The mechanism, at the cellular level, by which radiation may cause cancer and genetic harm is relatively well understood, certainly compared with how a large number of chemicals do the same thing. What is controversial is the numerical prediction of the resulting harm. The radiation, in passing through a cell and ionizing its component atoms (Chapter 2), damages the DNA (deoxyribonucleic acid) that is the genetic template that controls the development and reproduction of all living cells, and hence of humans and other beings. The damage may result in runaway reproduction of the cell, known as cancer or, if in the cells of reproductive organs, harm that would be passed on to offspring, i.e., genetic defects. If the mechanism were that simple the LNTH would follow logically: the greater the radiation damage the greater the harm, cancer or genetic defects, until a high enough dose is reached to kill the cell.
The first complication to the simple mechanism is that nature has provided all DNA with a repair mechanism. This seems only reasonable since normal processes occurring in all our bodies result in about 10,000 DNA lesions in each cell nucleus every hour. Without a repair mechanism none of us would survive for long. Of the 10,000, about three are due to the ambient level of radiation to which we are all exposed from natural sources. Further exposure to the annual limit of 1 mSv set by the CNSC would result in just over one more. Thus, as long as the dose is not delivered at a rate too fast for the repair mechanism to keep up, residual damage would not be expected to be detected.
Another complication is that cancer is believed to develop through a multistage process, with at least two stages, initiation and promotion. Thus damage to the DNA, by radiation, chemicals or any other agent, even if not repaired, does not necessarily result in a cancer. Also there can be a latency period of years, sometimes tens of years, following the original damage. For these reasons an absence of any harmful effects from radiation doses around 1 mSv, i.e., a threshold in the dose response, would be consistent with our understanding of the mechanisms involved.
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