At a glance
| Chemical family | Trihalomethane — water chlorination by-product and historical solvent |
| CAS number | 67-66-3 |
| Classification | IARC Group 2B (possibly carcinogenic to humans); EPA Group B2 (probable human carcinogen). Maximum contaminant levels set for total THMs in drinking water |
| Where you encounter it | Chlorinated drinking and bathing water (the dominant trihalomethane); indoor air after showering or bathing; historical anaesthetic and solvent (no longer used); swimming pool air |
| Sleep micro-environment relevance | Enters bedroom air from shower/bath water use in adjacent bathrooms. Not a mattress contaminant. Exposure during sleep is from residual indoor air chloroform after water use |
Regulatory & certification status
| European Union | Drinking Water Directive 2020/2184: total THMs limited to 100 µg/L. REACH registered substance. CLP Carc. 2 (H351), Repr. 2 (H361d). Regulatory |
| United States | EPA MCL for total THMs in drinking water: 80 µg/L (Stage 2 D/DBP Rule). California Proposition 65 listed (cancer, reproductive toxicity). OSHA occupational limit 50 ppm ceiling. Regulatory |
| Canada | Health Canada guideline for total THMs in drinking water: 100 µg/L. Regulatory |
| International | IARC Group 2B (possibly carcinogenic). WHO drinking water guideline: 300 µg/L chloroform. Regulatory |
What it is
Chloroform (trichloromethane) is a dense, volatile liquid historically used as an anaesthetic and industrial solvent. Today, the primary source of human exposure is not industrial — it is the inadvertent formation of chloroform during water treatment. When chlorine disinfects water containing organic matter, trihalomethanes (THMs) form, and chloroform is typically the most abundant. IARC classified chloroform as Group 2B based on sufficient evidence of carcinogenicity in animals (liver and kidney tumours) and inadequate evidence in humans. Chloroform is not genotoxic — its carcinogenic mechanism in animals is through sustained cytotoxicity and regenerative cell proliferation, which has a threshold. This means low-level exposure may not carry the same risk as genotoxic carcinogens.
Where it shows up in bedding
Chloroform is not a mattress or textile contaminant. Its bedroom relevance is indirect: showering or bathing in chlorinated water volatilises chloroform into bathroom air, which then disperses into adjacent rooms including the bedroom. Studies have measured elevated chloroform in indoor air for 30-60 minutes after a shower. For bedrooms adjacent to or sharing ventilation with bathrooms, this creates a transient exposure during the hours around bedtime. Swimming pool attendance also elevates chloroform body burden.
Citations
- IARC (1999). Chloroform. IARC Monographs Vol. 73. Source Peer-reviewed
- ATSDR (1997). Toxicological Profile for Chloroform. Source Regulatory
- EPA. Stage 2 Disinfectants and Disinfection Byproducts Rule. Source Regulatory
Frequently asked questions
Does showering increase chloroform exposure in the bedroom?
Yes. Showering in chlorinated water volatilises chloroform into bathroom air, and this disperses into adjacent rooms. Studies have measured elevated indoor air chloroform for 30-60 minutes after a shower. Running a bathroom exhaust fan during and after showering, or showering with good ventilation, reduces this transfer.
Is chloroform in tap water dangerous?
Drinking water chlorination is one of the most important public health interventions in history. The small amounts of chloroform formed as a by-product are regulated (80-100 µg/L total THMs) and the cancer risk at these levels is far smaller than the risk of waterborne disease from unchlorinated water. The main exposure route is actually inhalation during showering, not ingestion.
Related compounds
Embr is a sleep environment company researching and addressing the chemistry of the bedroom. Research and product development in progress.
Last reviewed 2026-07-07. If you find a factual error, contact us.