It's actually not clear how low doses of radiation over a long period of time affect human health. Estimates and standards are usually built assuming that a short, intense dose and a long, slow dose are the same as long as they contain the same quantity of radiation (the Linear no-threshold model). However, there is doubt about how valid this is for long-term doses.
There are a few places on earth that have naturally high radiation levels (up to 250mSv/year), including many places with large granite deposits or hot springs. However, these places often do not show any higher cancer incidence (or other relevant health problems) than other areas, possibly suggesting some sort of limit before radiation becomes harmful (the Threshold model).
Some studies actually show significantly lower cancer incidence than other areas. This has prompted the idea of Radiation hormesis, that low doses above background have some benefit. We do know that low doses do stimulate cell repair mechanisms in different ways than large doses do, so it does seem possible (although real-world data is mixed). The effects are not well understood at present, and it's an area that is under ongoing study. Unfortunately, it's an absolute devil to research!
There are a few places on earth that have naturally high radiation levels (up to 250mSv/year), including many places with large granite deposits or hot springs. However, these places often do not show any higher cancer incidence (or other relevant health problems) than other areas, possibly suggesting some sort of limit before radiation becomes harmful (the Threshold model).
Some studies actually show significantly lower cancer incidence than other areas. This has prompted the idea of Radiation hormesis, that low doses above background have some benefit. We do know that low doses do stimulate cell repair mechanisms in different ways than large doses do, so it does seem possible (although real-world data is mixed). The effects are not well understood at present, and it's an area that is under ongoing study. Unfortunately, it's an absolute devil to research!