- a type of chemical compound in which three of the hydrogen atoms in a methane molecule have been replaced by halogen atoms, esp by chlorine in drinking water. Trihalomethanes are thought to be carcinogenic
- Any of various organic compounds containing three halogen atoms substituted for three of the hydrogen atoms in a methane molecule. It can occur in chlorinated water as a result of reaction between organic materials in the water and chlorine added as a disinfectant. Chloroform is a trihalomethane.
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