Cytogenetic damage: threshold effects and sensitivities

It is concluded that the spontaneous rate of chromosomal mutations in our cells is high, and that a proportion of these is a consequence of exposure to mutagenic agents in the environment, one category of which is ionising radiation. Detectable chromosomal effects occur around the equivalent of 5 rads of X-rays. Variation in response at low levels is relatively small but an enhanced response is found in certain rather uncommon inherited diseases associated with reduced ability to repair mutagen-damaged DNA. The relationship between dose and chromosome damage with densely ionizing (high LET) radiations is linear and dose rate independent, detectable down to around 1 rad, with no evidence for a threshold or for cellular repair. This contrasts with sparsely ionizing X or γ-rays where the dose response is curvilinear and dose rate dependent. Damage from these radiations is subject to repair and little or no effects are demonstrable at doses below 5 rads. (U.K.)