Because two of them sound very similar and all of them poses a serious threat to the health of this country. I believe that anyone who uses tap water for drinking should know about the risks that are involved when they are exposed to these contaminants. The EPA currently tracks some 80 different contaminants that may or may not be found in the drinking water in the United States. As testing methods improve then more stringent regulations may follow.

Trihalomethanes (THM)
This poses one of the largest threats to large populations of people who reside in municipalities that use primarily surface water treated with chlorine. Trihalomethanes (THM's) are the byproducts of chlorine's use in treating drinking water, what is left of the chlorine after it kills bacteria reacts with the dead organic content in the water. Portions of the residual become trihalomethanes. Chloroform is the best known example. THM's cause 10,000 cases of colon and bladder cancer annually. The chlorine industry strenuously opposes closer regulation of THM's.

The detected increases of THM's presence in numerous large-scale water treatment systems have sparked the use of chlorimine for disinfecting. Chlorimines residual does not react with the remaining dead organic content like chlorine. The result is less THM's present in the finished drinking water. One drawback to the use of chlorimine is that it needs a longer contact time with granular activated carbon (GAC) filters to be removed from the water. This posed a huge problem to the kidney dialysis industry that uses treated tap water for process makeup water. Because many facilities would use water treated with GAC filtration sized for chlorine removal. The longer contact time for the removal of chlorimine required resizing of pretreatment.

Tetrachloroethylene (PCE)
This is used primarily in the textile industry, and as a component of aerosol dry-cleaning products. Major releases of tetrachloroethylene to air and water are from dry cleaning and industrial metal cleaning or finishing. Water pollution can occur from tetrachloroethylene penetrating the walls of some types of pipelines used for water distribution. From 1987 to 1993, according to EPA's Toxic Chemical Release Inventory, tetrachloroethylene releases to land and water totalled over 1 million lbs. These releases were primarily from the alkali and chlorine industries that use it to make other chemicals. The largest releases occurred in Louisiana and South Carolina. PCE released to soil will readily evaporate or may leach slowly to the groundwater where the break down by soil microbes is slow. PCE released to water will primarily evaporate and has little potential for accumulating in aquatic life. Continued exposure over many years could leave you with liver damage and you may have an increased risk of getting cancer.