Frequently Asked Questions about the Atomic-bomb Survivor Research Program

The study of long-term health effects in the survivors of the atomic bombings of Hiroshima and Nagasaki

Question 2. How many cancer deaths have occurred among atomic-bomb survivors and how many of these can be attributed to radiation?

Question 3. What is the relationship of radiation to cancer occurrence?

Question 4. Are radiation-induced cancers still occurring among atomic-bomb survivors?

Question 5. What health effects other than cancer have been seen among the atomic-bomb survivors?

Question 6. What health effects have been seen among persons who were exposed while still in their mothers' wombs?

Question 7. What health effects have been seen among the children born to atomic-bomb survivors?

Question 8. How many atomic-bomb survivors are included in the group being studied by RERF and how were they chosen for this study?

Question 9. What percentage of the original atomic-bomb survivor study population is still alive?

Question 10. How is information about the atomic-bomb survivors obtained?

Question 11. What is "significant" radiation exposure?

Question 12. Are Hiroshima and Nagasaki still radioactive?

Deaths caused by the atomic bombings include those that occurred on the days of the bombings resulting from the collapse of houses caused by the blast and from burns due to heat rays and fires as well as deaths that occurred later from burns and radiation exposure. However, the total number of deaths is not precisely known because records of military personnel in each city were destroyed; entire families perished, leaving no one to report the deaths; and forced laborers were present in unknown numbers. Recent estimates of total deaths occurring within two to four months after the bombings are shown in the Table 1.

Table 1. Estimated population size and number of acute (within
two to four months) deaths in Hiroshima and Nagasaki after the
atomic bombings
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          	   Estimated city population             Estimated number of
 City             at the time of the bombings              acute deaths
____________________________________________________________________________

 Hiroshima              310,000 persons                   90,000-140,000

 Nagasaki               250,000 persons                    60,000-80,000
____________________________________________________________________________

References on this subject

	Hiroshima and Nagasaki. The Physical, Medical, and Social Effects of the Atomic Bombings. Edited by the Committee for the Compilation of Materials on Damage Caused by the Atomic Bombs in Hiroshima and Nagasaki. English translation by Eisei Ishikawa and David L. Swain, copyright 1981 by Hiroshima City and Nagasaki City. 706 pp. (Originally published in Japanese by Iwanami Shoten Publishers, Tokyo, 1979.)
	Ohkita T: Annex 4. Health effects on individuals and health services of the Hiroshima and Nagasaki bombs. In Effects of Nuclear War on Health and Health Services, Report of the International Committee of Experts in Medical Sciences and Public Health to Implement Resolution WHA34.38. Geneva, Switzerland: World Health Organization, 1984, pp 101-5.
	Medical Effects of the Atomic Bomb in Japan. Edited by A.W. Oughterson and S. Warren. New York: McGraw-Hill, 1956, 477 pp. (This book was based on the 6-volume report of the Joint Commission for the Investigation of the Effects of the Atomic Bomb in Japan. Within a few weeks after the atomic bombings, the Joint Commission, composed of US military and civilian scientists, had been established and was at work in the field. Copies of the original 6-volume report are archived at the US National Academy of Sciences, Washington, DC, and at the RERF Hiroshima Library.

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Analyses of deaths due to cancer among the Life Span Study cohort of atomic-bomb survivors from 1950 through 1990, published in Radiation Research (146:1-27, 1996), are summarized in Table 2. These results are for survivors who were exposed to significant radiation doses (See Question 11).

Table 2. Summary of cancer deaths in the Life Span Study cohort of 
atomic-bomb survivors, 1950-1990
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 			          Estimated number     Percentage of deaths
 Cause of      Total number         of deaths due  	    attributable
  death         of deaths           to radiation            to radiation
___________________________________________________________________________

 Leukemia          176	                  89                     51%

 Other types
 of cancer*      4,687                   339                      7%

 Total           4,863                   428                      9%

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*Solid cancers, such as stomach, lung, breast, colorectal and liver cancers

The proportion of cancer deaths attributable to radiation exposure is higher among those who were exposed closer to the hypocenter, as in the case of deaths due to injuries from the blast, heat, or radiation. Table 3 presents data on the size of the studied population and the number of cancer deaths in relation to distance from the hypocenter for the approximately 50,000 survivors with significant exposures (See Question 10).

