Studies of third-faucet point-of-use treatment devices indicated that the bacteriological quality of the product water varies among units of the same design as well as among units of different design. The development of bacterial populations is affected by the length of time a filter cartridge is in service, water temperature, flow rate, materials of construction, and the quality of the influent water. Although some units contain silver as a bacteriostatic agent, its effectiveness is questionable because the concentrations of heterotrophic bacteria associated with such units are often as high as those from units that do not contain silver.