Low-level radiation--how dangerous is it?

The main delayed effect of ionizing radiation in the individual is the induction of cancer. However, for low doses (less than 100 mSv effective dose equivalent), there is no direct evidence of radiation-induced cancer, with the possible exception of childhood cancer following prenatal exposure to X-rays. The risk at low doses has to be estimated from the observed risk at high doses; there are considerable uncertainties in this extrapolation. Direct study of those exposed to low doses does not produce any more precise information, although it does impose boundary conditions on the risk. Currently the most important problem in environmental radiation exposure is the increased incidence of childhood leukaemia observed around some nuclear installations; the explanation for this remains unclear.