Table 3. Cancer deaths among atomic-bomb survivors, 1950-1990,
by distance from hypocenter
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                                  Leukemia                Other cancers*
                        _______________________     ________________________
 Distance
   from                                Percent                    Percent
 hypocenter   No. of      No. of     attributable     No. of    attributable
   (km)       persons     deaths     to radiation     deaths    to radiation
_____________________________________________________________________________

   <1            810        22   	 100%           128          42%

 1.0-1.5      10,590	    79   	  64%          1156	     18%

 1.5-2.0      17,370	    36   	  29%          1622	      4%

 2.0-2.5      21,343	    39   	   4%          1781	    0.5%
_____________________________________________________________________________

Because the Life Span Study cohort does not include all survivors (see Question 8), the number of cancer deaths that may be attributed to radiation among all survivors would be larger than the 428 shown in Table 2.

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In addition to analyzing cancer mortality (deaths due to cancer), RERF analyzes data on cancer incidence obtained through the cancer registries of Hiroshima and Nagasaki prefectures. Although mortality studies are valuable, the accuracy of cancer diagnoses from death certificates is limited and mortality studies provide little information about the occurrence of cancers with relatively high survival rates. Among 79,972 members of the Life Span Study cohort whose radiation doses were known, 8,613 first primary solid cancers were diagnosed between 1958 and 1987, about three-quarters with histological verification. Significant radiation associations were observed for stomach, colon, lung, breast, ovary, urinary bladder, thyroid, liver, and nonmelanoma skin cancers. Because factors other than radiation can increase the risk of cancer (such as smoking, certain chemicals, various viral and bacterial agents, and so on), RERF researchers are now examining individual types of cancer more carefully to determine what role radiation plays in the mechanism of cancer causation and how it might interact with these other cancer risk factors.

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Yes. The excess risk of leukemia, seen especially among those exposed as children, was highest during the first 10 years after exposure and has continued to decrease throughout the study period. However, the excess risk for cancers other than leukemia continues today, and it seems likely that this excess risk will persist throughout the lifetime of the survivors. About 16% of all cancer deaths and about 25% of the excess--or radiation related--cancer deaths for the period from 1950 through 1990 occurred from 1986 to 1990.

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The Life Span Study mortality analyses have revealed a statistically significant relationship between radiation and deaths resulting from causes other than cancer (so-called noncancer diseases). A total of 15,633 noncancer deaths occurred between 1950 and 1990 among the 50,113 persons with significant radiation doses. The overall risk for noncancer deaths is considerably smaller than that for cancer deaths, but because noncancer causes comprise a larger fraction of human deaths overall, the total number of estimated radiation related excess noncancer deaths is about 50-100% the number of estimated radiation related cancer deaths (the reason for the wide range is that the data do not yet clarify the shape of the dose response, and different estimates of number of excess, radiation related, cases result from various shapes of response that can be fit to the data).

Clinical researchers conducting the Adult Health Study of biennial clinical examinations have analyzed the relationship between radiation exposure and a number of selected nonmalignant (non-cancer) disorders. Statistically significant excess risks were detected for uterine myoma, chronic hepatitis and liver cirrhosis, thyroid disease, and cardiovascular disease. The results suggested that the thyroid gland in young persons may be more sensitive to radiation not only in the development of thyroid cancer, but also possibly in the development of nonmalignant thyroid disorders. Although cataracts are another condition related to radiation, their association with radiation seems to be limited to the early years following exposure, as there has been no indication of radiation dose-related excess cases in recent examinations.

Some non-cancer diseases may be associated with altered immune functions in A-bomb survivors. Immunological study of survivors demonstrated that the proportion of helper T cells was significantly decreased with increased radiation dose [see Department of Radiobiology/Molecular Epidemiology]. Furthermore, the prevalence of myocardial infarction was significantly higher in individuals with a lower proportion of helper T cells. These results suggest that myocardial infarction in A-bomb survivors is partly due to defects of helper T cells. Such defects may contribute towards a reduced immune defense against microbial infections, possibly leading to atherosclerosis.

