What are safe concentrations of radionuclides in air and water?

Drinking water and foodstuffs

Normal Conditions

Under normal conditions, the guidelines for drinking water have been calculated using the recommendations published by the International Commission on Radiological Protection (ICRP) in 1996. Maximum acceptable concentrations (MACs) in drinking water have been established for the natural and man-made radionuclides that are most commonly detected in Canadian water supplies, using internationally accepted equations and principles and based solely on health considerations. They are calculated using a reference dose level of 0.1 mSv for 1 year's consumption of drinking water, assuming a consumption of 2 L/day at the MAC.

(Source: FPT Committee on Drinking Water - Guidelines for Canadian Drinking Water Quality – Summary Table, December, 2010.) (PDF)

Air

• Read about radiological monitoring at Canadian Nuclear Power Plants.

Radioactive releases from nuclear facilities can expose the public through a number of pathways such as ingestion, food stuffs, and inhalation. Rather than apply specific limits to each of these, the CNSC requires that the dose from all of these routes, to the most exposed person, does not exceed the public dose limit (1 mSv) and be as low as reasonably achievable below the public dose limit. This is referred to as the As Low As Reasonably Achievable (ALARA) principle. This is done by setting limits, called Derived Release Limits (DRLs), for the airborne and liquid releases of radioactive materials under normal operation of nuclear facilities.

The DRL takes into account all exposure pathways; material falling onto the ground and absorption by crops and animals and subsequent ingestion by persons and inhalation of air.  The calculation of DRLs is based upon CSA standard N288.1-08: Guidelines for Calculating Derived Release Limits for Radioactive Materials in Airborne and Liquid Effluents for Normal Operation of Nuclear Facilities. The CNSC publishes the releases with respect to the DRLs in INFO-210: Radioactive Release data from Canadian Nuclear Power Plants.

The DRLs serve to apply control at the source of release. Consequently, the release limits, or DRLs as mentioned above, do not specifically limit radioactivity in ambient air, but they do limit rates of radioactivity released at the point of discharge into the environment. They account for all reasonably possible routes of exposure from airborne releases to members of the public, including inhalation.

For the purpose of providing context to measurements of radioactivity in ambient air (e.g., presented in units of Bq/m³), the following provides the concentration of selected relevant radionuclides in air such that if each were to be inhaled throughout a year (i.e., 24 hours per day every day), the resulting dose would be 0.1 mSv. The value of 0.1 mSv was selected as a reference point in this case so as to be consistent with the approach taken in Health Canada's recommended drinking water guidelines. These values were calculated on the basis of CSA N288.1-08 and assume that exposure to the listed radionuclides results only from inhalation. They account for all age groups. These values should not be construed as regulatory limits set by the CNSC, but rather as reference values provided for context.

(Source: CNSC)

Water and Food Guidelines during Nuclear Emergency Situations

Health Canada has produced a guideline for the control of radioactively contaminated drinking water and commercial foods, in the event of a nuclear accident that affects the source of drinking water. The guidelines are based on the limitation of this risk, recognizing the need to maintain the safety of, and public confidence in, the commercial food supply, consistent with the objectives of the Food and Drugs Act (Health Canada 1981/1998). They have been developed following a review of current recommendations on intervention published by various international agencies, including the Food and Agriculture Organization of the United Nations (FAO)/World Health Organization (WHO) Codex Alimentarius Commission (FAO/WHO 1995). Codex guidelines also apply to the international trade of food that has been contaminated with radionuclides in the first year following an emergency.

In these guidelines, intervention to protect the public from the health risks due to radiation exposure takes the form of limits placed on the levels of radionuclides allowed in commercial foods and public water supplies. Action Levels are the point at which the food or beverage is unsafe to the consumer. Food and water containing radionuclides at concentrations above these levels would normally be withdrawn from sale or distribution and substituted with alternate supplies. 

Action levels for food and water are based on an intervention level of 1 millisievert (mSv) applied independently to each of three food groups, assuming that the intervention is completely effective at averting dose. The intervention level is assessed over a period of one year for long-lived radionuclides and 2 months for radionuclides with half-lives less than about 300 hours (12-13 days). The following table has the action levels for those radionuclides thought to pose the greatest radiation dose.  

(1)Bq L^-1 for Drinking Water.
(Source: Health Canada (2000) - Canadian Guidelines for the Restriction of Radioactively Contaminated Food and Water Following a Nuclear Emergency) (PDF)

References

• FPT Committee on Drinking Water - Guidelines for Canadian Drinking Water Quality – Summary Table, December, 2010. (PDF)
• Health Canada (2000) - Canadian Guidelines for the Restriction of Radioactively Contaminated Food and Water Following a Nuclear Emergency (PDF)
• Canadian Standard’s Association (CSA) - N288.1-08: Guidelines for Calculating Derived Release Limits for Radioactive Material in Airborne and Liquid Effluents for Normal Operation of Nuclear Facilities.