ABSTRACT
The presence or absence of an observed threshold depends upon the patience of the observer, particularly as the delay in appearance of effect may increase as the dose decreases. The essential difficulty is to decide whether an apparent threshold is due to a genuine absence of effect at low doses or to a failure to observe the effect owing to the very low frequency with which it is occurring. In May 1960 the American Federal Radiation Council, set up to advise the American President, has attempted to introduce the concept of ‘radiation protection guides’ to avoid some of these difficulties. It was realized that the effects of X rays involved intense spots of localized energy absorption and thence disruption of biological structures, but somehow Nature repaired these micro-events and macroscopically all might still be well.