References on this subject

	Shimizu Y, Pierce DA, Preston DL, Mabuchi K: Studies of the mortality of atomic bomb survivors. Report 12, Part II. Noncancer mortality: 1950-1990. Radiation Research 152:374-89, 1999
	Wong FL, Yamada M, Sasaki H, Kodama K, Akiba S, Shimaoka K, Hosoda Y: Noncancer disease incidence in the atomic bomb survivors: 1958-1986. Radiation Research 135:418-30, 1993

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Among the approximately 3,000 in utero exposed atomic-bomb survivors, the following results have been observed: a reduction in IQ as radiation dose increases, a higher incidence of mental retardation among the heavily exposed, and some impairment in the rate of growth and development on average (not all exposed individuals demonstrated these effects). Many of these effects seem to be particularly pronounced among persons who had been exposed between the 8th and 15th weeks of gestation. Death rates and cancer incidence are being monitored in this group. The numbers of deaths or cancers are still too small to draw any firm conclusions, but the data to date suggest a trend of increasing risk as radiation exposure increases, similar in extent to the trend observed among those exposed to atomic-bomb radiation as children.

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This was one of the earliest concerns in the aftermath of the bombings. Efforts to detect genetic effects were begun in the late 1940s and continue to this day. Thus far, no evidence of genetic effects has been found. Recent advances in molecular biology may make it possible to detect genetic changes at the gene (DNA) level at some time in the future. RERF scientists are working to preserve blood samples that can be used for such studies as suitably powerful techniques are developed (see Repository of biological materials). Monitoring of deaths and cancer incidence in the children of survivors also is continuing.

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Current studies of deaths and cancer incidence are based on a sample of 86,500 atomic-bomb survivors who were alive and living in either Hiroshima or Nagasaki at the time of the Japanese national census in October 1950 and for whom radiation dose estimates can be computed. About 50,000 of these persons were exposed to significant radiation doses (Question 11) within a distance of 2.5 km from the hypocenter. The other 36,500 members of the study population were exposed beyond 2.5 km and received very low doses.

In the national census of 1950, approximately 280,000 people indicated that they had been exposed to the atomic bombs. The population studied by RERF probably includes about 50% of the proximally exposed survivors and about 25% of the distally exposed. However, these percentages are not precise because the census did not include recording of the place of exposure.

The cohort includes another group of 27,000 people whose family registry was in one of the cities and who lived in Hiroshima or Nagasaki at the time of the national census but were not exposed to the atomic bomb. However, this latter group is not usually included in analyses because of data suggesting that there are socioeconomic differences from the survivor population. These two groups together are formally known as the Life Span Study (LSS) cohort.

In addition to the Life Span Study, RERF scientists are involved in studies of several other populations. These include the Adult Heath Study (AHS), In Utero Exposed, and Second Generation Cohorts. The AHS population comprises 20,000 members of the LSS, who, since 1958, have been asked to participate in biennial clinic examinations carried out at RERF. About 3,000 people who were exposed to the bombings while in utero make up a second group. Finally, a group of about 80,000 constitute the F₁ cohort and include children born to parents with and without exposure to the bombs.

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In 1995, the average age of the surviving atomic-bomb victims was about 66 years. In 1990, about 54% of the RERF study population was still living, and as of 1995, slightly more than 50% were alive. Looking at vital status in terms of age at exposure, we find that at the end of 1990, more than 90% of the survivors exposed under the age of 20 were still living.

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To make monitoring easier in long-term follow-up of mortality and cancer incidence due to radiation exposure, approximately 90,000 people who were resident in Hiroshima or Nagasaki at the time of the national census on October 1, 1950 were selected from about 280,000 survivors who were directly exposed in Hiroshima City or Nagasaki City.

Information about radiation effects in the atomic bomb survivors is obtained in many ways, some utilizing the full Life Span Study population, others based on subsets of that population. One ongoing method is mortality follow-up, checking for registrations of death and the cause of death. A second ongoing method is checking registrations of cancer diagnoses made by local hospitals and physicians to the cancer registries in Hiroshima and Nagasaki prefectures. Mail surveys asking about lifestyle and other factors have been sent approximately every 10 years to the Life Span Study cohort members. A subset of the Life Span Study (the Adult Health Study) is examined clinically every two years; this clinically examined subset also forms the basis of cytogenetic, immunological, and molecular epidemiologic studies. Chromosome aberrations and blood proteins have been examined in a sample of approximately 8,000 and 24,000 children (respectively) born to one or two radiation exposed parents to assess possible genetic damage passed on to children. Approximately 76,000 children were examined at birth and at age 9 months, between 1948 and 1954, for possible congenital birth defects. Finally, plans are being made to assess the health status of children of atomic-bomb survivors based on mail questionnaires and clinical examinations.

