THMs have been implicated in a number of public health issues. Historically, concern centred on carcinogenesis, but more recent work suggests reproductive health effects such as low birth weight are a more likely outcome of exposure to high concentrations of THMs (see . Removal of organics at the works ensures that THM concentrations entering the system are reduced. However, the majority of THMs in these systems are produced in distribution network either through reaction between organics and residual chlorine as the water passes through the distribution system, or at secondary dosing points required to compensate for chlorine loss. While the organic content has reduced the resulting chlorine concentrations are higher. Hence it is not obvious what the total impact on THM concentrations is likely to be. In the case of Watchgate, less than 30 % of the THMs observed at customer taps was produced at the works. The size of the distribution system means that multiple rechlorinations can occur, each time giving the water an additional THM boost. A major cause of loss in chlorination is reaction with organics in the water. Removing organics reduces the chlorine demand and it penetrates further into the distribution system. Analysis shows that overall THMs have been reduced by 40 -55% (Table ) and has radically changed the average concentrations across the region (see Figure ). In addition, it is noted that the variability has also been reduced which reduces the risk of a high THM concentrations.