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.

Individual radiation exposures are calculated for different pathways (external radiation, internal radiation), different kinds of ionizing radiation (UV, X-ray, alpha, beta, gamma, neutron, cosmic rays - high energy protons or atomic nuclei), energy of the ionizing radiation, different weighting for different kinds of tissue.

https://en.wikipedia.org/wiki/Effective_dose_(radiation)

In the end you get the stochastic health risk to the whole body, which is the probability of cancer induction and genetic effects in Sieverts. 1 sievert (Sv) corresponds to a 5.5% chance of developing cancer.

In most cases you deal with mili Sievert amounts of dose, which correspond to very small increases in probability of cancer and they are hard to prove in medical studies because the base rate of cancer in humans is large and you have many factors that affect cancer rate: sex, weight, age, smoking, life style, environmental factors, food.

I read the article about higher incidence of cancer for the flight personnel of airlines, it would be interesting to compare it to risks of thermal spa workers. Or miners, there can be a lot of radon underground, especially with insufficient ventilation.