Draft Trihalomethanes Drinking-Water Guideline

Summary#

Health Canada has published a draft of the Guidelines for Canadian drinking water quality — Trihalomethanes. It proposes a maximum acceptable concentration for total trihalomethanes (THMs) of 0.100 mg/L (100 µg/L) and the draft is available from January 10, 2025 to April 4, 2025; written comments are accepted for 60 days after publication.

What it does#

• Proposes a single limit for total THMs (the sum of the four THM species: chloroform, BDCM, DBCM, and bromoform) of 0.100 mg/L (100 µg/L).
• Says that compliance should be measured as a locational running annual average using a minimum of quarterly samples taken at points in the distribution system where THMs are likely highest.
• Explains the scientific basis: Health-based values were calculated for some THMs (chloroform 1.4 mg/L, BDCM 0.100 mg/L) and the proposed total-THM number is set to be protective based on the lowest health-based value.
• Notes limited data for iodinated THMs (I-THMs) and for some brominated THMs, so no guideline values were derived for those.
• Gives practical guidance to reduce THMs: measure total THMs with available lab methods, reduce natural organic matter before disinfection, optimize or change disinfection approaches (without weakening disinfection), and manage distribution systems (flushing, lowering water age, switching to chloramines, aeration, modelling).

Who's affected#

• Municipal and private water utilities and their operators — they would need to monitor, report and manage THMs to meet the proposed approach.
• Public health and regulatory officials who set and enforce local drinking-water requirements.
• People who use municipal tap water, especially in systems with long water age or during summer/fall when THMs tend to be higher.
• Laboratories and companies that provide water-treatment or monitoring equipment and services.
• It is unclear from the notice whether the draft would immediately change legal requirements; this is a proposed guideline open for comment.

Why it matters#

• THMs form when chlorine reacts with natural organic matter in source water. Some studies link THM exposure to bladder cancer and certain reproductive or developmental effects, so the guideline aims to reduce health risks from drinking water.
• A clear numerical target (0.100 mg/L) gives utilities a measurable goal and could prompt changes in treatment and distribution practices to lower THM levels.
• Because THMs keep forming in pipes, the guideline emphasizes distribution system management, not just treatment at the plant.
• This is a draft guideline. If you want to comment, Health Canada asks for written feedback by email to water-consultations-eau@hc-sc.gc.ca during the consultation period noted above.