Draft Trihalomethanes Drinking Water Guideline

Summary#

Health Canada has published a draft called Guidelines for Canadian drinking water quality — Trihalomethanes and is asking for feedback. The draft proposes a maximum for total trihalomethanes (THMs) and is available for review from January 10, 2025 to April 4, 2025; comments are invited for 60 days after the notice.

What it does#

• Proposes a maximum acceptable concentration (MAC) of 0.100 mg/L (100 µg/L) for total THMs. Total THMs means the sum of chloroform, BDCM, DBCM and bromoform.
• Says the MAC would be assessed as a locational running annual average, based on at least quarterly samples taken at the places in a distribution system most likely to have the highest THM levels.
• Notes that analytical methods exist to measure THMs well below the proposed MAC and that measurements should report the four named THM species.
• Advises water systems on ways to lower THMs, including removing natural organic matter before chlorination, optimizing disinfection, considering alternative disinfectants (for example switching to chloramines), reducing water age, flushing, distribution-system management and, where appropriate, aeration — with the repeated caution that any control must not reduce disinfection effectiveness.
• Reports an assessment of iodinated THMs (I‑THMs) but does not propose a guideline for them because of limited data.
• Invites written comments by email to water-consultations-eau@hc-sc.gc.ca during the stated consultation period.

Who's affected#

• Municipal and regional water utilities and their operators who monitor and treat drinking water.
• Provincial and territorial regulators and public health authorities involved in drinking-water standards.
• Laboratories that test drinking water for THMs.
• People who get drinking water from systems that use chlorine and might have higher THM levels (for example systems with older water or during summer/fall).
• The general public, because the draft guides how water safety and treatment might change; if it is finalized, utilities may need to adjust monitoring or treatment.

Why it matters#

• THMs form when chlorine reacts with natural organic matter in raw water. People are exposed by drinking water and by breathing or absorbing THMs during showers and baths.
• Health studies link THM exposure with bladder cancer and possible reproductive effects. The proposed MAC is set to limit those risks based on available animal and human evidence.
• If finalized, the guideline could lead utilities to change monitoring and treatment practices to lower THMs. That can affect costs, operations and possibly local water taste or treatment methods — while keeping disinfection working properly.
• This is a draft guideline and not a regulation. People and organizations can comment during the consultation window (document available January 10, 2025 to April 4, 2025).