There are mountains of literature and regulations on this and the math just kind of works that way (up to some limit, beyond which acute radiation effects will begin to dominate)
There is now some thought that very low doses of ionizing radiation are better than no radiation at all (so called 'radiation hormesis'), but I don't believe that has been adopted as part of any specific standard.
FWIW an individual person may be experiencing up to a trillion DNA mutations per day from both external and internal mechanisms of action. We just have a bunch of systems built in to prevent them from getting out of control (most of the time).
https://en.wikipedia.org/wiki/Linear_no-threshold_model
There is now some thought that very low doses of ionizing radiation are better than no radiation at all (so called 'radiation hormesis'), but I don't believe that has been adopted as part of any specific standard.
FWIW an individual person may be experiencing up to a trillion DNA mutations per day from both external and internal mechanisms of action. We just have a bunch of systems built in to prevent them from getting out of control (most of the time).