Water fluoridation
Health and dental organizations worldwide have endorsed its safety and effectiveness. Its use began in 1945, following studies of children in a region where higher levels of fluoride occur naturally in the water. Further research showed that moderate fluoridation prevents tooth decay.
Water chlorination
Chlorine gas was first used on a continuing basis to disinfect the water supply at the Belmont filter plant, Philadelphia, Pennsylvania by using a machine invented by Charles Frederick Wallace who dubbed it the Chlorinator. It was manufactured by the Wallace & Tiernan company beginning in 1913. By 1941, disinfection of U.S. drinking water by chlorine gas had largely replaced the use of chloride of lime.