Chloroform

Chloroform was discovered in 1831 independently by several chemists and became famous as one of the first general anesthetics, used widely in surgeries beginning in the 1840s [1]. Its use as an anesthetic declined when safer alternatives became available, and its toxicity to the liver and kidneys was recognized. Today, chloroform is not deliberately added to anything — it forms as an unintended byproduct. When water utilities chlorinate drinking water (which is essential for killing pathogens), the chlorine reacts with naturally occurring organic matter in the source water — humic acids from decaying leaves, soil compounds — to form trihalomethanes, a group that includes chloroform, bromodichloromethane, dibromochloromethane, and bromoform [2]. Chloroform is also produced industrially as a precursor to HCFC-22 refrigerant and is used as a laboratory solvent. Swimming pools are a source: the chlorine that keeps pools safe also reacts with swimmers' body oils, sweat, and urine to form chloroform and related compounds in pool air [3].

Drinking chlorinated tap water is the primary exposure for most Americans — the EPA limits trihalomethanes (including chloroform) at 80 µg/L total in municipal water [1]. Showering and bathing in chlorinated water is a significant exposure route: chloroform is highly volatile and inhaled in hot shower steam, and it also absorbs directly through skin. Studies show that showering in chlorinated water may deliver more chloroform to your bloodstream than drinking equivalent volumes of the same water [2]. Indoor swimming pools are high-exposure environments, particularly for competitive swimmers and pool workers who spend many hours breathing chloroform-containing air. Chloroform is also present in industrial emissions and can contaminate groundwater near chemical manufacturing sites [3].

Chloroform is classified as a probable human carcinogen (Group 2A by IARC), primarily for colorectal cancer, bladder cancer, and rectal cancer based on epidemiological studies of people drinking chlorinated water [1]. The liver and kidneys are the primary organs of acute toxicity — chloroform damages cells in both organs, and the effects at environmental concentrations are thought to be a threshold phenomenon rather than a linear no-threshold cancer model. Central nervous system depression occurs at high concentrations, explaining why it worked as a historical anesthetic [2]. The bladder cancer association is the most consistent across epidemiological studies: people who have consumed chlorinated drinking water for decades have modestly elevated bladder cancer rates, with longer-duration use and higher trihalomethane levels conferring greater risk [3].

People who have spent a lifetime drinking chlorinated municipal water and those with higher-than-average water consumption face the most significant bladder cancer association [1]. Competitive swimmers — especially those who swim in indoor pools for many hours per week — have elevated genotoxicity markers compared to non-swimmers, reflecting elevated chloroform and related disinfection byproduct exposures [2]. Pregnant women may face additional concern: some studies link high trihalomethane exposures during pregnancy to small-for-gestational-age births and miscarriage, though evidence is mixed. Industrial workers involved in chloroform manufacturing and laboratory workers who use it as a solvent face occupational exposures [3].

Use a granular activated carbon filter (GAC) or point-of-use activated carbon block filter on your drinking water — these effectively remove trihalomethanes including chloroform [1]. Because showering exposure can exceed drinking exposure, consider an inline carbon shower filter to reduce chloroform in shower steam. Let hot water sit in a running shower for a minute before entering, and leave bathroom ventilation fan running during and after showering [2]. For swimming, outdoor pools have lower indoor air chloroform concentrations than indoor pools; competitive swimmers may benefit from monitoring their training environment's air quality. If you draw from a surface water source with high organic matter (common in the South and Mid-Atlantic), consider a whole-house filter to reduce all trihalomethane exposures from tap water [3]. Request your utility's annual water quality report, which must disclose total trihalomethane levels.