After studying this module, the reader will be able to
This module addresses the basics of radiation science and radiation health effects. The Hanford Site in south central Washington state released more than 100 radionuclides into the environment for more than forty years (See Module 3). These substances emitted different forms of radiation-alpha, beta, and gamma-and each form affects the body differently.
Radiation is the release of energetic particles and rays from atoms. This release occurs because some atoms, the basic building block for all substances, are unstable. The nucleus of the atom is made up of protons and neutrons. Electrons surround the nucleus. The term atomic number refers to the number of protons within the nucleus of an atom. The term atomic weight, or mass number, refers to the number of neutrons and protons in the nucleus of an atom.
When two substances have the same number of protons and the same atomic number, but have different numbers of neutrons and different atomic weights, they are called isotopes of one another. For example, uranium-235 and uranium-238 are isotopes of one another. Too many or too few neutrons in a nucleus make the atom unstable. An unstable atom is radioactive when it gives off energy as it tries to become stable. This process is called radioactive decay.
Iodine-131 and other radioactive materials give off energy in the form of ionizing radiation. Ionizing radiation transfers energy to the substances it strikes. This transfer of energy between the radioactive substance and living matter is, in general, a harmful process. The greater the energy transferred, the greater the injury. Ionizing radiation consists of either waves of energy or tiny particles. Radiation can come from alpha, beta, and neutron particles, and gamma and X-ray electromagnetic waves. Each form of radiation acts differently depending on the source.
Alpha particles are positively charged particles made up of two protons and two neutrons. The particles lose their energy quickly and do not penetrate the surface of the skin if the body is exposed externally. Material containing alpha radioactivity can enter the body through a cut in the skin, by ingestion, or inhalation. Uranium-238 and plutonium-239 are sources of alpha radiation.
Beta particles are fast moving electrons which are negatively charged. Beta radiation can penetrate a few millimeters in human tissue before losing all of its energy. Iodine-131, phosphorus-32, and strontium-90 are all sources of beta radiation.
Gamma rays are photons, or electromagnetic waves, that come from the nucleus of the atom. Gamma rays are uncharged and pass through humans at the speed of light. As gamma rays pass through the body, they may damage cells. Cobalt-60 is a source of gamma radiation.
Neutron particles are the uncharged particles in the nucleus of an atom. Neutrons may damage cells as they penetrate the body. Neutrons are commonly released in nuclear reactors.
X-rays are similar to gamma rays, but are produced outside the nucleus. Their properties are identical to those of gamma rays.
Several terms are used to describe radioactive materials and their effects:
Half-life: This is the amount of time it takes for a radioactive substance to lose one-half of its radioactivity. Iodine-131 has a half-life of eight days. At the end of eight days, half of the iodine-131 atoms have undergone decay and converted to stable xenon-131. Half of the remaining iodine-131 will decay into stable xenon-131 in another eight days, and so on. When an atom decays and becomes stable, it is no longer radioactive.
Some radioactive substances decay quickly into non-radioactive materials. Others decay over long periods of time into other radioactive materials which, in turn, undergo radioactive decay. For example, uranium-238 has a half-life of 4.5 billion years and undergoes more than a dozen changes before becoming a stable form of lead.
Curie: This a measure of radioactive material. It measures the number of atoms that decay each second. One curie is 37 billion atoms undergoing decay each second. Hanford released an estimated 739,000 curies of iodine-131 from 1944 to 1972. In contrast, the 1979 accident at the Three Mile Island nuclear power plant in Pennsylvania released and estimated 15 curies of iodine-131.
Rad: A rad is a unit used to measure the absorbed dose, or the amount of energy body tissues absorb. However, equal doses of different types of radiation may not have the same effects on the body. For instance, a dose of alpha particles is more damaging than the same dose of gamma rays or beta particles.
Rem: The rem is the unit of radiation which accounts for the different effects of different types of radiation. In order to calculate the equivalent dose in rem, absorbed dose must first be established. This number is then multiplied by a radiation weighting factor depending on the type of radiation. For beta particles and gamma rays, the weighting factor is 1. Most of the radioactive material released from Hanford emitted beta particles and/or gamma rays, so it is easy to convert directly from rad to rem: 1 rad is equal to 1 rem. Doses from alpha particles and neutrons have larger weighting factors.
Effective Dose Equivalent (EDE): This term is used because a radiation dose to one part of the body does not have the same potential health effect as a dose to another part. The EDE is used to put different types of radiation doses on an equivalent basis in terms of their potential for causing damage.
