The late astronomer and science communicator, Carl Sagan, once wrote, "We live in a society exquisitely dependent on science and technology. We have also arranged things so that almost no one understands science and technology. This is a prescription for disaster." One of the many ways in which this observation plays out is a phenomenon that some have called "radiophobia", an irrational, disproportionate fear of radiation, which exists in many countries, but in the United States in particular. This is not to say that people avoid exposure to radiation beyond the background levels present where they live. Rather they tend to fear certain types of radiation exposure while not hesitating to be exposed to other types. The so called "natural" food industry has campaigned fiercely against irradiation of foods, even though studies have demonstrated that irradiation makes certain foods, meat in particular, safer. At no time do we encounter more fear of radiation exposure than during pregnancy. Or at least to radiation from medical sources. I have witnessed patients and obstetricians agreeing that it is best to avoid exposing a woman's fetus to the miniscule dose ionizing radiation given during radiography of the pelvis to check for cephalopelvic disproportion, thus to be sure whether or not a cesarian section will be necessary, yet both patient and clinician smiling as the woman remarks that she'll be taking an intercontinental flight in the next few weeks.
Somehow, the obstetrician did not know that the flight would expose the fetus to a dose of radiation several times higher than the pelvic X-ray would. Not the exposure on the flight would be dangerous to the fetus either; though you may be exposed to a chest X-ray worth of radiation every 1-3 hours in the air on a commercial flight (depending on altitude and the route), there is no evidence that such exposures cause cancer. This is in the category of low dose radiation exposure, not much higher than the doses to which you are exposed simply by walking around on Earth every day. Our cells have repair mechanisms, various enzyme systems whose job is to look for mistakes in DNA sequences and damage to cell structure and fix them when they are brought on by radiation. There is evidence from animal studies that these mechanisms are induced by exposure to low levels of radiation and that avoiding such exposures weakens the cellular response to higher levels.
Most people are a little confused about what ionizing radiation is, an example of of Sagan's observation that society does not understand science and technology. But even those who were able to get through high school, college, and graduate school without taking any class in physics may recall from high school biology something about the effects of sunlight on skin cells. True, we have been inundated with warnings about Sun exposure and recommendations to use sunscreen. Nevertheless, you may recall that exposure to a type of solar radiation known as ultraviolet light stimulates cells known as melanocytes to produce melanin. This tans your skin and in doing so reduces the amount of ultraviolet light that can reach skin cells. It is an adaptive response, and it is protective, which is one reason why a little bit of sunlight every day, without sunscreen, is good for you (another reason is that it enables your cells to synthesize vitamin D).Perhaps, because society never was faced with movies in the 1950s about mutant giants being created as a result of heavy sunlight exposure, society does not fear radiation from the Sun the way it fears it from other radiation sources, like nuclear reactors.
Now, if you're in the process of buying a home, or moving into a home, or trying to make a very old home safer, it is likely that you are thinking a lot about radon. Unlike the Environmental Protection Agency (EPA) -which I suspect is going to change its mind on radon at some point (the question is just "when?")- I'm going to tell you that the question of whether radon causes lung cancer is very complicated and nuanced. Having read through the scientific literature regarding radon in homes and in mines, lung cancer, smokers vs. nonsmokers, and all of that, I'm rather startled that the EPA is so convinced that the levels of radon present in many homes in the United States causes lung cancer, because the evidence just isn't there. Yes, there have been several studies published during the last 15 years drawing such a conclusion, but they all use extrapolation as the method by which they arrive at their conclusion that the concentration of radon gas in the air of homes should be made as low as reasonably achievable.
What types of studies are we talking about? Essentially, all of the studies concluding that the levels of radon present in many homes cause lung cancer use data that come from men working underground in mines, where radon levels are ten times or more what they are in most homes. Eliminating the contribution of smoking, which of course causes most cases of lung cancer, various analyses have made it very clear that high levels of radon gas cause lung cancer, just as high levels of other types of radiation certain do cause various other cancers, including lung. Extrapolating backward from high doses to conclude that if high doses of radiation cause a lot of cancer then low doses of the same radiation must cause a little bit of cancer is known in epidemiology as the linear no-threshold model. It means that not matter how low the radiation dose gets, it always is bad. But in the mid 1990s the radiation physicist, Bernard Cohen, of the University of Pittsburgh, published a meta-analysis based on eight different epidemiological studies, using data low dose exposures. What Cohen found was that once you get below a certain concentration of radon, the association between exposure and cancer is not linear. Instead what happens, Cohen found, is that while the lung cancer rate goes down in a linear fashion as the concentration of radiation from radon decreases on the order of dozens of picocuries/liter (pCi/L), once you get below 8 pCi/L, the lung cancer death rate goes up again, suggesting that radon exposure in this range actually helps to prevent lung cancer rather than causing it. If correct, this would be an example of radiation hormesis, the phenomenon in which small doses of radiation act to prevent the same cancer than higher doses would cause.
The EPA recommends reducing levels of radon in the home to below 4 pCi/L, relying on those indirect, extrapolation studies, using data from high dose exposure in mines, and despite Cohen's findings which depended on actual low dose exposure data. If Cohen is correct then spending all of the time and money to clear homes of such levels actually would cause an increase in lung cancer incidence. On the other hand, Cohen's critics -many of the authors of the indirect studies- say that they do not agree with how he accounted for the smokers vs. the non-smokers in the studies that went into the meta-analysis. They may have a point, although they do not explain very well why they think that smokers are exposed to less radon than non-smokers. But whether they are right or wrong about the smoking issue, there still are no data to support their studies, which means that Cohen may very well turn out to be correct.
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