Chlorination is the process of adding chlorine to drinking water to disinfect it and kill germs. Different processes can be used to achieve safe levels of chlorine in drinking water. Chlorine is available as compressed elemental gas, sodium hypochlorite solution, or calcium hypochlorite. While the chemicals could be harmful in high doses, when added to water, they all mix in and spread out, resulting in low levels that kill germs but are still safe to drink. According to the CDC website, chlorine levels up to 4 milligrams per liter (mg/L) or 4 parts per million (ppm) are considered safe in drinking water. Chlorine was first used in the U.S. as a major disinfectant in 1908, and by 1995 about 64% of all community water systems in the United States used chlorine to disinfect their water.