RERF modifies estimated dose of exposure, no change made in risk assessment

by Michiko Tanaka, Staff Writer

On July 11, the Radiation Effects Research Foundation (RERF, located in Minami Ward, Hiroshima) released the recalculated results of the estimated exposure doses of about 82,000 atomic bomb survivors. RERF, which is administered by the governments of Japan and the United States, has been conducting follow-up surveys of these survivors since the 1950s. The distance from the hypocenter, which is closely related to the radiation dose, was carefully reviewed after roughly half a century. As a result, the radiation doses of 4,119 survivors were raised by 10 millisieverts or more from previous estimates. Among this group, there were discrepancies of 500 millisieverts or more in the doses of 35 individuals.

The total number of A-bomb survivors included in this study was 81,873, of whom 55,227 were survivors of Hiroshima and 26,646 were survivors of Nagasaki. The Atomic Bomb Casualty Commission (ABCC), the forerunner of RERF, interviewed survivors and by 1961 had identified the location where each person was exposed to the atomic bomb.

However, the map used for the survey, which was provided by the U.S. military, lacked accurate locations for roads and buildings. In addition, when it came to the distance from the hypocenter, distances of less than 100 yards (91 meters) were sometimes rounded off. The formula for calculating the estimated radiation dose was revised a number of times, but the positional data at the time of the bombing was left uncorrected.

RERF began their recalculations in 2007, identifying the distances from the hypocenter with as much accuracy as possible by using an elaborate aerial photograph from before the war. On average, the distance between the hypocenter and each person’s location was extended by 0.7 meters in the case of Hiroshima survivors, and shortened by 11.4 meters for survivors from Nagasaki.

The number of survivors whose estimated doses were raised by 10 millisieverts or more was 3,000 in Hiroshima and 1,119 in Nagasaki. Among them, those with an increase of 500 millisieverts or more totaled 11 in Hiroshima and 24 in Nagasaki. On the other hand, the number of survivors whose doses were reduced by 10 millisieverts or more was 7,357 in Hiroshima and 2,458 in Nagasaki, with the doses of 59 survivors of Hiroshima and 56 from Nagasaki reduced by 500 millisieverts or more.

RERF believes that a correlation exists between a survivor’s dose and the incidence of cancer when the dose exceeds 200 millisieverts. No great change was made in RERF’s risk assessment after the revised doses were factored in. RERF also plans to recalculate the doses of survivors who experienced the atomic bombing while in air raid shelters and other locations.

On request, RERF will disclose the data involving individual radiation doses to those who were surveyed and wish to learn the result.

(Originally published on July 12, 2013)