Trihalomethanes Drinking Water Guideline

Summary#

The federal government has posted draft Guidelines for Canadian drinking water quality — Trihalomethanes under the Canadian Environmental Protection Act, 1999. The draft proposes a maximum acceptable concentration of 0.100 mg/L (100 µg/L) for total trihalomethanes and is available for comment from January 10 to April 4, 2025 (comments accepted within 60 days).

What it does#

• Proposes a maximum acceptable concentration (MAC) of 0.100 mg/L (100 µg/L) for total trihalomethanes (THMs). Total THMs are the sum of chloroform, BDCM, DBCM and bromoform.
• Sets the measurement approach as a locational running annual average based on at least quarterly samples taken at points in the distribution system that tend to have the highest THM levels.
• Explains the scientific basis: Health Canada derived health-based values (HBVs) including 1.4 mg/L for chloroform and 0.100 mg/L for BDCM, and used the lowest HBV (BDCM) to set the MAC so it protects against the four common THMs.
• Notes that iodinated THMs (I-THMs) were reviewed but no guideline value was set because of limited data.
• Describes monitoring and control options:
  • Analytical methods exist to measure THMs well below the proposed MAC.
  • Ways to reduce THMs include removing natural organic matter before disinfection, changing or optimizing disinfection, using alternative disinfectants, or changing the source water.
  • Distribution-system steps include management plans, reducing water age, flushing, switching disinfectants (for example to chloramines), and aeration — with a clear warning that any step must not weaken disinfection.
• Invites public feedback: written comments can be sent to water-consultations-eau@hc-sc.gc.ca within 60 days after publication.

Who's affected#

• Municipal and regional drinking-water utilities and operators who monitor and treat source water.
• Provincial and territorial regulators that set or enforce water rules.
• Small systems and rural supplies that may need new monitoring or operational changes (the draft does not spell out costs).
• People who use treated water for drinking, showering or bathing — because THM exposure can come from drinking, inhalation and skin contact.
• Public health officials and water-quality consultants involved in testing and system planning.

Why it matters#

• THMs form when chlorine reacts with natural organic matter in raw water. They can increase in warm months and where water sits longest in pipes.
• Studies link THM exposure in drinking water to an increased risk of bladder cancer and possible reproductive effects; animal studies show effects on liver and kidneys. The guideline aims to limit those risks.
• If adopted, the guideline could lead utilities to change monitoring and treatment practices to keep THMs low while ensuring water stays properly disinfected.
• The draft is open for comment, so stakeholders and the public have a chance to weigh in before any final guideline is set.