Three types of millisieverts ...
Radiation protection distinguishes the source of exposures to radiation. There are no limits for millisieverts resulting fron natural radioactivity, because one can not do otherwise. There are also no limits for doses in medical imaging or therapies. On the contrary, regulations imposes a limit of 1 millisievert for miscellaneous other exposures, including those arising from accidents. This limit is far below the natural doses or those resulting from in imagery in medicine.
IN2P3