Frequently Asked Questions

(Taken from Radiation Effects Research Foundation Home Page)

How many persons died in the atomic bombings?

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.

Five years after the atomic bombings, in the 1950 Japanese national census, approximately 280,000 persons indicated that they "had been exposed" in Hiroshima or Nagasaki. (Although most of them were probably exposed in the former administrative districts of the cities, the census did not require recording the place of exposure.) The census total is a rough estimate of those who were exposed and survived the bombings. However, so-called "early entrants," those who had entered the cities after the bombings, are not included in the census.

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

Analyses of deaths due to cancer among the atomic-bomb survivors from 1950 through 1990 were recently completed and published in Radiation Research (146:1-27, 1996). Table 2 summarizes the results of the study of survivors exposed to significant radiation doses (See Question 10).

The number of cancer deaths among the 36,500 survivors who were exposed beyond 2.5 km is 3,177, including 73 leukemia deaths and 3,104 deaths from cancers other than leukemia.

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).

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

Cancers attributable to radiation are still occurring among atomic-bomb survivors. The excess risk of leukemia, seen especially among those exposed as children, was the 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 cancer deaths for the period from 1950 through 1990 occurred from 1986 to 1990.

What health effects have been found among persons who were exposed before birth?

Many health effects are associated with radiation exposure before birth. Among the in utero exposed atomic-bomb survivors, the following have been observed: a reduction in IQ as radiation dose increases, a higher incidence of mental retardation among the heavily exposed, and impairment in the rate of growth and development. Many of these effects seem to be particularly pronounced among those persons who had been exposed between the 8th and 15th weeks of gestation. Death rates and cancer incidence are being monitored for this group. To date, the data 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.

What health effects have been seen among children of atomic-bomb survivors?

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 have 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. Monitoring of deaths and cancer incidence in the children of survivors also is continuing.

How many atomic-bomb survivors are included in the group being studied by RERF?

In the Japanese 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.

RERF research includes the study of deaths and the incidence of cancer occurring among atomic-bomb survivors. Who is included in this study population?

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 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 (See Question 10) 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.

What percentage of the original atomic-bomb survivor study population are dead?

In 1995, the average age of the surviving atomic-bomb victims was about 66 years. In 1990, about 46% of the RERF study population was dead, and as of 1995, slightly fewer than 50% were dead. Looking at vital status in terms of age at exposure, we find that at the end of 1990, fewer than 10% of the survivors exposed under the age of 20 were dead.

How is information about the RERF study population obtained?

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. 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 LSS, RERF scientists are involved in studies of several other populations. These include the Adult Heath Study (AHS), In Utero Exposed, and F₁ 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.

What is meant by "significant" exposure?

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 maximum 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 half for every 200-meter increase in distance from the hypocenter.

Are Hiroshima and Nagasaki still radioactive?

No. There are two ways radioactivity is produced from an atomic blast. The first is due to fallout of the fission products or the nuclear material itself, i.e., uranium or plutonium that contaminate the ground. (The same ground contamination occurred as a consequence of the Chernobyl accident.) The Hiroshima and Nagasaki bombs exploded at 500 to 600 m of altitude, then formed huge fireballs that rose with ascending air currents. Subsequently, the material cooled down and started to fall with rain. Because of the wind, the rain did not fall directly on the hypocenter but rather in the northwest region (Koi, Takasu area) of Hiroshima and the eastern region (Nishiyama area) of Nagasaki. Nowadays, the radioactivity is so miniscule that it is difficult to distinguish from trace amounts of radioactivity caused by atmospheric atomic-bomb tests.

Another way radioactivity is produced is by neutron irradiation of soil or buildings. (Neutrons comprise 10% or less of A-bomb radiation; nonradioactive materials become radioactive after absorbing neutrons. In contrast, gamma rays--which comprise the majority of A-bomb radiation--do not cause ground materials to become radioactive.) However, most of the radioactivity decayed very quickly so that it now takes months to measure the radioactivity using highly sensitive equipment. Despite miniscule levels, these measurements are currently utilized to estimate neutron doses released from the bombs.

In both cases, the residual radioactivity is far less than the dose received from background radiation; hence, there are no detectable effects on human health.

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