One hundred years ago next month, on July 12, 1917, the Germans dispensed mustard gas for the first time on Belgian battlefields during World War I. Mustard was not used on the battlefields of World War II, but Allied armies, gearing for its possible use or other chemical warfare, conducted mustard gas experiments on their own soldiers during that war.

And, possibly, to a degree much larger than already disclosed, according to a new book, “Toxic Exposures,” by Susan L. Smith, a history professor at Canada’s University of Alberta in Edmonton.

Quoting recent public estimates of experiments conducted on more than 2,500 Canadians, 2,500 Australians, 7,000 Britons and 60,000 Americans, Smith writes: “These are all likely low estimates due to incomplete records and government restrictions on still-classified military records.”

The book, published by Rutgers University Press, comes at a poignant time. World War II veterans are fading away, and the number of Americans exposed and still alive may be 400 or fewer, according to Sen. Claire McCaskill, D-Mo.

She is sponsoring a bill, the Arla Wayne Harrell Act, which would lower the high barriers of proof for veterans who claim they were exposed to the mustard gas experiments but have had claims denied by the Department of Veterans Affairs.