Swimming pools often are purified with chlorine to reduce bacteria and make them safe for swimmers. In recent years, there has been some concern about the safety of chlorocarbons produced when chlorine reacts with suntan lotions which are common contaminants in swimming pools. Consequently, some swimming pools are now maintained by electrolytically producing small amounts of copper and silver. The small concentrations are toxic to bacteria and viruses, but not toxic to humans at these low concentrations.