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Ionising radiation

Environmental Radioactivity - Medicine - Occupational Radiation Protection - Nuclear Hazards Defence

Ionisierende Strahlung

Atomic bombings of Hiroshima and Nagasaki: significance for radiation protection

  • During the final stages of World War II in August 1945, nuclear weapons were used for the first and only time in a military conflict. The first of two American atomic bombs was released over the Japanese city of Hiroshima on 6 August 1945. The second bomb attack on the city of Nagasaki was conducted three days later.
  • Current knowledge on the health risks of ionising radiation is mainly based on observations of the survivors of atomic bombings, especially on the results from the so-called Life Span Study, an epidemiological cohort study on atomic bomb survivors.
  • The study results provide an important basis for radiation protection, for setting limit values in particular. Important findings from this study are also expected in future.

History

Peace memorial in Hiroshima: monument Peace memorial in HiroshimaPeace memorial in Hiroshima: monument for the first military use of the atomic bomb

During the Pacific War between Japan and China, the American government decided to restrict exports of petroleum and steel to Japan in order to prevent the war from spreading to South East Asia. This economic embargo resulted in the Japanese attack on Pearl Harbor on 7 December 1941 and the spreading of the Pacific War to America. After that in 1942, the USA began to develop and build the atomic bomb ("Manhattan Project") which was then successfully tested ("Trinity Test") in Los Alamos in July 1945.

Following almost four years of continuous warfare and after Japan had rejected the ultimatum to surrender, the US military leadership requested permission to use the atomic bomb. Although many scientists involved in its development had advised against its use, the decision to employ the atomic bomb was taken in 1945. Hiroshima was selected as target for the bombing of 6 August. It was the seat of the General Headquarters of Japan's Second General Army and a military storage point at the same time. Moreover, there was no prisoner-of-war camp (with US inmates). Kokura, an important city for the defence industry, had been the original target of the second bombing on 9 August. However, due to poor visibility Nagasaki, which was the seat of the arms company Mitsubishi at that time, was approached.

Atomic bombings: the effects

Within seconds after the bombings, 80 % of the city centres were completely destroyed through shock and heat waves (of at least 6,000 °C). The rising mushroom clouds following the blasts were composed of whirled up dust and ash with radioactive particles attached. This cloud of dust settled on the surrounding area as so-called radioactive fallout about 20 minutes later. The victims of the atomic bombings either died immediately due to the detonation or later as a result of the acute or delayed effects of ionising radiation. A clear attribution of causes of death to burns, injuries or radiation was impossible because the shock and heat waves also had to be taken into account.

As all important records and registers in the cities were destroyed, the exact number of deaths caused by the blast is not clear to this day. According to estimates, up to 80,000 people in Hiroshima and up to 40,000 in Nagasaki died directly and just as many were injured.

Estimation of the population and the number of acute death cases in both cities at the time of the bombing until 4 months afterwards
CityEstimated population at the time of the bombingsEstimated number of acute death cases
Hiroshima340,000 to 350,00090,000 to 166,000
Nagasaki250,000 to 270,00060,000 to 80,000

The number of survivors exposed to ionising radiation was estimated to about 280,000 people in a census conducted by the Japanese government. The mean weighted colon dose (weighting: gamma colon dose + 10*neutron colon dose) is used by the Radiation Effects Research Foundation (RERF) as a measure for the radiation exposure of the survivors. The exposure depends on the location of the survivor at the time of the detonation and increases rapidly with proximity to the centre of the detonation (the so-called hypocentre).

