Man's sensitivity to bone marrow failure following whole body exposure to low LET ionising radiation: inferences to be drawn from animal experiments.

Evidence concerning the sensitivity of man to bone marrow failure following exposure to brief but substantial doses of ionising radiation is sparse. There is, however, a relatively substantial body of information on such effects in large animals. Reported experiments on six species where exposure to low LET radiation was uniform to the whole body and of brief duration (exposure times of the order of one hour or less) have been reanalysed both in terms of exposure and of midline tissue dose. The results indicate a marked lack of homogeneity among values for LD50 within species thus questioning the applicability of LD50 as a species dependent constant. It is, however, suggested that on a purely empirical basis these large animal data suggest that the dose killing 'most' (where 'most' is between 90 and 95 per cent) is about twice that killing 'few' (where 'few' is between 5 and 10 per cent). For man, where there is evidence that the dose killing few is unlikely to be less than 3 Gy, this relationship might indicate a gradient of mortality with dose between 3 and 6 Gy.