Low doses of ionising radiation: definitions and contexts

The term 'low dose' is applied to different levels of dose depending on the circumstances of exposure, with the potential for confusion unless the reasoning is clear. The United Nations Scientific Committee on the Effects of Ionising Radiation has defined low absorbed doses of ionising radiation as below about 100 mGy, and low dose rates as below 0.1 mGy min-1 (6 mGy h-1). These values relate to the interpretation of scientific evidence from epidemiological and biological studies. The International Commission on Radiological Protection has used similar values of 100 mSv and 5 mSv h-1 and applied this categorisation directly to the specific situation of patients undergoing diagnostic procedures: doses below 100 mSv were referred to as 'low' and doses below 10 mSv as 'very low'. Consideration of other exposure situations suggest that the same terms can be used for exposures received by emergency workers. However, for workers and members of the public in planned exposure situations, it is suggested that the term 'low dose' applies to doses below 10 mSv and 1 mSv, respectively - that is, below the dose limits. In each case, dose is being used as a surrogate for risk - risks at low doses are uncertain and estimates may change, but order of magnitude considerations are sufficient in most cases. Doses of < 100 mSv, < 10 mSv and < 1 mSv correspond to life-time cancer risk estimates of the order of < 10-2, < 10-3 and < 10-4, respectively. .