August 2010 marks the 65th year since the 1945 atomic bombings that devastated the cities of Hiroshima and Nagasaki, ending World War II with Japan. Those atomic bombs were the first used in wartime and, hopefully, the last.
Many of the survivors of those bombings have generously agreed to become part of the most extensive studies of health effects in a human population ever conducted, making their experiences available for the betterment of humankind. Those studies were begun in 1947 by the Atomic Bomb Casualty Commission (ABCC), which was established by the National Academy of Sciences at the request of President Harry Truman. The studies have been continued by the Radiation Effects Research Foundation (RERF), which was established in 1975 by the governments of Japan and the United States. Through studies of atomic bombing survivors and their children, RERF has examined the links between radiation exposure and disease, cell and genetic damage, and other factors.
I’d like to dedicate this issue to the researchers and survivors involved in that effort and to share the important things that they have learned.
Early Effects of the Atomic Bombs
Most of the deaths caused by the atomic bombings occurred on the days of the bombings due to the overwhelming force and heat of the blasts and in the following days and weeks from injuries and exposure to radiation. In Hiroshima, an estimated 90,000 to 166,000 deaths occurred within two to four months of the bombing out of a total population of 340,000 to 350,000. In Nagasaki, 60,000 to 80,000 died out of a population of 250,000 to 270,000. The precise number of deaths is not known because military personnel records were destroyed, entire families perished leaving no one to report deaths, and unknown numbers of forced laborers were present in both cities.
One thing to understand about the health effects from radiation exposure is that it depends on the dose a person receives. The dose depends on several factors, the most important of which is the distance from the radiation source. Through interviews with survivors shortly after the bombings, researchers estimated the distance from the bomb explosion at which half of people survived to be 1,000 to 1,200 meters (about two-thirds to three-fourths of a mile) in Hiroshima and 1,000 to 1,300 meters in Nagasaki. The closer people were to the explosion, the greater the dose of radiation (see Figure 1), as well as the severe effects of the blast and heat; there is no information on classification of immediate deaths.
Radiation damages organ tissues and can lead to organ failure. Illnesses collectively called “acute radiation syndrome” may occur a few days after exposure to high doses of radiation (of about 1 Sievert or greater, see Figure 1). Principal signs and symptoms are nausea and vomiting, diarrhea from damage to the intestines, reduced blood cell counts and bleeding from damage to bone marrow, hair loss due to damaged hair-root cells, and temporary male sterility.
The immune system is also vulnerable to radiation immediately after exposure. In people who received large doses of radiation, two vital parts of the immune system–lymphocytes and bone marrow stem cells–were severely damaged. Two months after exposure, marrow stem cells recovered and death due to infection generally ended.
Delayed Effects: The Study of Survivors
At the heart of RERF’s research programs is a group of about 120,000 atomic bomb survivors who were still living in Hiroshima and Nagasaki in 1950, known as the Life Span Study cohort. About 90,000 of these people were within 10 km (6 miles) of the bombsites, roughly half within 2.5 km (the core group) and the other half between 2.5 and 10 km where radiation exposures were much lower. This group has undergone long-term population health and individual clinical studies that have helped researchers to study the delayed health effects of radiation.
Link to Leukemia
Excess leukemia was the earliest delayed effect of radiation exposure seen in atomic bomb survivors, first noted by a Japanese physician in the late 1940s. A registry of leukemia and related disorders was established to track cases.
Because leukemia is a rare disease, the absolute number of leukemia cases among atomic bomb survivors is relatively small even though the percentage increase in risk is high. Leukemia accounts for only about 3% of all cancer deaths and less than 1% of all deaths. As of 2000, there were 310 leukemia deaths among 49,244 Life Span Study survivors with a bone marrow dose of at least 0.005 Sv. The group experienced 103 deaths beyond expected deaths from leukemia, which means that 33% of the cases were attributable to radiation, but for those with a bone marrow dose of 2 or more Sv, 95% of the leukemias were radiation associated.
