By Moira Warburton

WASHINGTON (Reuters) -Legislation to improve health care and benefits for veterans exposed to toxic military burn pits in Iraq and Afghanistan advanced with bipartisan support in the U.S. Senate on Wednesday, clearing the way for a final vote.

The legislation, which would cost $180 billion over four years, would cover injuries suffered by service members exposed to toxic smoke from the burn pits, which has been linked to respiratory illnesses and rare cancers.

It would also expand federal research on the impact of the burn pits, which were used by the U.S. military until the mid-2010s to dispose of plastics, chemicals, human waste and other garbage on foreign bases in Iraq, Afghanistan and other countries.

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The legislation passed the House last year but the Senate modified it to phase in coverage more gradually. It will have to pass the House again before Biden can sign it into law.

Senate Majority Leader Chuck Schumer said on the Senate floor ahead of the vote that 3.5 million servicemembers had been exposed to the toxic burn pits since Sept. 11, 2001, and called the years of denials of coverage "a confounding indignity" to veterans

The bill will also expand coverage of health issues caused by Agent Orange, a toxic deforestation spray used by the U.S. military during the Vietnam War.

President Joe Biden highlighted the legislation in his State of the Union speech in March. His son, Beau, died of a rare brain cancer in 2015 after serving for a year in Iraq in 2009.

(Reporting by Moira Warburton in Washington; Editing by Mark Porter)