Trihalomethanes (THMs) in drinking water can be harmful, as they are suspected to cause cancer. These chemicals form when chlorine is added to water in treatment plants. The level of precursors in raw water affects THM formation, which can be reduced by using an oxidant like ozone. THM production is also influenced by the amount of chlorine added and the water's pH. Ways to lower THM levels include using disinfectants other than chlorine, changing where chlorine is added in the treatment process, and removing precursors and THMs after formation. Some California utilities are successfully reducing THM levels.