The annual worker dose limit of 20 mSv has been set at a level which represents a 1 in 1000 risk of contracting cancer. That is to say, if 1000 people were exposed to 20 mSv in one year (on top of our annual background dose) then we would expect one person to develop a cancer later in life as a result of this exposure. This limit of 20 mSv has been arrived at by reference to evidence from the Japanese bomb blast survivors and other groups exposed to radiation. However, for exposures in the workplace this dose limit is seen as a backstop and is the very maximum that is permissible. The over-riding principle which is applied in the UK is that all radiation doses should be kept ‘as low as reasonably practicable’ (ALARP).