With increasing residence time, concentrations of THMs increased, but concentrations of HANs, HKs, CP, and HAAs decreased. Increases in the concentration of disinfection by‐products (DBPs) with increasing residence time in distribution systems have been predicted from laboratory studies but not conclusively demonstrated in the field. DBP concentrations were measured in a central New Jersey water distribution system during one year at four locations with nominal water residence times of zero, one, two, and at least three days. The water was disinfected with chloramine so that finished water left the plant with a free chlorine residual of 0.5 mg/L. Concentrations of trihalomethanes (THMs) increased with increasing residence time in the distribution system, but the concentrations of haloacetonitriles, haloketones, chloropicrin, and haloacetic acids decreased with increasing residence time. Concentrations changed more rapidly during warm months than cold months; temperature and chlorine residual were the most important parameters controlling DBP concentrations.

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Exposures to chlorination by-products (CBP) within public water supplies are multiroute in water. Cold water is primarily used for ingestion while a mixture of cold water and hot water is used for showering, bathing others, dish washing, etc. These latter two activities result in inhalation and dermal exposure. Heating water was observed to change the concentration of various CBP. An increase in the trihalomethanes (THM) concentrations and a decrease in the haloacetonitriles and halopropanones concentration, though an initial rise in the concentration of dichloropropanone, were observed. The extent of the increase in the THM is dependent on the chlorine residual present. Therefore, estimates of total exposure to CBP from public water supplies need to consider any changes in their concentration with different water uses. The overall THM exposures calculated using the THM concentration in heated water were 50% higher than those calculated using the THM concentration present in cold water. The estimated lifetime cancer risk associated with exposure to THM in water during the shower is therefore underestimated by 50% if the concentration of THM in cold water is used in the risk assessment.