Simple Nature - Light and Matter

tion better against diptheria; increases fertility in trout and mice;improves fetal mice’s resistance to disease; increases the life-spans offlour beetles and mice; and reduces mortality from cancer in mice.This type of effect is called radiation hormesis.There is also some evidence that in humans, small doses of radiationincrease fertility, reduce genetic abnormalities, and reducemortality from cancer. The human data, however, tend to be verypoor compared to the animal data. Due to ethical issues, one cannotdo controlled experiments in humans. For example, one of the bestsources of information has been from the survivors of the Hiroshimaand Nagasaki bomb blasts, but these people were also exposed tohigh levels of carcinogenic chemicals in the smoke from their burningcities; for comparison, firefighters have a heightened risk of cancer,and there are also significant concerns about cancer from the 9/11attacks in New York. The direct empirical evidence about radiationhormesis in humans is therefore not good enough to tell us anythingunambiguous, 3 and the most scientifically reasonable approach is toassume that the results in animals also hold for humans: small dosesof radiation in humans are beneficial, rather than harmful. However,a variety of cultural and historical factors have led to a situation inwhich public health policy is based on the assumption, known as“linear no-threshold” (LNT), that even tiny doses of radiation areharmful, and that the risk they carry is proportional to the dose.In other words, law and policy are made based on the assumptionthat the effects of radiation on humans are dramatically differentthan its effects on mice and guinea pigs. Even with the unrealisticassumption of LNT, one can still evaluate risks by comparingwith natural background radiation. For example, we can see thatthe effect of a chest x-ray is about a hundred times smaller thanthe effect of spending a year in Colorado, where the level of naturalbackground radiation from cosmic rays is higher than average, dueto the high altitude. Dropping the implausible LNT assumption, wecan see that the impact on one’s health of spending a year in Coloradois likely to be positive, because the excess radiation is belowthe maximum beneficial level.d / A typical example of radiationhormesis: the health ofmice is improved by low levelsof radiation. In this study, youngmice were exposed to fairly highlevels of x-rays, while a controlgroup of mice was not exposed.The mice were weighed, andtheir rate of growth was takenas a measure of their health. Atlevels below about 50,000 µSv,the radiation had a beneficialeffect on the health of the mice,presumably by activating cellulardamage control mechanisms.The two highest data pointsare statistically significant at the99% level. The curve is a fit toa theoretical model. Redrawnfrom T.D. Luckey, Hormesis withIonizing Radiation, CRC Press,1980.8.5.5 The green case for nuclear powerIn the late twentieth century, antinuclear activists largely succeededin bringing construction of new nuclear power plants to ahalt in the U.S. Ironically, we now know that the burning of fossilfuels, which leads to global warming, is a far more grave threat tothe environment than even the Chernobyl disaster. A team of biologistswrites: “During recent visits to Chernobyl, we experienced3 For two opposing viewpoints, see Tubiana et al., “The Linear No-ThresholdRelationship Is Inconsistent with Radiation Biologic and Experimental Data,”Radiology, 251 (2009) 13 and Little et al., “ Risks Associated with Low Dosesand Low Dose Rates of Ionizing Radiation: Why Linearity May Be (Almost) theBest We Can Do,” Radiology, 251 (2009) 6.Section 8.5 Biological Effects of Ionizing Radiation 501