MODULE 8
After studying this module, the reader will be able to
People exposed to Hanford's radioactive releases have many questions and concerns about radiation's effects on their personal and family health. A key concern is whether radiation from Hanford caused genetic effects or birth defects.
This module discusses genes, mutations and birth defects, and how radiation can harm a cell. Summaries of studies of the effects of pre-conception parental radiation exposure, the effects of in utero radiation exposure, and results from new laboratory research are included. The findings of these studies are considered in relation to Hanford's releases.
The deoxyribonucleic acid (DNA) in each cell of an individual consists of segments called genes. Genes are part of the 23 pairs of chromosomes found in human cells. Mutations in the genes can arise spontaneously or as a result of exposure to radiation or chemical and physical agents. When these changes result from radiation exposure they are called radiation-induced mutations.
A germline mutation, or inheritable genetic effect, occurs when the DNA of a reproductive cell is damaged. A somatic mutation, which is not inheritable, occurs when the DNA of a non-reproductive cell is damaged.
Radiation-induced germline mutations may cause health problems which include miscarriages, stillbirths, congenital defects, neonatal or infant death, chromosomal abnormalities and cancer in later life. Radiation-induced somatic mutations affect only the exposed individual and may also cause health problems (see Module 2).
Birth defects can arise spontaneously, through radiation-related impairment of normal developmental processes, or can be induced by other toxic exposures. A birth defect caused by a germline mutation from parental pre-conception exposure is an inherited genetic effect. In utero fetal exposure can result in errors in development that may manifest themselves as retardation in physical growth or mental development.
Birth defects that may result from in utero fetal exposure to radiation include a reduction in standing and sitting height, severe mental retardation, microcephaly and impairment of brain development, which may indirectly reduce an individual's intelligence quotient (IQ) and school performance.
When a radioactive particle or wave hits a cell in the body, one of four things may occur:
Most research of radiation and genetic effects and birth defects involves exposure to external radiation, such as X-rays. In contrast to this external exposure, nearly all of the dose from Hanford came from internal exposure. That is, people were exposed to this radiation through the food and water they consumed and the air they breathed.
The results of studies of external exposure may not apply to people exposed to internal radiation. Also, an internal exposure from a radioactive substance may give a dose mainly to one organ, such as iodine-131 gives to the thyroid. Internal radiation exposure may have different genetic effects than those of external radiation exposure.
Studies by Sever and others [1988] reported an association between neural tube defects and the radiation dose fathers received before their children were conceived. This effect was observed in children whose fathers received low doses (10 rem or less) of external whole-body radiation while working at Hanford. These results were not supported by studies of children born to atomic bomb survivors who received higher doses of radiation.
Other research suggests that pre-conception parental radiation exposure can increase the frequency of birth defects; further studies are underway. One study being conducted around the Hanford Site is investigating the relationship between parental exposure to radiation and leukemia in their children.
Sever and others [1988] also conducted a study of birth defects in Washington's Benton and Franklin counties near Hanford. The researchers examined the number of cases of certain birth defects between 1968 and 1980. There were more neural tube defects than expected when the county rates were compared with rates from Washington, Oregon, and Idaho. Cleft lip was reported less often in Benton and Franklin counties than in the three-state area.
Using information from a study of Hanford workers, the researchers concluded that the increase in neural tube defects was not explained by parental employment at Hanford or by occupational exposure to radiation. The researchers also concluded it was unlikely that exposure of the general public to radiation from Hanford operations caused the increase in neural tube defects. This conclusion was based on a dose estimate of slightly more than 1 rem for the years 1974-1980.
This Hanford study includes only a few years relevant to the Hanford Health Information Network, with its Congressional mandate to focus on 1944-1972, the years of the largest releases. In addition, the dose estimate for the public includes only the years 1974-1980, during which there were limited Hanford operations. Also, the study was conducted prior to any dose estimates being available from the Hanford Environmental Dose Reconstruction Project (HEDR).[+] (For a description of HEDR, please see Module 1.)
Otake [1989, 1993], Yoshimoto [1988, 1990], Schull [1984], and others have reported on genetic studies of children whose parents were exposed to the Hiroshima and Nagasaki atomic bombs. There was essentially no difference between the rate of inherited birth defects in children whose parents were exposed to radiation and in controls whose parents were not exposed. These researchers, however, believe that genetic damage did occur because of the radiation exposure. Animal research and laboratory experiments suggest that inherited genetic effects from radiation exposure should occur in humans. It is possible that current research methods may not be able to detect the genetic effect in humans.
Mulvihill and Byrne [1987] conducted a follow-up study of cancer survivors who had undergone radiotherapy or chemotherapy. They investigated whether the offspring of the cancer survivors had higher rates of genetic disease than children of parents without cancer. People in the study group were diagnosed with cancer before the age of 20 and had survived for more than five years. The researchers compared the study group to a control group. The rates of genetic diseases were the same in both the group of cancer survivors and the control group indicating that there was not a higher rate of genetic disease in children of cancer survivors who had undergone radiotherapy or chemotherapy or both.
Research suggests there is a relationship between in utero X-ray exposure and development of childhood cancer. A large study by Stewart [1956] and another by MacMahon [1962] found an association between medical X-ray exposure before birth and childhood cancer. These findings indicate that the most sensitive period of exposure for developing leukemia is the first half of the third trimester of pregnancy. The first and second trimesters of pregnancy are the most sensitive for developing all cancers except leukemia.
Studies of children born to mothers who received whole-body radiation doses of between 50 and 100 rad following the Japanese atomic bombing showed that the children experienced an increased risk for microcephaly and mental retardation. This was especially true for those women who were 8 to 15 weeks pregnant at the time of exposure. Compared with non-exposed children, these children exposed to whole-body radiation doses during this period in utero had lower intelligence test scores and performed less well in school. Atomic bomb survivor studies also suggest that children exposed to radiation in utero have cancer rates equal to or higher than children who were exposed from ages one to nine.
