Water systems add chlorine to kill bacteria.
Filters can remove both chlorine and THMs. But much of the chlorine escapes if tap water is left sitting overnight, says Richard Benzie, a water-quality specialist with the Michigan Department of Environmental Quality. Detroit's water system will cut chlorine use in half when the city opens its new water-treatment plant in 2002, says spokeswoman Mary Mazur. The plant will use ozone, a byproduct of oxygen, to disinfect drinking water. Lead seeps into water from the plumbing systems of older homes that have antiquated lead service lines feeding water from a water main or whose plumbing joints are fused with lead-solder, now illegal for use on water systems.