My Final Question

Suppose I died tomorrow. When my soul came before that great scientist in the sky, she might say “because of your contributions to Intermediate Physics for Medicine and Biology, I’ll answer one question for you. What is your question?” I can think of many things I would like to know, but the question I’d ask is: “is the linear no-threshold model appropriate at low doses?”

What’s the linear no-threshold model, and why’s it so important? Russ Hobbie and I explain it in Chapter 16 of IPMB.

In dealing with radiation to the population at large, or to populations of radiation workers, the policy of the various regulatory agencies has been to adopt the linear no-threshold (LNT) model to extrapolate from what is known about the excess risk of cancer at moderately high doses and high dose rates to low doses, including those below natural background.

If the excess probability of acquiring a particular disease is αH in a population N [where H is the equivalent dose per person in sieverts and α is a proportionality constant], the average number of extra persons with the disease is

                 m = α N H.         (16.42)

The product NH, expressed in person Sv, is called the collective dose. It is widely used in radiation protection, but it is meaningful only if the LNT assumption is correct. Small doses to very large populations can give fairly large values of m, assuming that the value of α determined at large doses is valid at small doses.”

Let me give you an example of why this question is so consequential. Should our society spend its time and money trying to reduce radon exposure in people’s homes? Radon is a radioactive gas that is produced in the decay chain of uranium. This noble gas can seep into basements, where it may be breathed into the lungs. The decay of radon and its progeny can cause lung cancer. However, the typical yearly dose from radon is very low, about 2 mSv. For an individual the resulting cancer risk is tiny, but if the linear no-threshold model is correct then when multiplied by the population of the United States (over 300,000,000) there can be tens of thousands of cancer deaths each year caused by radon. On the other hand, if a threshold exists below which there is no risk of cancer, then radon probably causes few if any cancer deaths. So, from a public health perspective, the answer to my question about the validity of the linear no-threshold model is crucial.

Other examples are

• The severity of low-dose, widespread exposure to radiation caused by a terrorist attack, such as a small amount of radioactivity dissolved in the water supply of a major city, 
• The hazard caused by low-dose x-ray backscatter scanners used at airports for security, 
• The danger from the release into the Pacific Ocean of minuscule amounts of radioactivity in water leftover from the Fukushima nuclear accident, or 
• The risks associated with storing radioactive waste from nuclear power plants in underground storage facilities.

All these situations have one thing in common: small individual doses to a large number of people. Public health officials need to know whether or not the collective dose is significant, and is it something we should spend our scarce resources trying to minimize.

On that fateful day I fear that even if she answers my last question, I won’t be able share it with you, dear readers (I don’t think they allow blogging down there). We’ll just have to examine the evidence available today.