What most people want to know when they read about a reactor accident like Fukushima is how dangerous it is, particularly to themselves. The media responds poorly to the desire for that information, not without good reason.
The link between radiation and cancer risk is complex and difficult to understand, particularly for low doses of radiation, which is what most people, including reactor operators, will receive over their lives. Setting the standards for acceptable dose is a matter of judgement based on not enough data, with large uncertainties. It usually involves balancing increased dose against other circumstances, as, in Japan today, being removed from one's home.
How dangerous is radiation? Normally, forty-two people out of a hundred will develop cancer during their lifetimes. The upper ranges of low doses, more than most people's exposures, may add one more person to that.
Since the Japanese earthquake and the events at Fukushima Dai-Ichi in March, I've been trying to unscramble all this in such a way that I feel confident I understand how the numbers are derived and what they mean. For those who want to follow what I've done, here are the relevant posts:
"Exposed to Radiation"
Doses and Dose Rates
If You're Anywhere But Japan, Don't Take Potassium Iodide!
Radiation Exposure Standards – Making Hard Judgements
The BEIR VII Report
Radiation Dose and Cancer Risk: Some Numbers
Limitations of BEIR VII Estimates of Radiation Risk
Increased Cancer Risks From Radiation for Workers and Children in Japan