Dose matters

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Do you ever think about how much radiation you receive when you fly long-distance, have a dental X-ray or undergo a CT scan?

Much of what we know about the risks arising from radiation exposure come from studies on the Hiroshima and Nagasaki bombings during WWII. Despite only having data for high doses, it was concluded that all doses, no matter how small, were harmful and that the risk of cancer increases exponentially with dose. This is termed the linear no-threshold model (LNT). LNT is in contrast with what radiation biologists have demonstrated experimentally for more than two decades, that below a certain dose, where the LNT is extrapolated, risk is reduced and often the low dose appears to have a ‘protective effect’. This has been termed the radioadaptive response.

ionizingvsnon-ionizing

Radiation can be ionising (e.g. X-rays) or non-ionising (e.g.. microwaves). Non-ionising radiation does not have enough energy to displace electrons and therefore cannot cause damage to cells and other molecules such as DNA.

Why is this important? Understanding safe low dose exposure is an increasing concern where legislation and safe work practices are concerned. Increasing availability to the masses of medical imaging devices such as CT scans or dental X-rays, airport whole body scanners, increased air travel as well exposure to background radiation in mines means that greater understanding of safe radiation exposures is required.

So many factors can confound the determination of safe levels. At the simplest level it can be whether the exposure is single or repeated, whether it is whole body or targeted at an individual organ (important as different organs display different radiation sensitivities). And then, there is the agreement on what a low dose of radiation is. Even in research laboratories this can differ. Further adding to this is the consideration of natural background radiation arising from naturally emitting materials such as granite, or radon carried in the air. Even bananas are radioactive! Did you know there are some countries in the world where the natural background radiation is as high as receiving a CT scan or a dental X-ray, without increased rates of cancer or other radiation-induced problems?!

My own PhD lab has demonstrated that a low dose of X-rays can protect against DNA damage induced by a high dose, when given before or after the high dose exposure, while others have demonstrated that the incidence of tumours in radiation-sensitive mice can be reduced if the mice are given a low dose first.

Recently, a study into the deaths of nuclear industry-workers in France dating back 60 years reported that there was an increased, proportional risk in leukaemia. But only for extremely low doses. These individuals were exposed to doses just a little higher than background radiation doses, but accumulative. The only problem with studies like this, and it was pointed out by the authors, is that the risk was calculated based on mathematical model and the accumulated radiation doses cannot be directly linked to the deaths.

Based on increasing evidence that the LNT model cannot be used to justify exposures, there are many who say that a new model for radiation exposure and risk needs to be proposed. Public understanding of radiation, and what constitutes damaging radiation, also need to be addressed by scientists and legislators.

The take home message should be that radiation is harmful, but the question is still “at what dose does risk become neglible, if at all?”

Sources

http://www.nature.com/news/researchers-pin-down-risks-of-low-dose-radiation-1.17876

http://www.ansto.gov.au/NuclearFacts/AboutNuclearScience/NaturalBackgroundRadiation/index.htm

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4030667/

http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/34/086/34086353.pdf

http://atomicinsights.com/science-falsified-no-safe-dose-hypothesis-radiation-now/

Hooker et al. Radiation research 162.4 (2004): 447-452.
Dayet al. Radiation research 167.6 (2007): 682-692.
Mitchel et al. Radiation research 159.3 (2003): 320-327.