Trihalomethanes. Toxic chemical substances that consist of a methane molecule and one of the halogen elements fluorine, bromine, chlorine and iodine attached to three positions of the molecule.High concentrations of Trihalomethanes can have detrimental effects on humans when exposed such as increases in cancers or bladder problems. They usually have carcinogenic properties, and are most efficiently removed through the use of a carbon filter. However, you must ensure that your filter is certified to remove THM’s and there is an instruction set that tells you when to replace it.