Modern science is constantly under attack from political forces, often fueled by fear. A big fear is radiation exposure – a fear made only too real by the devastation of the atomic bombs dropped on Japan to end Word War II, and the aftereffects of several extensive nuclear accidents around the world in the last few decades. But, while high doses of radiation are known to be harmful to human health or even deadly, the effects of low doses are controversial.
For many years, the prevailing wisdom in the scientific community about radiation protection has been that there is no safe dose of ionizing radiation. This belief is enshrined in the so-called LNT (linear, no-threshold) model used to estimate cancer risks and establish cleanup levels in radioactively contaminated environments. The model dates back to studies of irradiated fruit flies in the 1930s, and subsequent formulation of the LNT dose-response model by American geneticist and Nobel laureate Hermann Muller.
The LNT model assumes that the body’s response to radiation is directly proportional to the radiation dose. So any detrimental health effects – such as cancer or an inheritable genetic mutation – go up and down with dose (and dose rate), but don’t disappear altogether until the dose falls to zero.
A very different concept that is gaining acceptance among radiation workers is the threshold model. Unlike the LNT model, this assumes that exposure to radiation is safe as long as the exposure is below a threshold dose. That is, there are no adverse health effects at all at low radiation doses, but above the threshold there are effects proportional to the dose, as in the no-threshold model.
A new variation on the threshold model is hormesis, which hypothesizes that below the threshold dose, beneficial health effects actually occur. Hormesis has been championed by Edward Calabrese, an environmental toxicologist at the University of Massachusetts Amherst who has long been critical of the LNT approach to risk assessment, for both radiation and toxic chemicals. In 2015, a petition was submitted to the U.S. NRC (Nuclear Regulatory Commission) to adopt the hormesis model for regulatory purposes.
Which model is the correct picture of how the human body is affected by radiation? The scientific evidence isn’t all that clear.
Even when the LNT model was proposed, only very limited data was available at low doses, a situation that’s unchanged today. This means that the statistical accuracy of individual data points at low doses is poor, and much of the data could equally well fit the LNT, threshold or hormesis models. Two major pieces of evidence that a U.S. NAS (National Academy of Sciences) committee formerly relied on to buttress the LNT model – a study of Japanese atomic bomb survivors and a 15-country study of nuclear workers – are in fact compatible with either the threshold or the LNT model, more recent analysis has shown.
The threshold model may seem more intuitive, since it’s well known for chemical toxins that any substance is toxic above a certain dose. “The dose makes the poisin,” as medieval Swiss physician Paracelsus observed. But the biological response to radiation isn’t necessarily the same as the response to a toxin.
Evidence in support of the hormesis model, however, includes numerous studies showing that low radiation doses can activate the immune system and thereby protect health. And no increase in the incidence of cancer has been observed among those Japanese bomb survivors exposed to only low doses of the same radiation that, in higher doses, sickened or killed others.
Scientific opinion is divided. The once strong consensus on the validity of the LNT model has evaporated, 70% of scientists at U.S. national laboratories now believing that the threshold model more accurately reflects radiation effects. A similar percentage of scientists in several European countries hold the same view.
Whether or not low doses of radiation are protective, as the hormesis model suggests, no adverse health effects have ever been detected from exposure to low dose, low dose rate radiation. But the public clings to the outmoded scientific consensus of the LNT model that no dose is safe. So society at large is unnecessarily fearful of any exposure to radiation whatsoever, when in reality low doses are most likely benign and could even be beneficial.
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