When chlorine is used to disinfect water, it reacts with organic matter in the water to create volatile by-products known as trihalomethanes (THMs). One particularly dangerous THM is chloroform, deemed by the EPA to be a probable human carcinogen. According to the 2010 report of the President's Cancer Panel, long-term exposure to chlorine disinfection by-products such as chloroform may increase cancer risk.
Showering with chlorinated water is of particular concern. According to the Centers for Disease Control, chloroform easily enters the body through the skin and you can breathe chloroform that evaporates from hot shower water. Chloroform that is absorbed through the skin or inhaled quickly enters the bloodstream and is dispersed throughout the body. It affects the brain, liver and kidneys. In addition to being linked to various cancers, chloroform has been linked to reproductive disorders, including miscarriage.
Thus, experts recommend shower filters to reduce chlorine and chlorination by-products in shower water as much as possible. According to this article from Green America, the best type of filter to remove chlorine and its by-products is a combination carbon/KDF adsorption filter. The article recommends Bestfilters.com as a resource. However, NSF, a not-for-profit organization that provides independent third-party testing and certification for water treatment units, does not currently certify any shower filters to reduce chloroform and other THMs. NSF only certifies shower filters for chlorine reduction.
If you do not have a shower filter, or if you don't trust that your filter is in fact reducing certain dangerous chlorination by-products, you can take steps to reduce your exposure to those by-products. Take shorter, cooler showers and ensure that your bathroom is ventilated as well as it can be. Also, don't use a mist or fine spray setting on your shower head, as that can increase the amount of chloroform inhaled.
As for those of us whose water is now treated with chloramine--according to the the EPA, chloramine is more difficult to remove from water than chlorine. For example, showerhead filters and bathtub filter balls do not remove chloramine. In addition, the by-products of chloramine disinfection have not been studied and their health effects are unknown. According the the National Resources Defense Council, they may be as bad, or worse, than THMs. Stay tuned for a future post about chloramine.