There is a much-challenged concept called "radiation hormesis," in which low doses of radiation actually have a tonic and health-promoting effect. This is as opposed to the "no safe dose of radiation" standard model.
I'm not a radiation scientist, but from what I've read the evidence points toward the hormesis group being closer to the truth.
Similar effects have been claimed for many toxins and pollutants.
Which is not to say that the levels of radiation at issue here fall into the hormesis range.