Comment by adrian_b

Yes, there is a great uncertainty about which is the level where radiation becomes measurably harmful.

Nonetheless, I think that I have seen recently right here on HN comments about an article showing that there is a higher incidence of cancer for the flight personnel of airlines, which is likely to be caused by the long time spent during their careers at high altitudes with increased radiation level.

In the case of high altitude, exactly like for the weakly radioactive elements that stay inside our body, the exposure to radiation is certain and permanent for everyone.

For the people who live in geographic areas with higher radiation levels, the difference in individual radiation exposures can be very great, because most of the extra radiation exposure would come from ingestion or inhalation of dust, as otherwise the alpha and beta radiation from the soil or rocks would not penetrate the body.