Mutagen X, a by-product of chemicals used to disinfect public water supplies, is not monitored or regulated in the U.S. water supply. A new report from researchers at Harvard’s School of Public Health provides the first broad evidence that Mutagen X, first discovered in Finland’s drinking water more than ten years ago, also lurks in chlorinated U.S. drinking water. Researchers discovered small amounts of Mutagen X, which is a powerful genetic mutation-causing agent, in 36 Massachusetts towns served by 23 different public water systems. The findings may increase support for pending national regulations to tighten drinking water standards and refuel local efforts to identify safer ways to disinfect tap water. Nine years ago, the World Health Organization recommended that Mutagen X be limited in drinking water. The Massachusetts findings were published in the February 2002 issue of the journal Environmental Health Perspectives. The study is part of the doctoral thesis of Michael Wright, the first author and now an Oak Ridge Institute for Science and Education postdoctoral fellow at the Environmental Protection Agency in Cincinnati.