Research on A-bomb-related leukemia showed that the incidence of leukemia rose almost in direct proportion to dose; that the risk for leukemia was much higher for those exposed as children than for those exposed as adults; and that the incidence of radiation-related leukemia peaked at 8-10 years after exposure.
Link to Cancers: Linear but not Large Effects
By about 1956, researchers found an increase in rates for many other types of cancers. One of the most important findings is that exposure to radiation increases rates of most types of cancer, basically in proportion to radiation dose. That’s an important finding, because it means that even exposure to a very small amount of radiation will cause a very small increase in the risk of getting cancer. These results have direct implications for us today.
As of 2003, over 8% of cancers observed in the population of life Span Study survivors were attributable to radiation. There were 6,308 solid cancer deaths among the 48,102 Life Span Study survivors with a dose of 0.005 Sv or greater, which was 525 more solid cancer deaths than would have been expected in a similar, but unexposed, population. For the average radiation dose of survivors within 2,500 meters (about 0.2 Sv), there is about a 10 % increase above normal age-specific rates.
It is not possible to distinguish whether a cancer in a particular person is caused by radiation or other factors. In contrast to early effects of radiation that damage organ tissues, late radiation effects result from genetic changes in living cells. The exact mechanisms that lead to cancer are not clear, but it is believed that the process requires a series of genetic mutations accumulated over periods of years. Therefore, excess cancers attributable to radiation (except leukemia) are often not evident until decades after exposure.
Radiation exposure increases the risk for the following types of cancers: esophagus, stomach, colon, rectum, liver, gall bladder, pancreas, lung, breast, uterus, ovary, prostate, and bladder.
Small Non-cancer Effects of Radiation
RERF researchers also have analyzed the relationship between radiation exposure and a number of noncancer disorders. Radiation effects found in the Life Span Study survivors include relatively small but statistically significant excess risks for cardiovascular, digestive, respiratory and non-malignant thyroid diseases. In particular, radiation accounts for nearly one-third as many excess cardiovascular-disease deaths as cancer deaths. Studies also show a pattern of growth retardation for survivors who were exposed to the bomb’s radiation in childhood. Investigations of possible accelerated aging have shown some increased risk with radiation exposure for arteriosclerosis.
The considerable differences in the timing and increased risk of radiation-related leukemia, solid cancers and non-cancer diseases are illustrated in Figure 3.
Good News for Children of Survivors
One of the earliest concerns in the aftermath of the atomic bombings was how radiation might affect survivor’s children who were conceived and born after the bombings. Efforts to detect genetic effects caused by radiation damage to sperm and ovarian cells in survivors children began in the late 1940s. Recognizing the need for continued follow-up on children of survivors, RERF established the F1 study of 77,000 children, of which about 30,000 have at least one parent who received a radiation dose greater than 0.005 Sievert.
So far, no evidence of inherited genetic effects has been found. RERF is now using recent advances in molecular biology to confirm those results at the DNA level. Monitoring of deaths and cancer incidence in the children of survivors continues, and a clinical study is being undertaken to evaluate any potential radiation effects on late-onset genetic disorders.
Using RERFs Work
RERF’s important work has become the world’s primary guide for radiation-induced health effects, especially cancer. It has been used to develop standards for occupational exposures and to assess risks from medical exposure sources such as CT scans and other diagnostic procedures. The studies have also been vital in illuminating potential health effects in victims of nuclear accidents, current and former workers at nuclear facilities, and other exposed populations.
Many of the survivors who were children during the atomic bombings are still alive today and are now reaching their peak cancer years (see Figure 3). As of 2003, more than 40% of the survivors were alive, but more than 90% of those exposed under the age of 10 were still living. Projections suggest that in 2020 those percentages will be about 20% and 60% respectively. Consequently, RERF’s important mission to track the health of the survivor population and their children will continue for at least another two decades.
You can visit RERF’s website to find a wealth of information about its findings, its history, and general information about radiation, including a recently published Basic Guide to Radiation and Health Sciences.