The head (or former head) of the radiation protection division of U.S.-NRC once stated (jokingly) at an IAEA reception in Vienna: There are three types of photons, namely ‘green’ ones, ‘yellow’ ones and ‘red’ ones.
The ‘green’ ones are plentiful and of natural origin. We are not concerned about them and we don’t regulate them.
The ‘yellow’ ones come from medical applications. They are usually less plentiful, but we are a bit concerned about them and thus we regulate them somewhat.
The ‘red’ ones are very rare, they find their origin in nuclear energy applications. We are very concerned about them and consequently we regulate the hell out of them.