The effects of gamma rays on longevity.

A number of animal model studies have assessed the capacity of long-term whole body gamma rays to affect life span. The initial goal of such studies was to establish the equivalent of a no observed adverse effects level (NOAEL) that would provide a toxicological foundation for deriving an acceptable worker exposure standard. In the course of initial studies to establish such a 'tolerance threshold', data emerged suggesting that low dose rates/cumulative doses enhanced longevity in mice and guinea pigs of both sexes. Extensive large scale follow-up investigations with other mouse strains and rats revealed what appear to be inter-strain/species differences in response with some models providing strong evidence for a low dose increase in longevity. The subsequent positive studies in mouse models were generally well designed, well conducted and used extensive numbers of mice. In all experiments that displayed enhanced longevity the average life span was enhanced by 10-30% but not the maximum life span potential. The underlying mechanisms affecting the apparent enhancement in longevity are believed to result from the stimulation of hematopoietic and immune systems following an initial low level chronic injury to the bone marrow.