring access to medical treatment.

"Sometimes military service can result in increased health risks for our veterans and some injuries and illnesses, like asthma, cancer and others, can take years to manifest," the White House said.

"These realities can make it difficult for veterans to establish a direct connection between their service and disabilities resulting from military environmental exposures such as burn pits -- a necessary step to ensure they receive the health care they earned."

It described the bill as the "most significant expansion of benefits and services for toxic exposed veterans in more than 30 years."

Biden believes the pits are at the root of the brain cancer that claimed the life of his son Beau, who served in Iraq and died in 2015 at age 46.

Recounting his own many visits to US troops in Iraq as a senator and vice president, Biden described seeing "burn pits the size of football fields" filled with the "incinerated waste of war."

Subjected to "toxic smoke," many of the "fittest and best warriors that we sent to war" came home and "were not the same," encountering symptoms as varied as headaches and numbness, as well as serious diseases, Biden said.

"My son Beau was one of them."