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No. 13 – The Linear Model Controversy

January 20, 2011
The concept of the linear no-threshold dose model has probably done more to arouse people’s fears of radiation risks than anything else we have done in the name of radiation safety. According to this concept, each increment of radiation dose carries with it an increment of risk. Therefore, when we are asked about radiation safety, we have to say there is a probability of risk down to zero dose.

The linear model has led to the notion that a single atom of plutonium, for example, is enough to cause lung cancer and death. Thus, citizens loudly proclaim that even a few atoms of man-made radioactive material are unacceptable. People want zero radiation risk, which requires zero radiation dose, and demand protection accordingly.

We have used the linear model for so many years as a tool for estimating radiation risks, that most people (including some health physicists) have forgotten that it is only a model. The public has also readily adopted the model as reality because it fits the popular perception of radiation risk. Namely, if radiation is there, it is bad for you.

The news media has fueled such perceptions by the common use of two-word risk assessments. Radiation is often referred to as “lethal radiation,” “deadly radiation,” “dangerous radiation,” or “toxic radiation.” News stories seldom evaluate radiation risks in terms of the amount of radioactive material, how it is contained, where it is located, the pathways for exposure, or the doses that may be received.

When local citizens near the campus of the National Institutes of Health heard that radioactive materials were released from the stack of the incinerator, they strongly objected. These citizens were not interested in the fact that public doses would only be fractions of a mrem per year or that an NRC inspector said he would be willing to sit on the stack and breath the air. The bottom line was that radioactive atoms were being released and that was not acceptable. By the way, the NIH stopped incinerator operations in response to these community concerns.

The use of the linear model has forced health physicists to say risks are a matter of probability. We cannot say absolutely that any dose is safe or not safe. The public then translates population risk probabilities into individual probabilities and concludes that no probability of risk is acceptable. Risk communication would be much simpler if we could say no risks are expected below some threshold dose. However, even if we discontinue using the linear model for low doses, it will be very difficult to change strongly entrenched risk perceptions based on the linear model.

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