The government should revise its formula to calculate radiation doses of atomic bomb survivors because the current method does not properly incorporate internal radiation exposure from "black rain" and other factors, scientists said.
Black rain refers to the downpour containing radioactive substances that fell in and around Hiroshima immediately after the city was leveled by an atomic bomb on Aug. 6, 1945. The black rain covered an area that is at least 11 kilometers wide east-west and 19 km long north-south.
A revision of the calculation method could lead to the certification of additional atomic bomb survivors as “hibakusha,” who are eligible for financial assistance to treat radiation-related illnesses.
A group of researchers from Hiroshima University and other institutions tracked about 37,000 holders of Atomic Bomb Survivor's Certificates in Hiroshima Prefecture, who were within 2 km of ground zero and were alive as of Jan. 1, 1977.
According to studies, which continued until the end of 2010, 44 of the hibakusha died from solid tumors, which do not include leukemia.
The epidemiological survey found the levels of risks of death from solid tumors did not form concentric circles around ground zero. For example, the risk of death was up to about 1.7 times higher in one area 2 km west-northwest of ground zero than in an area 2 km east of it.
The government’s formula to evaluate radiation doses of atomic bomb survivors to authorize A-bomb-related syndromes and for other purposes was drawn up by Japanese and U.S. experts and is called the Dosimetry System 2002 (DS02).
But the DS02 can only work out direct radiation doses from gamma rays and neutron beams; it does not take into account the effects of beta rays, indirect exposure and internal exposure.
Atomic bomb survivors may have received additional doses from radioactive dust they inhaled and radioactive water they ingested, including substances from the black rain, said Megu Ohtaki, a professor of biometrics at Hiroshima University's Research Institute for Radiation Biology and Medicine who is a member of the research team.
Masaharu Hoshi, a professor emeritus of radiation biology at Hiroshima University who also belongs to the team, said he plans to call on relevant organizations to have Japanese and U.S. experts review the DS02 formula, which he helped to draw up.
Hoshi was to present the latest research results and recommendations on July 25 during an annual meeting of the Health Physics Society in the United States.
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