The treatment plants stopped using chlorine because it creates byproducts that are linked to cancer with chronic exposure. Water experts say further study is needed before they decide whether chlorine will play a role in solving the city's lead problem.

Because chlorine creates byproducts that are linked to cancer, the EPA recently tightened permissible levels of those byproducts in drinking water.

The surprising drop in lead levels announced yesterday not only feeds the theory that chloramines drove lead levels up but also shows that chlorine might lower them. Some scientists believe that chlorine may offer protection against lead. "This is the first time there is real good data from a real-system situation" on that theory, [Rick Rogers] said.