Cohen's conclusion: High level radioactivity kills, but low level radioactivity does not; it is beneficial.
Everett DeJager
Cincinnati, OH
To test the linear-no threshold theory (LNT), I developed a compilation of radon measurements from available sources which gives the average radon level in homes for 1,729 U.S. counties, comprising about 90 percent of the total U.S. population. Plots of age-adjusted lung cancer mortality rates vs. these radon levels show an unquestionable tendency for lung cancer rates--with or without correction for smoking prevalence--to decrease with increasing radon exposure, in sharp contrast to the increase expected from the fact that radon can cause lung cancer.
In quantitative terms, the theory predicts an increase at a rate of 7.3 percent per unit (pCi/L) of radon exposure, whereas the data indicate a decrease of 7.7 percent, with an uncertainty of only 0.5 percent. This finding started a study that extended over many years.
One problem was that this is an "ecological study," relating the average risk of groups (county populations) to their average exposure. In general, the average dose does not determine the average risk, and to assume otherwise is what epidemiologists call "the ecological fallacy." However, it is easily shown that the ecological fallacy does not apply in testing LNT. All other problems with ecological studies that have been discussed in the epidemiological literature have also been investigated and found not to be applicable here.
All explanations for the discrepancy that we could develop or that have been suggested by others have been tested and found to be grossly inadequate. Three independent sources of radon data have been used, but all give the same result. Three different sources of data on smoking prevalence similarly fail to explain away the finding. In fact, even a perfect negative correlation between radon and smoking prevalence cannot eliminate the discrepancy.
Effects of confounding were studied for over 500 potential confounding factors, but these did little to explain our discrepancy. For example, the strong negative correlation between lung cancer rates and radon exposure is found if we consider only the very urban counties or only the very rural; if we consider only the richest counties or only the poorest; if we consider only the counties with the best medical care or only those with the poorest medical care; if we consider only the wettest counties or only the driest; if we consider only the warmest counties or only the coolest; and so forth for all 500 potential confounding factors. It is also found for all strata in between--for example, considering only counties of average wealth, only counties of average medical care, only counties of average temperature, etc. It is also found if we consider only counties in a given section of the country.
The only plausible explanation I can find for this discrepancy between LNT and the observations is that the linear-no threshold theory fails, grossly overestimating cancer risk in the low dose, low-dose rate region. There are no other data capable of testing the theory in that region.
Bernard L. Cohen
Physics Department
University of Pittsburgh
Pittsburgh, PA
Note: This is a condensation of a paper by Cohen, "Test of the Linear-No Threshold Theory of Radiation Carcinogenesis for Inhaled Radon Decay Products," Health Physics, 68 (1995), pp. 157-174.
The article about radon was an excellent contribution, but I have one possible correction. I believe that, although radon is an alpha emitter, this radioactivity is not the source of its suspected harmful effect.
I believe the danger from radon, and the reason it is more dangerous in the lungs, is due simply to the chemical nature of its ultimate daughter. Radon emits an alpha and mutates to polonium. Polonium is also an alpha emitter and, in turn, quickly mutates (with a half-life of several minutes or less, depending on the isotope) to lead. I believe it is the deposition on lung tissue of metallic lead from radon that is believed to cause lung cancer, not genetic mutation caused directly by energetic alpha particles.
Thomas Cunningham
NASA Jet Propulsion Laboratory
Pasadena, CA
Kenneth Silber replies: My article's fourth paragraph pointed out the correlation between high residential radon levels and low lung cancer rates in regions such as New England and the Rocky Mountain states. I also noted that some scientists speculate that radon can help prevent lung cancer. While I did not specifically cite Bernard L. Cohen's research, I did take it into account, along with other studies of radon's health effects.
In reply to Thomas Cunningham's letter, both radon's alpha emissions and its daughter products have been cited as possible health threats.
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