Health Canada has updated its technical guidance on radionuclides in drinking water, setting maximum acceptable concentrations (MACs) for the radioactive substances most commonly found in Canadian water supplies and outlining monitoring, treatment, and risk management expectations for utilities.
The guideline establishes MACs for three naturally-occurring radionuclides: lead-210 (Pb-210), radium-226 (Ra-226), and radium-228 (Ra-228), all of which are identified as the most significant contributors to radiation from drinking water in Canada. The limits are based on a reference radiation dose of one millisievert per year (mSv/y), assuming an average daily water consumption of 1.53 litres.
Health officials emphasize that exceeding a MAC does not pose an immediate health risk, but it does trigger further investigation and follow-up by water system operators.
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The guidance applies to water as consumed at the tap, meaning sampling and monitoring should reflect finished drinking water rather than untreated source water. For systems that treat water prior to distribution, monitoring should occur after treatment, as many radionuclides can be reduced through treatment processes. In most cases, radionuclide concentrations are not expected to change significantly within the distribution system, making post-treatment or reservoir sampling appropriate.
The technical document was developed in collaboration with the Federal-Provincial-Territorial Committee on Drinking Water and draws on international health risk assessments, including dosimetric models released by the International Commission on Radiological Protection in 2007.
Radionuclides occur naturally in the environment, and Canadians are routinely exposed to background radiation from cosmic and terrestrial sources. While natural sources account for more than 98% of radiation exposure (excluding medical sources), drinking water is typically a minor contributor. The guidelines apply under routine operating conditions for both existing and new water systems and do not address nuclear accidents, which fall under provincial emergency response plans.
A screening criterion equivalent to a dose of 0.3 mSv/y has also been established for adults. If this level is exceeded, the water remains acceptable for children over one year of age and adults, but Health Canada recommends using an alternative water source — such as bottled water — for the preparation of infant formula.
While elevated levels of radionuclides other than Pb-210, Ra-226, and Ra-228 are rare in Canadian drinking water, the guideline includes reference concentrations for additional natural radionuclides, such as polonium-210 and radon-222, as well as several artificial radionuclides. These substances are generally present at very low levels in the Canadian environment.
Higher concentrations of natural radionuclides may occur in limited areas, particularly where groundwater interacts with uranium-bearing rock or where industrial activities such as uranium mining are present. Artificial radionuclides — originating from historical nuclear weapons testing or nuclear facilities — are typically detected only at trace levels in Canada.
The primary health concern associated with long-term exposure to low-level radiation is an increased risk of cancer, as ionizing radiation can damage DNA if cellular repair mechanisms fail.
At the municipal scale, treatment technologies such as reverse osmosis, ion exchange, and lime softening can effectively remove most radionuclides from drinking water. Utilities are advised to consider that treatment residuals may contain low-level radioactivity, requiring appropriate handling and disposal. Similar treatment options are available at the residential level, with no special precautions generally required for homeowners.
The guideline also highlights the potential for radionuclides to accumulate in distribution system piping under certain conditions. Utilities are encouraged to include radionuclides in monitoring and distribution system management plans where source water concentrations warrant it, and to apply best practices that limit corrosion, maintain stable water quality, and reduce disturbances that could release accumulated contaminants.
Radon remains the largest contributor to radiation exposure for most Canadians, primarily through inhalation rather than drinking water. Because radon in indoor air can originate from multiple sources, Health Canada does not set a drinking water MAC for radon and instead recommends indoor air testing and mitigation when levels exceed national guidelines.
Overall, regulators note that radionuclide concentrations above MACs are expected to occur in only a small number of Canadian drinking water systems, and that effective treatment and management options are widely available.