Estimation of the mean weighted colon dose of the survivors in relation to the distance from the hypocentres in both cities
Weighted colon dose in gray (Gy)Distance hypocentre HiroshimaDistance hypocentre Nagasaki
0.005 Gy2,500 m2,700 m
0.05 Gy1,900 m2,050 m
0.1 Gy1,700 m1,850 m
0.5 Gy1,250 m1,450 m
1 Gy1,100 m1,250 m

Epidemiological studies

In order to investigate the effects of ionising radiation on humans, a cohort study (Life Span Study) on about 120,000 survivors was initiated in 1950. In addition, the following smaller cohort studies were conducted with subgroups of this cohort:

  • a study with 20,000 participants who receive regular health examinations (The Adult Health Survey)
  • a study with 77,000 descendants of survivors (F1 Study)
  • a study with 3,600 participants who were exposed (in utero) to ionising radiation before they were born (in-utero study) as well as
  • a study using blood samples from 1,703 survivors for investigating genetic alterations.

On account of its large study population, relatively precise individual dose estimation, a long observation period and the observation of numerous diseases, the Life Span Study is of great importance for investigating the health effects of ionising radiation.

More than 90 % of the survivors were under 10 years old at the time of the bombings. In 2007, altogether about 40 % of the study population were still alive (average age 74 years).

Acute radiation damage (deterministic radiation effects)

Immediately after the atomic bombings, those affected suffered acute radiation injuries, so-called deterministic radiation effects. These are tissue reactions caused by massive cell death and occur only above a threshold dose. The deterministic effects include, for example, acute radiation syndrome and malformations following radiation exposure in utero.

Acute radiation syndromeshow / hide

Acute radiation syndrome is one of the early tissue reactions. It includes diverse symptoms (such as hair loss or nausea). Atomic bomb survivors, who had been exposed to high radiation doses of about 1 to 10 gray, suffered from the syndrome. At very high doses it leads to death within days to months.

Deterministic radiation effects in children exposed in uteroshow / hide

In children exposed in utero between the 8th and 15th week of pregnancy, a statistically significant increased number of mental disabilities was observed. This increase was less significant in children exposed between the 16th and 25th week of pregnancy. It is assumed that there is a certain threshold dose in these (radiosensitive) phases of pregnancy under which no detectable damage occurs in the brain.

Delayed damage (stochastic radiation effects)

Years or decades after the atomic bombings, delayed damage, so-called stochastic radiation effects (such as cancer, leukaemias and genetic effects) occurred in the survivors. These can also be induced by radiation doses below the threshold for deterministic radiation effects. They stem from DNA mutations which can cause cancer or leukaemias, manifesting themselves clinically only after years. Mutations in germ cells can result in malformations and hereditary diseases in the following generations.

These stochastic radiation effects are investigated in epidemiological studies.

Leukaemiashow / hide

A significant increase in cases of leukaemia had already been observed 2 years following the atomic bombings, reaching a peak 6 to 8 years afterwards. According to most recent estimates, about 30 % of the cases of leukaemia among the survivors (94 out of 312 in the observation period 1950 - 2001) are attributed to radiation exposure due to the atomic bombs, For the dose groups >0.005 Gy, 94 out of 192 cases of leukaemia are attributed to radiation, that is, about 49 %1. The higher the radiation dose and the lower the age of an individual at the time of radiation exposure, the higher the probability of developing leukaemia due to radiation. In this case the dose-response relationship is assumed to be non-linear as the additional relative risk at low doses is clearly lower than at high doses (linear-quadratic relationship).

Tumoursshow / hide

Solid tumours

In contrast to leukaemia, an increased risk of solid tumours was only observed about 10 years after the bombings. According to most recent estimates, about 5 % of all solid tumours in the survivors (853 out of 17448 in the observation period 1958 - 1998) are attributed to radiation exposure due to the atomic bombs. For the dose groups >0.005 Gy 850 out of 7851 cases of cancer are attributed to radiation, that is, about 11 %.2 A significantly increased risk of cancer was observed particularly in the following organs: stomach, lung, liver, colon, bladder, breast, ovary, thyroid, oesophagus and skin. There was also an increased risk of other types of cancer (such as pancreas, rectum, uterus and prostate), which however was not statistically significant.

