SHORTLY AFTER 1900 physicians noted that high exposure to penetrating radiation could produce injury. It was later shown that injuries can also occur eventually as a result of somewhat lower, but still comparatively high, exposures over a much longer period. If the exposures are sufficiently low, no effect at all can be determined. Recognition that everyone receives continuous lowlevel background radiation exposure led to the concept that exposures below a so-called threshold level will not be injurious. The conservative position has been taken that exposures should be limited to as low values as feasible, although it is recognized the data do not necessarily justify stringent and expensive controls. This position has recently been restated by the Federal Radiation Council which reported, that exposure potential should be "balanced" against benefits. In 1956, it was stated that all radiation, even the most minute, is hazardous. This was based on some published