A major new study, which has involved more than 407,000 workers in the nuclear industry, provides a new clue.
Ionising radiation is measured in millisieverts, or mSv, and health authorities reckon that the public shouldn't be exposed to more than 1 mSv a year. But background radiation emitted naturally from decaying radioactive substances in the earth alone reaches 3 mSv every year. An average patient who has a CT scan once a year is exposed to an additional 10 mSv a year, while a radiograph emits another 4 mSv. If those are extrapolated over a five-year period, the average citizen is exposed to 85 mSv, which is 15 mSv lower than someone who works for five years in a nuclear power plant, and way above the levels considered safe by our governments.
An exposure of 100 mSv over five years increases - by nearly 10 per cent - the number of deaths from all cancers other than leukaemia and lung cancer and, with the latter two included, the mortality rate from all cancers rises to 19 per cent but, in some sections of the study, rises as high as 84.7 per cent (BMJ, 2005; 331: 77-80).