Natural gamma rays linked to childhood leukaemia. June 12 http://www.ox.ac.uk/media/news_stories/2012/120612.html
[Excerpt] "The team found that there was a 12% increase in the risk of childhood leukaemia for every millisievert of natural gamma-ray dose to the bone marrow. While this finding was statistically significant, even with a study of this size there is still some uncertainty around the size of the effect. The relative risk increase is likely to lie within a range from 3% to 22% per millisievert."
hat tip: Paul Langley