Low-Level Radiation: How the Linear No-Threshold Model Keeps Canadians Safe

December 16, 2009

Radiation protection risk models

The Linear No-Threshold model (LNT) is the most widely accepted risk model used by health agencies and nuclear regulators around the world, including the CNSC. For radiation protection practices, it assumes a direct and proportional relationship between radiation exposure and cancer risk with all radiation doses.

The risks from radiation have been largely derived from atomic bomb survivor studies, where the incidence of disease (principally cancer) was plotted against radiation dose.  Where there was data, the dose response was linear, meaning that as dose increased, so did cancer risk.  However, below the lowest data point, the natural incidence of the disease masked any effects that may have been caused by radiation.  Because of this, the LNT assumes that cancer incidence as it relates to radiation dose behaves in the same way as at higher doses; that is, in a linear manner. 

The adoption of the LNT model led to the development of the ALARA principle, which is a fundamental aspect of the CNSC’s approach to radiation protection.

However, given that there is little evidence of adverse health effects at chronic doses below 100 mSv, there is no certainty to the LNT risk model.  In response to this, some scientists have proposed alternative risk models.   

Figure 1 illustrates some of the models proposed by scientists to estimate the radiation risk below the lowest doses known to show excess cancer (about 100 mSv). 

• The LNT model is the straight line that runs to zero, meaning that cancer risk will rise with increasing dose starting at background levels. 
• The Threshold model implies that below a certain dose, there is no risk.
• The Hypersensitivity model suggests a greater risk at lower doses. 
• The Hormesis model suggests that low radiation doses may even be protective and beneficial. 
• The data points represent known epidemiological data at 100 mSv and above.

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Figure 1: Radiation Protection Models

The 2006 BBC documentary entitled Nuclear Nightmares* challenged the accuracy of the LNT model, leaning in favour of the Threshold and Hormesis models, which it admitted were controversial.

Reports by the International Commission on Radiological Protection (ICRP 99, 2006) and the U.S. National Research Council of the National Academies (BEIR VII, 2006) stated that while evidence supports other models, the LNT model provides the best overall fit for radiation protection purposes. 

In November 2009, the Electrical Power Research Institute (EPRI) published a comprehensive evaluation of scientific literature on the health effects of low doses of radiation*, with emphasis on new information. It came to three key conclusions:

1. Risks due to low dose-rate effects may be over-estimated.
2. Individual radiation doses of less than 10 rem (100 mSv) in a single exposure are too small to detect any statistically significant excess cancer.
3. Research into health effects of low-dose radiation needs to continue, combining the latest radiobiology and epidemiology results to produce a comprehensive understanding of radiation risk.

* Important: Unless specifically stated, the CNSC does not endorse the viewpoints, opinions or conclusions reached in any referenced third-party materials.

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The CNSC’s perspective

The CNSC would not license nuclear activities that are unsafe; the LNT model provides one way to ensure safety.

The CNSC applies the LNT in its approach to radiation protection, as a prudent way to protect Canadians from radiation. If new research revealed another model that provided the same degree of safety as the LNT and that was supported by the international radiation protection community, then the CNSC would consider adopting such a model. For now, however, the CNSC keeps pace with its international peers by using the LNT.

The CNSC would not license nuclear activities that are unsafe, and the LNT model provides one way to ensure safety. The model provides an understanding of radiation health risks that allows CNSC and other regulatory bodies to set dose limits and regulations that limit exposure to an acceptable level.  Radiation doses at or below these limits are considered "safe" in that there is no direct medical or scientific evidence to show that they cause harm. To protect nuclear workers and members of the public, the CNSC licenses and monitors nuclear activities to control doses.

Regulatory radiation dose limits, for workers at nuclear facilities and members of the general public, are set to restrict exposures to acceptable levels. The CNSC also requires all licensees to keep radiation levels As Low As Reasonably Achievable (ALARA), economic and social factors being taken into account.
 
The regulatory dose limit for a member of the public in Canada is 1 millisievert (mSv) per year, while the dose limit for a nuclear energy worker is 50 mSv per year and 100 mSv over a five-year period. These limits are based on international standards that have been derived from the best scientific knowledge available.

Canada’s nuclear facilities have a strong track record of keeping radiation doses far below the limits set by the regulations. Figure 2 shows that the annual radiation doses to members of the public living near nuclear power plants were under one percent of the 1 mSv limit.

Figure 2: Public doses around Canadian nuclear power plants as a percentage of annual public dose limit of 1 mSv.

Questions or Comments?

The CNSC welcomes questions and comments from all its stakeholders. It is committed to its mandate to regulate the nuclear industry to protect Canadians and the environment, which includes disseminating scientific information to the public about how it carries out its work.

Related Links

• Electrical Power Research Institute report (Source: EPRI)
• BBC’s Nuclear Nightmares (Source BBC Web site)
• Read “Highlight 1: Explaining Radiation Doses” of CNSC’s 2008-2009 Annual Report (p. 20)
• Health related studies and information documents
• Radiation, People and the Environment (Source: IAEA)
• International Commission on Radiological Protection
• United Nations Committee on the Effects of Atomic Radiation
• Beir VII: Health Risks from Exposure to Low Levels of Ionizing Radiation
• ICRP Publication 99:  Low-Dose Extrapolation Of Radiation Related Cancer Risk

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