Hey everyone, this week's blog is a bit different to normal because it's based on a body of research rather than a single recent paper. I'll admit, I got a bit behind this week so I've recycled a post from my Sci Comm class last year. It's a little longer than normal but well worth the read in my opinion - let me know what you think! Radiation can be a pretty scary thing. After all, the effects of an overdose of radiation can be devastating. Radiation overexposure can cause radiation sickness, burns, cataracts, infertility and cancer. Some of these effects can take years to manifest. The reason radiation is so dangerous is because it is able to damage cells by disrupting the chemical bonds between molecules inside the body. If a cell is greatly damaged it will die, and the death of many cells can lead to radiation sickness and even death. Alternatively, if radiation damages a cell’s DNA it can cause it to mutate and become cancerous. But radiation is an everyday part of life. On a daily basis we are exposed to radiation from space, air, rocks, buildings, food, and even from our own body. Radioactive elements are everywhere, and there’s no way of avoiding them completely. What matters is the dose Generally we think of radiation exposure as directly dose dependent -- that is, any radiation is harmful, with no safe level of exposure. This is called the linear no-threshold model and this is the model used by regulators and policy-makers to determine radiation guidelines. However, there’s evidence that this might be wrong. While there’s no question that high levels of radiation leads to a greater cancer risk, there’s less data about how low levels of exposure might affect this risk. This is partly because our natural incidence of cancer is fairly high and can vary greatly depending on lifestyle factors, making it hard to untangle the exact effects of chronic low-dose radiation. Despite this, there are some studies that show exactly the opposite of what you would expect: that low doses of radiation might be good for you. In Taiwan, during the 1980s, a batch of recycled steel was accidentally contaminated with radioactive cobalt. This steel was used to make over 180 buildings containing roughly 1700 apartments. Approximately 10,000 residents of the apartments were exposed to higher than normal levels of radiation over 9-20 years. Within this group of residents, the cancer mortality rate was much lower than in the general population – in fact, it was only 3% of the expected rate. The rate of congenital birth defects was also significantly lower – only 6.5% of the expected rate for the general population. This is possibly the best example of a case study on low-dose radiation because the sample size is large, and other factors such as lifestyle and environment are likely to be consistent between the sample population (the residents) and the general population of Taiwan. But it is not the only example. A number of other studies have shown that people living in areas with naturally higher background radiation have a lower cancer mortality rate than the general population. So, it appears chronic low doses of radiation may actually lower the risk of cancer. This phenomenon is termed ‘radiation hormesis’. It’s not certain why radiation hormesis may exist, but several explanations have been proposed.
It’s possible that radiation hormesis is an adaptive response to damage. Cell exposed to a very low dose of radiation, given a few hours to recover, and then exposed to a higher dose of radiation are less likely to sustain DNA damage than unexposed cells. This means cells that are pre-exposed are more able to protect themselves against higher levels of radiation. Other effects of low dose radiation include:
Animal studies have shown support for radiation hormesis. One experiment using mice predisposed to cancer found low doses of radiation helped slow down the rate of cancer growth. However, analysis of data in humans suggests a mixture of effects. Phillipe Duport, the founder of the Centre for Low Dose Research at the University of Ottowa says “A little radiation may be good for some people but bad for others.” Some scientists believe there is already enough evidence to accept the radiation hormesis hypothesis, and go so far as to say the linear no-threshold model is ‘scientifically unfounded’. Others believe proponents of the hormesis theory have chosen to ignore works that support the linear no-threshold model. However, most of the data supporting the linear no-threshold model is from survivors of the Japanese atomic bomb. These results may be complicated by other factors, such as exposure to cancer-causing chemicals released during the bombing. The conflicting evidence demonstrates a need for further research on humans, but this poses an obvious ethical dilemma. A strong experimental design would require a large number of subjects willing to be exposed to radiation over a long period of time. Unfortunately such a study is unlikely to be approved by an ethics committee, so we may have to rely on data from ‘happy accidents’ like the Taiwan apartments. Currently, advisory bodies like the National Council on Radiation Protection and Measurements are sticking to the linear no-threshold model. On one hand, acceptance of the hormesis model would redefine ‘safe’ levels of radiation exposure, and potentially save millions of dollars in the clean-up of radiation accidents. On the other, regulators tend to err on the side of caution, and mistakes in this area could be devastating to the lives of the people affected. Nevertheless, there’s stacks of evidence to support radiation hormesis, and pressure is growing on policy makers. So perhaps we will see a change sometime soon. And maybe you won’t run away screaming next time you see a radiation sticker.
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March 2018
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