Health effects at Hiroshima, Nagasaki

ON THE 6th August 1945, exactly 56 years ago, an atom bomb destroyed Hiroshima. Nagasaki faced devastation three days later. The Imperial Headquarters requested Dr. Y.Nishina (famous for Klein-Nishina formula for Compton scattering of photons) to go to Hiroshima. He concluded that the damage was due to an atomic bomb.

When the bombs were dropped, these cities had an estimated population of 310,000 and 250,000 respectively. About 90,000- 140,000 in Hiroshima and 60,000- 80,000 people in Nagasaki died immediately or within two to four months after bombing, resulting from collapse of houses caused by the blast and from heat rays and fires and radiation exposure. In the 1950 Japanese national census, nearly 280,000 persons stated that they "had been exposed" in the two cities.

Radiation fogged X-ray films stored at the Hiroshima Red Cross Hospital. Exposed material became radioactive. Kimura and Tajima of the Institute of Physical and Chemical Research determined that the Hiroshima bomb exploded at a height of 577 m. They used an electric furnace and a carbon arc furnace to simulate charring observed on the surface of a wooden board of Japanese cypress and estimated that the bomb had a radiant energy of 4.6 million million calories.

Scientists could enter the bombed area only after 3 to 4 weeks because of frequent typhoons. They measured higher radiation levels due to induced radioactivity near the epicentre. Additional radiation levels were 10-15 per cent of the natural background.

On November 18, 1946 Harry Truman authorised the National Research Council to establish an agency "to undertake a long range, continuing study of the biological and medical effects of the atomic bomb on man". This organisation grew into the Atomic Bomb Casualty Commission (ABCC). ABCC continued to work till 1975 when Radiation Effects Research Foundation (RERF) was set up as a bi-national endeavour.

Many believe in the myth that birth defects are more common among the children of the survivors of the atomic bombings at Hiroshima and Nagasaki. In my interaction with general physicians and other professional groups the one topic, which elicits maximum queries, is " genetic effects of radiation". Radiation can cause mutations in genes. Mouse and in fruit flies exposed to radiation suffer genetic mutations. The changes may show up in the offspring as colour of the coat, shape of the wings, etc. But such markers are not available in the human species. We require very sophisticated laboratory methods to demonstrate subtle changes in exposed individuals.

Physicians appointed by ABCC examined 76,626 infants conceived and born in Hiroshima and Nagasaki over a period of six years starting from the late spring of 1948. The researchers did not see statistically demonstrable increase in major birth defects in these infants. While the survey started, certain dietary staples were rationed in Japan. Pregnant women had special provisions. Because of this, the surveyors of new-borns could identify 90 per cent of the pregnancies that persisted for at least 20 weeks of gestation.

Physical examination of the new born and autopsies on as many stillborn infants revealed that neither the frequency of major birth defects nor the frequency of the most common birth defects differ significantly with radiation exposure of parents. The researchers examined some 21,788 infants shortly after birth and re-examined them eight to ten months later. The study covered 65,431 registered pregnancy terminations and appropriate control populations.

In a brief review, the Radiation Effects Research Foundation cautioned, "The absence of a statistically significant effect of ionising radiation on the frequency of major birth defects should not be construed as evidence that mutations were not induced by parental exposure to atomic radiation". The researchers saw mutations in every animal and plant species studied.

Then how can we presume that human genes are not mutable when exposed to ionising radiation? The magnitude of a difference between two or more groups that can be detected statistically depends upon the number of observations made and on the "natural" frequency of the event under scrutiny as well as the difference between the groups resulting from exposure. The RERF study had the statistical power to detect a doubling of the rate of major congenital malformations, if such defects had occurred. The need for prudence is obvious.

Long term study of the survivors of the atom bombing of Hiroshima and Nagasaki showed that high radiation exposures cause excess cancer in the exposed individuals. Till 1990, RERF recorded 7827 cancer deaths in the survivor population of just over 86,000. Radiation exposure might have caused 421 excess cancers. Of the 17 types of cancers considered, survivors suffered excesses in 16; at lower radiation levels the risk of cancer has not been demonstrated. The cellular repair mechanisms play a role at all levelsA few `mavericks' believe that low level radiation exposure is beneficial to man. They assert that radiation safety professionals have a vested interest in arguing that radiation is risky at any level. They ignore the fact that the assumption of risk at low level radiation is advocated as a matter of abundant caution.

Studies on 1600 children who were irradiated while they were in their mother's womb during the atomic bomb explosions in the two cities revealed that 30 of them suffered clinically severe mental retardation. Between 0 and 7 weeks post conception mental development was not affected. Between 8 and 15 weeks the sensitivity for mental retardation was maximum. This is possibly because neuronal proliferation and cell migration in the cortex is most active during this period. From 15 weeks to 25 weeks the incidence of metal retardation was clearly lower. In general, mental retardation was found to depend on radiation dose. There was no detectable threshold dose below which the effect was zero. But a threshold of 100 milligray cannot be ruled out. (milligray is a unit of radiation dose; the skin dose in some medical x-ray examinations can be as high as 1 milligray).

Recent RERF studies suggest that there may be a small radiation associated increase in the risk of death for diseases other than cancer. The incidence of myocardial infarction, chronic liver disease, thyroid diseases and uterine myoma has increased slightly among A-bomb survivors. There is some evidence that radiation caused an increase in cancer deaths among in survivors who got exposed in their mothers' wombs. The number of such deaths is still small.

International organisations use the RERF data about diseases and deaths among the atomic bomb survivors to establish radiation protection guidelines for radiation workers and the general populations. Whether low level radiation exposure will cause harmful effects in humans has not been demonstrated conclusively. Irrefutable evidence on the harmful effects, if any, due to low levels of radiation exposure is unlikely to emerge in the near future. Evidently, it is prudent to reduce all radiation exposures to, as low a value as is reasonably achievable.

K. S. Parthasarathy

Secretary, Atomic Energy Regulatory Board

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