Alpha particles are extremely damaging to developing hemopoiesis compared to gamma irradiation.

Estimates of risk of stochastic effects from contamination with alpha-particle-emitting radionuclides are based on equivalent doses which take into account the RBE of the high-LET radiation. ICRP has recommended a dose-weighting factor, wR, of 20 for alpha-particle radiation. It is assumed that the RBEs for deterministic effects are considerably less than those for stochastic effects. However, the offspring of mice injected with 30 Bq g-1 239Pu at 13 days gestation develop a persistent deficit in hemopoietic stem cells which is primarily the result of damage to their regulatory microenvironment. Their spatial distribution in the marrow is also perturbed, and recent observations on those mice suggested a considerably higher factor than 20. To define a more realistic RBE for hemopoiesis, the effects of external gamma irradiation during the fetal development period have been compared directly with those of 239Pu incorporated via placental transfer on the development of hemopoietic tissue. Pregnant mice were irradiated with 60Co gamma rays (a) continuously from day 13 of gestation to birth at 0.15 or 0.6 Gy/day; (b) six repeated acute doses (0.6 Gy/min) at 0.1 or 0.3 Gy from day 13 of gestation; (c) one acute dose of 0.6 or 1.8 Gy on day 15 of gestation. The spatial distribution of hemopoietic stem cells in 8-week-old offspring was then determined and compared to that resulting from alpha-particle irradiation. In each case, the higher dose was required to match the results for alpha particles, suggesting an RBE for developing hemopoiesis of 250-360 compared to a continuous gamma-ray dose and a rather lower value of 130-180 compared to a single acute dose of gamma rays. This contrasts greatly to values for direct irradiation of the stem cells but argues that the effective RBE, measured for long-term effects in vivo, is the more realistic. It is concluded that an all-embracing factor can be grossly misleading in the specification of protection guidelines and can greatly underestimate the risks of exposure to alpha particles.