Ionizing radiation can damage living organisms, that’s clear. But there are big questions over the validity of the linear no-threshold model (LNT), which essentially states that the risk of cancer from radiation and carcinogens always drops linearly with dose. The LNT model implies, in other words, that any amount of radiation is always dangerous and that zero risk is present only at zero dose.

But it is not the only dose-response model that exists. The “threshold model”, in contrast, says there is a dose below which there is no perceptible risk, implying that small exposures to radiation are harmless. Then there is the “hormesis model”, which says a small dose at or slightly above natural background levels can, in fact, trigger beneficial repair mechanisms.

Developed almost 80 years ago, LNT has been integral to the US government policy on radiation protection over the last half century, and many other countries have followed suit. Even if LNT is incorrect in detail, it may appear sensible to err on the side of caution concerning matters of human health. But adopting it in this way, as a kind of “precautionary principle”, can have unforeseen and undesirable consequences.

The LNT can also promote a false sense of security by suggesting that the removal of minute doses guarantees safety

Incorporated into policy the LNT model can require companies and government agencies to clean up tiny and perhaps non-hazardous amounts of radiological material, consuming funds that might be better spent tackling more dangerous sites. The LNT can also promote a false sense of security by suggesting that the removal of minute doses guarantees safety despite the presence of other toxins already in the environment.

The model may even discourage use of radioactive material for beneficial uses, such as X-ray imaging, stress tests, and medical diagnosis and treatment. As a result, the LNT has become a hotly debated topic among historians, public policy makers, toxicologists and medical physicists. One paper in the Journal of Nuclear Medicine (58 1) has even accused the LNT model of leading to “needless public and professional radiophobia”.

Complex beginnings

That radiation can cause genetic mutation in living organisms was discovered by the US geneticist Hermann Muller (1890–1967). In 1927 he published a paper in Science (66 84) entitled “Artificial Transmutation of the Gene”, his title ambitiously and audaciously invoking the artificial transmutation of elements, which had been discovered a few years earlier.

Muller wrote that his discovery in fruit flies that “relatively heavy doses of X-rays induced the occurrence of true ‘gene mutations’” might explain the mechanism for evolution and the ability of X-rays to cause cancer. Over the next few years, he cited his own studies to argue for the proportionality of radiation dose and damage, which soon became known as LNT.

In 1946, Muller was awarded the Nobel Prize in Physiology or Medicine “for the discovery of the production of mutations by means of X-ray irradiation”. Based at the time at Indiana University in Bloomington, Muller proclaimed in his Nobel lecture that there is “no escape from the conclusion that there is no threshold dose”. Individual mutations, he explained, result from individual “hits”, producing genetic effects in their immediate neighbourhood.

Muller’s work made him a celebrity, and opened up the field of “X-ray genetics”. Scientists now began to look in more detail at Muller’s methods and assumptions and to conduct further experiments on fruit flies and also mice. Some researchers questioned whether his X-rays were really inducing mutations or just knocking out pieces of chromosome. Others questioned his key assumption that the genetic damage does not depend on dose rate delivery.

What’s more, after the 1953 discovery by Francis Crick and James Watson of the structure of DNA – which carries genetic information – scientists found that the molecule often repairs itself after being buffeted by chemicals and radiation already present in the environment. Still other scientists pointed out that LNT is not an empirical fact but a hypothesis generated by extrapolation from massive to minute doses.

For a while, US policy for setting levels of radiation protection in the workplace was based on a threshold model. Then, in 1955, the US National Academy of Sciences established the Committees on the Biological Effects of Atomic Radiation (BEAR). Its genetics panel, whose 17 members included Muller, recommended switching from a threshold to a linear dose response model when estimating risk assessment.

Its report, published in 1956, stated that radiation from any source, including natural environmental background radiation and X-rays, is “harmful to life”. The report was highly influential and made the front page of the 13 June 1956 edition of the New York Times, among other newspapers. The report led to changes in public perception and to acceptable radiation-protection levels.

Continued questions about the validity of the use of LNT in radiation protection have led to the Million Person Study

In the 1960s, rising fears of low levels of radiation led the US Congress to set up the Biologic Effects of Ionizing Radiation (BEIR) committee, which in 1972 published a report that essentially endorsed LNT.  In 1975 the US Environmental Protection Agency (EPA) began using the LNT model to fix clean-up levels for radiologically contaminated environments.

Chernobyl’s legacy and why assessing radiation risk is so difficult

In the past decade, however, continued questions about the validity of the use of LNT in radiation protection have led to the Million Person Study (MPS) of US radiation workers and veterans. A joint effort by several US universities and national labs to study the health impacts of exposure to low doses of radiation, the MPS has enrolled a million people to evaluate effects involving different cancers, types of radioactivity and differential impacts on men and women.

The MPS faces huge challenges. Dose response at minute levels can be masked by noise and drowned out by the many other sources of damage such as those posed by genetics, diet, lifestyle, oxyradicals, carcinogens and background sources of radiation from earth and sky. But success would help frame more responsible policies and better protect workers and the public.

The critical point

One of the many reasons for the need to study the validity of LNT is that convictions of its accuracy continue to be used as an argument against nuclear power plants, in connection with their operation as well as their spent fuel rods. Nuclear power may be undesirable for reasons other than this. But the critical need to find a workable alternative to fossil fuels for energy production requires an honest ability to assess the validity of this model.

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• Source: https://physicsworld.com/a/how-sound-is-the-model-used-to-establish-safe-radiation-levels/