TTHM levels are affected by several factors: changes in the quality of the water that has to be disinfected (and this can be affected by drought or sudden rainfall or by changes in the source of the water); by temperature; by the quality of drinking water storage tanks and pipelines; and...
U.S. EPA says it regulates disinfection byproducts such as TTHM "to address health risks posed by a potential association between chlorinated drinking water and cancer, particularly bladder cancer."
Some of that variation has been minimized over the years through the standardization of drinking water treatment regulations. Current emphasis in many plants focuses on reducing disinfection by product (DBP) levels with the upcoming EPA Stage II regulations. DBP's are a result of the interaction ...