In 1990, Gardner and colleagues published the results of a study of leukemia and non-Hodgkin's lymphoma among young people born and living near the Sellafield nuclear power plant in West Cumbria, United Kingdom. The researchers concluded that leukemia in children was linked to their fathers' exposure to external whole-body radiation before conception of the child.
For children whose fathers worked at the nuclear facility, the rate of childhood leukemia was twice as high as normal. There was also an eightfold increase of leukemia in children whose fathers received a lifetime dose greater than 10 rem or a dose greater than 1 rem within the six months before the children's conception. Leukemia, however, was also found more often than expected in children whose fathers were farmers or worked in the steel or chemical industries.
Interpretation of this finding includes consideration of the very small number of fathers whose children had leukemia. Only 4 out of 46 fathers who worked at Sellafield and had a radiation dose greater than 10 rem were compared to 3 out of 276 control group fathers.
Several scientists attempted to reproduce the results of Gardner's study. A study by McKinney [1991] indicated a 2.5-fold increase in leukemia in children whose fathers had radiation doses similar to those in the Gardner study. But Urquhart [1991] found that there was a 42 percent reduction in leukemia in children of exposed fathers compared with unexposed fathers.
Other scientists have developed different explanations about the results of Gardner's study. Evans [1990] found that most of the children had a genetic disorder that caused acute lymphatic leukemia and that this disorder was not related to a father's radiation exposure. Kinlen [1990] suggested that the increase in leukemia was due to a virus and found increased childhood leukemia in children born in other towns.
A number of scientists have concluded that Gardner's finding is not biologically plausible. Although the researchers Doll and Darby [1994] believe that Gardner's finding is biologically possible, they disagree with Gardner's conclusion. They argue that the conclusion is not supported by what is currently known about radiation genetics, or the inherited nature of childhood leukemias or studies of the children of atomic bomb survivors or nuclear facility workers. Doll and Darby conclude that the association between a father's radiation exposure and leukemia is a chance finding.
Sorohan and Roberts [1993] evaluated the relationship between childhood leukemia and a father's pre-conception radiation exposure. Using data already collected for the Oxford Survey of Childhood Cancer, researchers estimated a father's radiation dose based on his reported occupation. The researchers found little support for a father's exposure to external whole-body radiation in the six months before a child's conception as a risk factor for childhood cancer. The study suggested, however, that a father's internal exposure to radionuclides was connected with childhood cancer risk more often than exposure to external whole-body radiation.
Laboratory experiments suggest that plutonium-238 may produce genetic damage in cells. Kadhim [1992] reported that alpha particles from plutonium-238 produced a high frequency of chromosome damage in descendants of cells grown in the laboratory. Research by Hatsumi and Little [1992] indicates that alpha particles from plutonium-238 can cause genetic damage in the chromosomes of a cell for doses as small as 0.03 rem (30 mrem).
These findings from laboratory studies suggest that plutonium-238 can possibly induce genetic effects in humans from small doses of radiation. Two factors need to be considered when interpreting these findings: (1) genetic damage is constantly being repaired by the cells themselves, and (2) laboratory experiments with cells cannot be used to predict exactly what might occur in cells inside the human body.
Tawn and colleagues [1985] studied the chromosomes of white blood cells in plutonium workers. Chromosomal aberrations, or changes, were found. This suggests a relationship between plutonium exposure and genetic effects.
Scientists do not agree on the significance of some chromosomal aberrations. One perspective is found in a report issued by a National Academy of Sciences committee [1990] that studied the biological effects of ionizing radiation. The scientists commented that the implications, if any, of an increase in chromosomal aberrations in white cells are not clear. Another perspective is offered by Gofman [1992]. He argues that if aberrations increase in white cells, they also increase in other cells including reproductive cells. In Gofman's opinion, many birth defects considered to be of unknown origin result from chromosomal damage induced by radiation.
Animal studies, mainly using mice, have detected genetic effects not detected in human studies. This may suggest that humans are less sensitive to radiation than mice. Since genetic mutations are found in all animal species studied, it is expected that mutations do occur in humans.
A
I
In April 1994, HEDR released a draft of dose estimates for representative individuals. According to these estimates, even people who received the highest exposures were in the low-dose category for whole-body exposure (below 50 rem).
HEDR developed dose estimates for six radioactive substances released into the air: iodine-131, plutonium-239, ruthenium-103, ruthenium-106, strontium-90, and cerium-144. Iodine-131, which concentrates in the thyroid gland, accounts for most of the dose to most people from the air pathway. The highest estimated dose to the thyroid was for a child between 1944 and 1951 and was 870 rad. This is equivalent to an estimated whole-body dose of 29 rem EDE (Effective Dose Equivalent). A typical person who was an adult by 1944 had a cumulative estimated whole-body dose from exposure to all six air pathway substances of 1 rem EDE from 1944 to 1972.
For releases into the Columbia River, HEDR made dose estimates for five radioactive substances: zinc-65, phosphorus-32, neptunium-239, sodium-24 and arsenic-76. The highest estimated cumulative dose to an adult's red bone marrow was 2.8 rem EDE, and 4.8 rem EDE to the lower large intestine. The highest estimated cumulative whole-body dose for an adult was 1.4 rem EDE.
NOTES
The HEDR Project was formed in 1987 to estimate radiation doses the public may have received as a result of releases of radioactive materials from the Hanford Site. The Project was initially funded by the U.S. Department of Energy (DOE) and later funded by the U.S. Centers for Disease Control and Prevention (CDC).
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