Chlorine is a disinfectant added to water to reduce or eliminate microorganisms, such as bacteria and viruses, which can be present in water supplies. Disinfecting our drinking water ensures it is free of the microorganisms that can cause serious and life-threatening diseases, such as cholera and typhoid fever. To this day, chlorine remains the most commonly used water disinfectant, and the disinfectant for which we have the most scientific information. Chlorine is added as part of the drinking water treatment process. However, chlorine also reacts with the organic matter, naturally present in water, such as decaying leaves. This chemical reaction forms a group of chemicals known as disinfection by-products. The most common of these by-products are trihalomethanes (THMs), which include chloroform. The amount of THMs found in drinking water depends on a number of things, including the season and the source of the water. For example, THM levels are generally lower in winter than in summer, because the amount of natural organic matter is lower and less chlorine is needed to disinfect at colder temperatures. THM levels are also low when wells or large lakes are the drinking water source, and higher when rivers or other surface waters are the source, because they generally contain more organic matter. Current scientific data shows that the benefits of chlorinating our water (less disease) are much greater than any health risks from THMs and other by-products.