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In the discussion of cancer risks presented here, attention is focused on survivors with estimated doses greater than 5 millisieverts (mSv; 0.005 Sv). While no excess risks of cancer or other diseases are detectable among survivors with doses at the low end of this range, a dose of 5 mSv is several times higher than the typical annual background radiation level to which people are exposed in daily life (1-2 mSv), or about one-fourth of the currently accepted average annual dose allowed for radiation workers (20 mSv). Survivors with doses of 5 mSv or more were typically within about 2.4 km of the hypocenter in Hiroshima and within 2.6 km of the hypocenter in Nagasaki. The average dose received by the group of survivors considered here is about 200 mSv. The radiation dose decreases by about one-half for every 200-meter increase in distance from the hypocenter. (see Dose estimates by distance).

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The practical answer is, "No."

People often ask, "If uranium and plutonium pose a potential hazard in nuclear waste sites and were present at dangerous levels in the environment following the Chernobyl nuclear accident, why aren't Hiroshima and Nagasaki still uninhabitable?"

There are two ways residual radioactivity is produced from an atomic blast. The first is due to fallout of the fission products or the nuclear material itself--uranium or plutonium (uranium was used for the Hiroshima bomb whereas plutonium was used for the Nagasaki bomb)--that contaminate the ground. Similar ground contamination occurred as a consequence of the Chernobyl accident, but on a much larger scale (click here for more-detailed explanation). The second way residual radioactivity is produced is by neutron irradiation of soil or buildings (neutron activation), causing non-radioactive materials to become radioactive.

Fallout.The Hiroshima and Nagasaki bombs exploded at altitudes of 500 to 600 meters, then formed huge fireballs that rose with the ascending air currents. About 10% of the nuclear material in the bombs underwent fission; the remaining 90% rose in the atmosphere with the fireball. Subsequently, the material cooled down and some of it started to fall with rain (black rain) in the Hiroshima and Nagasaki areas, but probably most of the remaining uranium or plutonium was dispersed widely in the atmosphere. Because of the wind, the rain did not fall directly on the hypocenters but rather in the northwest region (Koi, Takasu area) of Hiroshima and the eastern region (Nishiyama area) of Nagasaki. As for contamination with plutonium, early measurements in Nagasaki showed levels of radioactivity far below the levels of radioactivity from cesium-137, which is one of the fission products of uranium or plutonium and contributed most to radiation from fallout. Attempts to measure uranium in Hiroshima were difficult to interpret due to low levels of radioactivity. Nowadays, the radioactivity is so miniscule that it is difficult to distinguish from trace amounts (including plutonium) of radioactivity caused by worldwide fallout from atmospheric (as opposed to underground) atomic-bomb tests that were conducted around the world in past decades, particularly in the 1950's and 1960's.

Neutron activation. Neutrons comprised 10% or less of the A-bomb radiation, whereas gamma rays comprised the majority of A-bomb radiation. Neutrons cause ordinary, non-radioactive materials to become radioactive, but gamma rays do not. The bombs were detonated far above ground, so neutron induction of radioactivity on the ground did not produce the degree of contamination people associate with nuclear test sites (Nevada test site in Southwest US, Maralinga test site in South Australia, Bikini and Mururoa Atolls, etc.). As for Hiroshima and Nagasaki proper, the longest-lasting induced radionuclide that occurred in amounts sufficient to cause concern was cesium-134 (with a half-life of about 2 years). Most of the induced radioactivities from various radionuclides decayed very quickly so that it now takes considerable time and effort to measure it using highly sensitive equipment. Despite such miniscule levels, measurements of residual radioactivity using recently developed ultra-sensitive techniques have been utilized to estimate neutron doses released from the bombs and have formed part of the basis of the latest atomic-bomb dosimetry (DS02).

Although the levels of residual radioactivity in Hiroshima and Nagasaki were initially high, they declined quickly and are now far less than the dose received from background radiation. Hence, there is no detectable effect of present-day residual radiation on human health. In fact, today both Hiroshima and Nagasaki are thriving cities with large populations.

References on this subject

	Shigematsu I, Ito C, Kamada N, Akiyama A, Sasaki H (Editors): Effects of A-bomb Radiation on the Human Body (Overview, pages 6-7). Harwood Academic Publishers, Bunkodo Co., Ltd., Tokyo, Japan, 1995
	Okajima S, Fujita S, Harley JH: Radiation Doses from Residual Radioactivity. In: Roesch WC, editor: US-Japan Joint Reassessment of Atomic Bomb Radiation Dosimetry in Hiroshima and Nagasaki, Volume 1, Chapter 6. Radiation Effects Research Foundation, Hiroshima Japan, 1987

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