To help patients understand why cancer and thyroid disease are of concern, it is useful to discuss how radiation can cause harm to the body. The following description provides this information in general terms. When radiation enters the body and hits a cell, one of four things can happen:
If the radiation passes through the cell without doing damage or the cell repairs itself successfully (numbers 1 and 2 above), there is no lasting damage or health effect. If the damage is passed on when new cells are formed (number 3 above), there may be a delayed health effect, such as cancer or genetic effects. Any dose of radiation may produce a delayed health effect. Delayed effects from radiation exposure may occur months, years, or decades later. It is not possible to predict if or when these effects will occur.
If the damage to a cell is not repaired and is passed on to new cells (number 3 above), a cancer can begin to grow. It may take years or even decades (the latent period) for a cancer to grow large enough to be discovered. The latent period varies for different types of health effects and different types of radiation doses.
When radiation kills a cell (number 4 above), there will be acute (immediate) health effects if the dose is high and many cells die. Death may occur within days or weeks from radiation sickness, as happened to the highly exposed people in the atomic bombings in Japan. Other acute effects include vomiting and loss of hair. From what is currently known, doses to people from Hanford's environmental releases were not enough to produce immediate or direct effects.
Radiation exposure may be internal or external. Internal exposure comes from eating or drinking contaminated food or water, or from breathing contaminated air. A radioactive substance can also enter the body through cuts in the skin. Alpha and beta radiation contribute to internal exposure. External exposure can come from beta, gamma and X-ray radiation that penetrates the body. Both internal and external radiation exposure can directly harm cells. Exposure to Hanford's radiation was primarily internal. Exposure from the atomic bombings in Japan was primarily external.
Whether or not exposure to radiation will cause cancer depends on a variety of factors. These include: the amount and type of radiation dose; individual characteristics that make some people more susceptible to cancer than others; age; gender; whether the exposure occurred over a short or a long time; and the presence of other substances that enhance the cancer-causing power of radiation.
There has been much controversy over the extent to which low-dose radiation causes cancer. One of the more widely-known reports was published in 1990 by the Fifth Committee on the Biological Effects of Ionizing Radiations (known as BEIR V) [1]. BEIR V concluded that information from scientific studies about people receiving low doses was insufficient to determine cancer risk.
Overall, BEIR V concluded that cancer risk from radiation exposure is higher than regulatory and advisory groups had previously described. BEIR V estimated cancer risk but acknowledged uncertainty concerning these risk estimates. BEIR V estimated that for every 10,000 adults exposed over a short time period to 1 rem of radiation, eight would die from radiation-induced cancer.[2] If the exposure took place during childhood, the risk for fatal cancer was estimated to be twice as high. BEIR V also concluded that when the dose was received over a long time, the lifetime risk of death from cancer was lower by a factor of 2 or more than if the same dose had been received over a short time. Most Hanford exposures occurred over long times (months, years, or decades).
Other scientists have drawn quite different conclusions, arguing that BEIR V either overestimated or underestimated the risk of radiation-caused cancer. For instance, a team of scientists found that radiation doses received by survivors of the atomic bomb dropped on Hiroshima were higher than current estimates.[3] If this is true, BEIR V cancer-risk estimates may be too high, as they are strongly influenced by the Japanese survivor studies.
Others argue that the BEIR V report underestimates the risk of radiation-caused cancer. Among these scientists is Dr. John Gofman. He concluded that for every 10,000 adults exposed to 1 rem of radiation, 26 would die from radiation-induced cancer.[4] Gofman pointed out that about 2,200 of these 10,000 adults will die from cancer induced by all causes. Gofman also said that the risk is even higher for children.
Contrary to BEIR V, Gofman believes that receiving a low dose of radiation over weeks or months (such as in the Hanford situation) does not lower the risk for radiation-induced cancer. In fact, he argues that a dose of radiation given over a longer time will produce a greater cancer risk than the same dose given over a short time.
Additionally, two other scientists have been sharply critical of BEIR V. Rudi H. Nussbaum and Wolfgang K�hnlein have pointed out a number of inconsistencies within the BEIR V report. They also argue that studies published after BEIR V support the position that there is a greater risk of health effects from chronic low doses than is reflected in current radiation protection regulations.[5]
Researchers are unable to determine with certainty the relationship between cancer and radiation exposure. Many people find this frustrating. However, it is important to know that there are three key factors that complicate this scientific research. First, there are many things that can cause cancer besides exposure to radiation, making it difficult to measure which ones were caused by radiation exposure. Cigarette smoking, exposure to pesticides and other toxic chemicals, and random genetic mutations also can cause cancer. Second, people receive radiation from sources other than Hanford, such as background radiation and medical procedures. Third, not everyone exposed to radiation gets cancer.
