Sen. Claire McCaskill, D-Mo., plans to introduce legislation today to help World War II veterans who were exposed to mustard gas. The vets were used in classified experiments conducted by the U.S. military, and were sworn to secrecy about their participation for a half-century.

Last year, an NPR investigation found the Department of Veterans Affairs failed to notify thousands of mustard gas test subjects of their eligibility to apply for compensation — and that it routinely denied claims from veterans who qualified. In many cases, the VA has said veterans don't have enough evidence of their participation in the tests to get benefits — even though the tests were kept off official records.

McCaskill is naming the bill the Arla Harrell Act, after a man thought to be the last surviving Missourian who served as a mustard gas test subject. Harrell, 89, lives in a nursing home. His repeated claims for compensation have been denied by the VA, as recently as last month.

Roughly 60,000 Army and Navy troops were used in the experiments, which sought to prepare the U.S. military to face mustard gas in battle. Mustard gas is known to cause serious illnesses, including leukemia, skin cancer and chronic breathing problems.

McCaskill's office launched its own investigation into the treatment of mustard gas test subjects after NPR published its report. With cooperation from the VA and the Pentagon, McCaskill found that only 40 living veterans are currently receiving compensation for injuries resulting from the experiments, and that 90 percent of veterans' claims for mustard gas-related injuries have been denied.

The bill calls for the VA and the Department of Defense to establish a new policy for processing claims for mustard gas exposures, and to reconsider all previously denied claims. In doing so, the bill says the VA should presume that veterans were exposed to the toxic agents "unless either agency can definitely prove otherwise."

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