Enhanced mutagenicity of internal sequentially decaying beta-emitters from second event effects

Calculations using simple probability theory suggests that dose for dose, external irradiation has thousands of times less probability of causing the same Second Event sequence than internal beta emitters. Even at natural background radiation levels, cells receive on average one hit the cell per year and the Second Event interception of a critical window in a replicating cell is very unlikely. This highlights a possible error in the present understanding of risk from this source and supporting evidence is presented, from animal studies and also from higher cancer incidence responses to exposure to Strontium-90 in weapons fallout. (orig.)