Two Marines spent years firing heavy weapons. Then came headaches and hemorrhage.

"Based on the research, that's justified," says Stephen Ahlers, a neuroscientist at the Naval Medical Research Command... There's growing evidence that repeated exposure to blast waves can alter blood vessels in the brain."

"It might happen," says Dr. Ibolja Cernak, an expert on blast injury... We are gathering more and more information that primary blast does cause vascular changes in the brain."

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