The type of radiation that caused the highest doses downwind from Hanford, iodine-131, concentrates in the thyroid gland. Exposure to some types of radiation has been shown to cause thyroid disease, including cancerous and noncancerous thyroid growths. The Hanford Thyroid Disease Study (HTDS)[+] is gathering information on all types of thyroid disease, whether or not previous studies have suggested links between radiation exposure and thyroid disease. While the HTDS will not be completed until late 1998, thyroid disease studies from other types of radiation exposures may offer some comparisons to the Hanford situation.
Because people downwind from Hanford were exposed to airborne releases of iodine-131, studies of other people who were exposed to airborne releases of iodine are of interest. The situations of the Nevada-Utah downwinders, the Marshall Islanders, and children exposed as a result of the Chernobyl accident have some similarities with the Hanford situation (mainly exposure to iodine-131). However, there are also some important differences that limit comparisons with Hanford, including
People who lived downwind (downwinders) from the Nevada Test Site were exposed to nuclear fallout, including iodine-131, caused by atmospheric testing of nuclear weapons. A study of these downwinders suggests a dose-response relationship between the occurrence of thyroid growths (nodules and cancer) and iodine exposure. The investigators who did the study concluded that the radioactive iodine exposure "probably caused" between one and 12 of the 19 cases of thyroid growths among the study population of about 2,500.[6]
In 1954, Marshall Islanders were exposed to radioactive fallout from a nuclear weapon test in the South Pacific. They were exposed to some iodine-131, but most of the thyroid exposure came from other radioactive forms of iodine. The Marshall Islanders suffered both acute and delayed effects from radiation. Eight years after the blast, some Marshall Islanders developed thyroid disease. After 27 years, the Marshall Islanders had an increased rate of hypothyroidism (underactive thyroid gland) and both noncancerous and cancerous thyroid growths. It is difficult to say that it was the iodine-131 or the other radioactive iodines alone that caused these thyroid problems because the Marshall Islanders also received external radiation.
In 1995, scientists reported that the rates of thyroid cancer were significantly increased among young people who were exposed to Chernobyl's radioactive fallout [7]. Before the 1986 accident, childhood thyroid cancer in the areas around Chernobyl was rare. The current rates are up to 200 times higher than normal. The rates in the table below[8] are the number of thyroid cancers per million people. Childhood thyroid cancers are those diagnosed before the children turn 15 years old.
Most (about 85 percent) of the Chernobyl thyroid dose came from iodine-131 and was received over a short time. The rest of the thyroid dose came from other radioactive isotopes of iodine. At Hanford, nearly all of the thyroid dose was from iodine-131 and was received over a number of years. The Hanford Environmental Dose Reconstruction Project (HEDR) [*]. estimated that children living downwind from Hanford received total thyroid doses in the range of 3 to 235 rad for the period 1944 through 1951. Because of uncertainties, the estimated dose could have been as high as 870 rad [9] .
Until further studies around Chernobyl are completed, it is not clear if radioactive iodine was the only cause of the high rates of thyroid cancer. Among other possible contributors were an iodine deficiency in the exposed population before the accident and a higher-than-normal sensitivity to the harmful effects of radiation exposure among some of those exposed [10]. Another contributor could have been the greatly increased number of thyroid examinations after the accident [11].
| TABLE 1 Childhood Thyroid Cancer Near Chernobyl (before and after the 1986 accident) |
|||||||
| 1981-1985 | 1986-1990 | 1991-1994 | |||||
|---|---|---|---|---|---|---|---|
| No. of Cases |
Rate* | No. of | Rate* | No. of Cases |
Rate* | Thyroid Dose Estimate | |
| Gomel region, Belarus | 1 | 0.5 | 21 | 10.5 | 143 | 96.4 | 15 to 570 rad |
| Northern Ukraine | 1 | 0.1 | 21 | 2.0 | 97 | 11.5 | 5 to 200 rad |
| Bryansk and Kaluga regions, Russia | 0 | 0 | 3 | 1.2 | 20 | 10.0 | 6 to 180 rad |
*number of thyroid cancers per million people
[adapted from Stsjazhko et al. 1995]
Much of what is currently known about the health effects of iodine-131 comes from studies of the medical uses of iodine-131. One group of people exposed to iodine-131 received a one-time high dose (thousands of rad) to treat hyperthyroidism (an overactive thyroid gland). Another group received a one-time low dose (50-100 rad) of iodine-131 for tests to diagnose thyroid disease. Studies of these two groups of people do not show any link between iodine-131 and thyroid cancer.