A linear-no-threshold dose-response model is assumed for solid tumours. This means that the probability of developing cancer due to radiation increases with the radiation dose and that any radiation exposure increases the lifetime risk of developing solid tumours. The lower the age of an individual at the time of radiation exposure, the greater the probability of developing the disease.

Benign tumours

An association between benign tumours and radiation exposure was observed for thyroid, salivary gland and uterus.

Other diseasesshow / hide

A statistically significant dose-dependent increase of the mortality rate also for diseases other than cancer was observed in the atomic bomb survivors. About 20,000 out of 48,000 individuals exposed to radiation doses of more than 5 mGy died from non oncological diseases during the period from 1950 to 20033. Among these were, for example, heart diseases, diseases of the respiratory, digestive and nervous systems, strokes and infectious diseases. According to estimates 1 or 2 % of these deaths can be attributed to radiation exposure from the atomic bombs. Not included are non-oncological diseases of the blood-forming system, for which a dose-response relationship was also observed. However, further studies are needed to clarify a causal relationship.

Yet, the survivors were not only struggling with the health consequences of radiation, but especially with psychological and social problems as well as physical disabilities due to injuries.

Stochastic radiation effects in individuals exposed in uteroshow / hide

The risk of developing adult cancer is not higher for individuals exposed in utero than after early childhood exposure. Furthermore, dose-dependent deficits in school performance and lower intelligence quotients were observed in children exposed between the 8th and 25th week of pregnancy.

Genetic radiation effects in descendants of survivorsshow / hide

When the genetic material in reproductive cells (germ cells) is damaged during radiation exposure, this damage in the genetic material can be passed on to the offspring. No statistically significant increase in malformations was observed in the descendants of atomic bomb survivors. There was neither any indication of an association between radiation exposure and chromosomal anomalies nor of a change in the gender ratio of the descendants. In 2007, there was no significant increase in cancer frequency or mortality in the descendants who had an average age of 47 years at that time. Whether parental radiation dose has an influence on the disease patterns of descendants cannot yet be stated as the statistical power is insufficient.

Significance for radiation protection

Data from various epidemiological studies are analysed by national and international scientific panels such as the Japanese-American Radiation Effects Research Foundation (RERF) and play an important role in the assessment of radiation risk, e.g. by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) and also by the German Commission on Radiological Protection (Strahlenschutzkommission - SSK). The findings from the Life Span Study, the largest study on atomic bomb survivors, provide an important basis for estimating radiation-induced risks and for deriving limits to radiation exposure and regulations for radiation protection.

However, as the atomic bomb survivors were exposed to acute high-level radiation, it is difficult to estimate the risks of low or chronic radiation exposure (which are more relevant today) based on these data and it is being controversially discussed to this day.

As the observation period grows, the power of the study is expected to increase and to result in an even more accurate description of the dose-response relationship (for example with regard to age- and gender-related differences in the effects of ionising radiation).

Literature

1 Hsu, W. L., D. L. Preston, M. Soda, H. Sugiyama, S. Funamoto, K. Kodama, A. Kimura, N. Kamada, H. Dohy, M. Tomonaga, M. Iwanaga, Y. Miyazaki, H. M. Cullings, A. Suyama, K. Ozasa, R. E. Shore and K. Mabuchi (2013). The incidence of leukemia, lymphoma and multiple myeloma among atomic bomb survivors: 1950-2001. Radiat Res 179(3): 361-382.
2 Preston, D. L., E. Ron, S. Tokuoka, S. Funamoto, N. Nishi, M. Soda, K. Mabuchi and K. Kodama (2007). Solid cancer incidence in atomic bomb survivors: 1958-1998. Radiat Res 168(1): 1-64.
3 Ozasa, K., Y. Shimizu, A. Suyama, F. Kasagi, M. Soda, E. J. Grant, R. Sakata, H. Sugiyama and K. Kodama (2012). Studies of the mortality of atomic bomb survivors, Report 14, 1950-2003: an overview of cancer and noncancer diseases. Radiat Res 177(3): 229-243.

State of 2017.09.18

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