Trihalomethanes (THMs) are by-products of disinfection, created when disinfectants added to protect public health (particularly chlorine) react with natural organic matter (such as leaves, earth and sediment) that can be present in source water. Drinking water systems in South Australia are carefully managed so that any concentrations of THMs are as low as possible, aiming to consistently be within the national health limit of 250 micrograms per litre, whilst ensuring the important disinfection process making water safe to drink is not compromised.