However, the length of time people were studied varied. The longest study followed people an average of 20 years. Investigators believe that the latent period for thyroid cancer can range from 5 to more than 40 years. They believe that the very high doses of iodine-131 used to treat people with hyperthyroidism result in killing off cells so that cancer cannot develop.
While there is not conclusive evidence linking iodine-131 and thyroid cancer, there is a link between thyroid cancer and exposure to X-rays and gamma radiation. Studies of people who received X-ray treatments of the head and neck show that X-rays can cause thyroid cancer. Thyroid cancer was the first solid tumor to show an increased rate in Japanese atomic bomb survivors who were exposed to gamma radiation.
Parathyroid glands help maintain the level of calcium in the body and are located around the thyroid. Studies of people receiving X-ray treatments to the head and neck have demonstrated a higher rate of hyperparathyroidism than expected. Further, those people who had hyperparathyroidism and a history of radiation treatments also had a greater frequency of thyroid disease than those who had hyperparathyroidism but did not have radiation treatments[12]. Radioactive iodine in the thyroid exposes the parathyroid and may cause tumors in the parathyroid glands. HTDS is investigating whether hyperparathyroidism is increased among people exposed to Hanford's radioactive releases.
Although cancer is the most studied of all radiation health effects, exposure to radiation can harm the human body in other ways. The following are brief summaries of some other radiation health effects. Publications are available from the Network on some of these health effects.
Studies have shown that radiation exposure can weaken the immune system [13]. While there are no studies concerning Hanford and autoimmune diseases, some Hanford-area residents are concerned that their exposure to radioactive materials has triggered such diseases. They believe that there are a higher-than-usual number of autoimmune disease cases among those who were exposed.
Genetic effects of radiation exposure occur when radiation damage to a parent's DNA code is transmitted to a child. Genetic effects caused by radiation fall into two categories: (1) effects that appear in the children of an exposed parent and (2) effects that appear in later generations. Birth defects can arise spontaneously or through harm to normal developmental processes by radiation or by other toxic exposures. For more information about possible genetic health effects, see Module 8.
Module 9 describes the possible nervous system diseases related to high-dose and low-dose radiation exposure. Past studies on radiation effects involving the nervous system are summarized.
The secrecy surrounding the Hanford releases, the involuntary nature of the exposure and the lack of information about radiation health effects have left some people understandably frustrated, mistrustful, and angry. Many people report feeling that the emotional and economic toll has been great. This is especially true for those who have thyroid diseases and other illnesses and whose family members, friends, and neighbors are ill or have died.
About 2 million people were exposed to environmental releases of radiation from Hanford's nuclear weapons operations from 1944 to 1972. Radiation can cause health effects, including cancer and thyroid disease. It is not known now what the health impact has been from the Hanford releases. More information will be available when the HTDS is completed in late 1998. However, given the uncertainties, the full impact of Hanford's releases will probably never be known.
For further reading about Hanford:
Many callers to the Hanford Health Information Lines have questions and concerns about the release of plutonium and other radioactive materials from Hanford. Some downwinders have health problems and believe that they are, or might be, related to Hanford. The personal perspectives within this monograph are offered to help readers understand these experiences and concerns.
When I arrived in Richland in 1954, I was healthy, happy, full of energy, and a bride of two weeks. It wasn't long before I began having horrific migraines, and unexplained attacks of vomiting and diarrhea that sent me to the hospital because I was dehydrated. Tests could not explain my symptoms--yet they persisted. I was weak to the point of exhaustion. And I lost an alarming amount of weight.
"Within a few years it became impossible for me to participate in family and social events. More often than not, I stayed home and on more than one occasion, my husband and children went on vacation trips without me. Two of my pregnancies ended in miscarriages. By my early 30s, I was a semi-invalid. I was diagnosed with endometriosis. When I was 35, I was rushed to the hospital unconscious and hemorrhaging. An emergency hysterectomy saved my life. Seven years ago, I was diagnosed with fibromyalgia. Was it connected to living there (near Hanford)? The doctors didn't connect it--yet?
"Both of our children were born with immune dysfunctions. A simple cold was an alarming matter. They were often anemic and our pediatrician tested them for leukemia. Both had skin cancer. My adult daughter has endometriosis. Connected? I wonder. . . Without warning, my husband was diagnosed with prostate cancer. It had already metatasized to his kidney, then to his liver. He died in 1990. His question was, "Are our medical problems because we lived in Richland for 25 years?" It weighs heavily upon my heart. Is there a connection? Studies and medical monitoring may one day answer his question. We greatly miss his loving presence in our lives."
