There is no Henry's constant for the group defined as total trihalomethanes (TTHM). The EPA values for Henry's Law constants for the four regulated trihalomethane species in units of M/atm are:
chloroform: 3.0 x 10-1
dichlorobromomethane: 4.6 x 10-1
dibromochloromethane: 1.2
bromoform: 1.8