Answer to Question #413 Submitted to "Ask the Experts"

Q

Does the Health Physics Society have a position on the issue of linear vs. threshold curves for dose vs. biological effect? If so, what is it? Is there any credible scientific data to justify extrapolation to zero dose? If so, what is it?

A

The Health Physics Society (HPS) does not have an explicit statement regarding the appropriateness of the linear no-threshold (LNT) theory in radiation protection. However, the HPS has issued two position statements that are closely related to the LNT question. These statements, "Risk Assessment" and "Radiation Risk in Perspective," deal with the important question of extrapolating risk to dose levels typically encountered in occupational and environmental settings. These statements may be downloaded from the HPS website (Risk Assessment and Radiation Risk in Perspective). There is credible evidence to support a linear extrapolation to zero dose. There is also credible evidence to support a threshold and also curvilinear dose responses. The problem is that most of the evidence is at dose levels much higher than doses of interest in radiation protection, making it difficult to select one theory to the exclusion of other biologically plausible theories. Regarding support of the LNT theory, some evidence does exist at doses less than 100 mGy from cellular studies. You may wish to look at some of the papers from Eric Hall's lab at Columbia University. Extrapolating cellular effects to cancer and other whole organism effects is difficult. Thus, the relevance of cellular studies to risk at low doses is questionable.

Kenneth L. Mossman