It may look clean, but odds are your tap water contains unhealthy chemicals.

RODALE NEWS, EMMAUS, PA—Ever wonder what's in your glass of water—besides water? Unfortunately, the answer may be "plenty." The laws designed to guarantee clean drinking water are getting weaker by the day, and a new report from the advocacy-oriented nonprofit Environmental Working Group (EWG) reveals just how much contaminated drinking water is pumping through our nation's municipal water pipes.

THE DETAILS: The EWG reviewed 20 million water-quality tests from the past five years, and found that the water flowing to about 85 percent of the population contains 316 contaminants. Of those, 202 chemical contaminants are not regulated by the Environmental Protection Agency, meaning that any water supply can have any level of the pollutants and a municipality doesn't need to take action to remove them. In some cases, those pollutants are at low levels and don't cause harm to human health. In others, however, the problems could be serious. For example, perchlorate, a component of rocket fuel that was recently detected by the Centers for Disease Control and Prevention in nearly all Americans tested, is one of those unregulated chemicals, and it can interfere with the body's thyroid-hormone production, which could lead to metabolic disorders and diabetes.

EWG also used its data to rank cities with the best and worst water supplies, based on the number and concentrations of hazardous chemicals found. At the top of the list were Arlington, Texas; Providence, Rhode Island; Fort Worth, Texas; Charleston, South Carolina; and Boston, Massachusetts. The five worst? Reno, Nevada; Riverside County, California; Las Vegas, Nevada; Riverside (city water supply), California; and Pensacola, Florida.