The findings, published in the latest issue of Environmental Toxicology and Chemistry, found that low, chronic doses of gamma radiation at 50 to 200 times background levels had beneficial effects on the stress axis and the immune axis of natural populations of meadow voles. The paper provides evidence of hormesis from the only large-scale, long-term experimental field test ever conducted on the chronic effects of gamma radiation on mammals.

Source:
http://www.sciencedaily.com/releases/2005/01/050128222047.htm

Do keep in mind the use of the word "may". The jury is still out on this one and will be for a very long time.
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DA Morgan