What it is:

  • Trihalomethanes (THMs) are formed along with numerous other disinfection byproducts when chlorine reacts with organic matter in water (chlorination).
  • THMs were regulated primarily as indicators of the bulk of disinfection byproducts that are not readily measurable, so that reductions of formation of THMs (and also haloacetic acids, HAAs) would simultaneously reduce the formation of other unregulated DBPs.
  • THMs and HAAs may account for about half of the total organic halogen product mass.
  • They consist of four chemicals: Chloroform, bromodichloromethane, dibromochloromethane and bromoform; some mixed iodinated THMs are also formed at very low levels when iodide ion is present in the water.
  • Chloroform is the most prevalent THM in water systems.
  • Brominated THMs are formed from bromide ion in water whereby the bromide is first oxidized to HOBr or OBr- by chlorine and then competes effectively with chlorine in the halogenation processes.

Occurrence:

  • In the presence of free chlorine residuals, THMs continue to form during water distribution to consumers.
  • While drinking water consumption is the principal source of THM exposure to the general population, showers are another way people can be exposed because the THMs are VOCs and somewhat hydrophobic.
  • Frequent swimmers, especially in indoor pools that have been chlorinated, can also receive substantial exposure to THMs from inhalation, dermal contact and some ingestion of water while swimming.

Health effects:

  • The International Agency for Research on Cancer (IARC) classifies chloroform and bromodicloromethane as Group 2B, "possibly carcinogenic to humans," and bromoform and dibromomethane as Group 3, "not classifiable as to carcinogenicity to humans."
  • The World Health Organization has calculated Tolerable Daily Intakes (TDIs) with safety factors, similar to reference doses (RfDs), for chloroform, bromoform and dichlorobromomethane.

Water treatment:

  • The Environmental Protection Agency (EPA) considers that THMs are best managed by using treatment techniques that reduce the formation of THMs rather than to remove them after formation. Some of these include: Chloramine for combined chlorine residual, chlorine dioxide and ozone followed by chloramine.

Regulation:

  • THMs were originally regulated in the late 1970s at 100 ppb. The current MCL of 80 ppb implemented by quarterly averages at each sampling site has been established by the EPA under its Disinfectants/Disinfection Byproducts Rule. The reduced MCL was based upon technical and economic feasibility.
  • EPA does not consider chloroform to be a non-threshold carcinogen and it has an MCLG of 0.07 mg/L.

Sources: Environmental Protection Agency, NSF, World Health Organization, International Agency for Research on Cancer, Environmental Working Group, New Hampshire Department of Environmental Services.