New dosimetry of atomic bomb radiations.

The reassessment of the radiation dosimetry from the Hiroshima and Nagasaki atomic bombs is almost complete. Since atomic bomb survivors provide a major source of data for estimates of risk of cancer induction by radiation the impact of the new dosimetry on risk estimates and radiation protection standards is important. The changes include an increase of about 20% in the estimated yield of the Hiroshima bomb and a reduction in the estimated doses from neutrons in both cities. The estimated neutron dose for Hiroshima is about 10% of the previous estimate. The neutron doses are now so small that direct estimates of neutron relative biological effectiveness may be precluded or be much more difficult. There is little change in most of the gamma ray organ doses because various changes in the new estimates tend to cancel each other out. The new estimate of the attenuation of the free-in-air kerma by the walls of the homes is about twice that used in the previous dosimetry. But the transmission of gamma radiation to the deep organs such as bone marrow is significantly greater than earlier estimates. Probably future risk estimates for radiogenic cancer will be somewhat higher because of both the new dosimetry and the new cancer mortality data. New risk estimates should be available in 1988.