Public health officials are becoming concerned about chlorine, a chemical that is added to many municipal water supplies to reduce bacterial growth. New evidence shows that the rate of a first-trimester miscarriage is increased significantly when a pregnant woman drinks five or more glasses of chlorinated water per day. Lead, which has been shown to cause brain damage in small children, is another main contaminant in urban water systems. Rising fears about these and other problems—such as toxic bacteria, cysts, and algae—have caused an increase in the number of alternate methods of water filtration that are available to the general public. The passage is chiefly concerned with