Over 90 million rounds of the "green" training ammunition has been used in the United States, since its introduction. It relies on a blend of tungsten and nylon, or tungsten and tin. That gives the bullets the same density and firing properties as the original, but without using lead. Tungsten was considered non-toxic. And, besides, it was believed that it was "non-mobile", unlikely to dissolve and travel, so it wouldn't get into the groundwater.
But new research by University of Arizona Research Professor of Pediatrics Mark Witten points to a different conclusion: that tungsten may elevate the risk for cancer.