The Basics of Radiation and Health
Ionizing radiation comes from an instability in the fundamental building block of all matter--the atom. It is a phenomenon involving the interchangeability of matter and energy first described by Einstein's Theory of Relativity. Einstein understood that small amounts of mass can be converted to very large amounts of energy--with the conversion ratio described by the very large number of the speed of light squared.
This energy, in turn, can be lethal to the human body--in particular the cell structure.
A stable atom is made up of negatively charged electrons that revolve in orbit around a nucleus composed of an equal number of protons. Also contained in the nucleus are neutrons, which have no electrical charge but which are endowed with a "binding energy" that keeps the nucleus together. Protons and neutrons account for more than 99.9 percent of the atom's weight and determine the basic properties of the element involved.
When an atom has an imbalance between protons and electrons it is considered unstable, or radioactive. Unstable atoms are called radioisotopes or radionuclides. In the process of achieving stability a part of the nucleus of a radioisotope disintegrates and emits particles and energy. It does this until it reaches stable equilibrium and is no longer radioactive. Thus radioactive elements travel through a "decay chain," emitting particles and energy until they transform into lighter, stable elements at the end of their chain.
The half-life of a radioactive substance describes the time it takes for one half of any quantity of it to decay into the next lighter element along its decay chain. Often, complete radioactive decay involves very long periods of time. For example, uranium 238 takes about twenty-eight billion years for half of it to decay into a stable form of lead.
Radiation is ionizing when it has enough energy to remove one or more electrons from an atom with which it comes in contact. When this occurs, the ionized atom is made chemically reactive and capable of damaging living tissue. Nonionizing radiation--as in the form of microwaves--falls on the other end of the electromagnetic spectrum and does not have sufficient energy to physically displace electrons of atoms. It can also, however, be damaging to human health.
Types of Radiation
There are essentially five types of ionizing radiation with which we are concerned here:
- Alpha radiation is created when two protons and two neutrons are emitted from the nucleus of an atom. Alpha particles have the same nucleus as the helium atom but lack the two electrons that make helium stable. Alpha particles travel at speeds up to ten thousand miles per second. Because they are so large in "subatomic" terms, alpha particles have been likened to large-caliber bullets. They tend to collide with molecules in the air and are easily slowed down. A thin sheet of paper or two inches of air can usually stop an alpha particle.
Unfortunately so can a human cell. When alpha-emitting elements are inhaled or ingested into the body, the high-energy particles they emit can rip into the cells of sensitive internal soft tissues, creating serious damage.
Alpha particles are emitted by a wide array of heavy elements, including plutonium, a by-product of nuclear fission; and radon, which seeps into the environment from the uranium-mining and -milling process; and radon gas, whose decay or "daughter" elements are carried into the atmosphere from uranium-mining wastes.
- Beta radiation is composed of streams of electrons that often travel at close to the speed of light. In some cases beta particles are emitted from a nucleus when a neutron breaks down into a proton and electron. The proton stays in the atom's core while the electron shoots out. Because they move faster than alpha particles, and weigh much less, beta particles are far more penetrating than alpha particles. Sheets of metal and heavy clothing are required to stop them.
Beta emissions to the skin can lead to skin cancer. And like elements that emit alpha particles, beta-emitters can be very dangerous when inhaled or ingested into the body. Beta radiation can be emitted from many substances released by nuclear bombs and power plants, including strontium 90 and tritium.
- Neutron emissions occur when the nucleus of an atom is struck by a particle that causes the unsticking of the "binding energy" in the atom's core. The resulting disequilibrium causes neutron particles to be shot out in a way that makes them capable of penetrating solid steel walls. Several feet of water or concrete are required to stop most of them.
Because of their tremendous penetrating ability, neutrons can be very damaging to the human body, a fact well known by the U.S. military, which is developing a bomb designed to kill people (but preserve property) by emitting large quantities of lethal neutron fragments. When neutrons strike atoms of elements that are not fissionable, they can render them radioactive by changing their atomic structure. For example, in a building near a neutron bomb explosion, the neutrons can change stable cobalt in the steel girders to cobalt 60, an emitter of highly penetrating gamma radiation.
- Gamma radiation is a form of electromagnetic or wave energy similar in some respects to X rays, radio waves, and light. Like X rays, gamma radiation is highly energetic and can penetrate matter much more easily than alpha or beta particles. Gamma rays are usually emitted from the nucleus when it undergoes transformations. An inch of lead or iron, eight inches of heavy concrete, or three feet of sod may be required to stop most of the gamma rays from an intense source.
- X rays are produced whenever high-energy electrons are accelerated or decelerated as they penetrate matter. X rays are produced by machine when electrons are accelerated to extremely high speed and are then crashed into a solid target. They are also produced in nuclear fission when electrons are accelerated out of the fissioning nucleus and are then slowed down by air and other materials. The energy released in the collision is a form of electromagnetic radiation, and is comparable in penetrating power to gamma rays. Because X rays can expose film after passing through some substances--such as human flesh and some building materials--they have been widely used in medicine and some industrial processes.
It is believed by many that because they are directly applied to the human body, medical X rays are at present the single greatest source of external exposure to human-made radiation. But unlike radioactive products that can escape into the environment and concentrate in the food chain, X-ray exposure can be controlled more easily than the fallout from a nuclear bomb or power plant.
Radiation and Human Health
Radiation attacks the human body at its most basic level--the cell structure.
Cells carry out the vital functions necessary to sustain and develop all living creatures. Over ten trillion cells make up the human body. The cell takes in food, gets rid of wastes, produces protein vital to life, and reproduces itself. Just as all living things are made up of cells, so every new cell is produced from another cell.
The nature of the cell is determined by the genetic material in its nucleus. Enormously complex, and not fully understood as yet, the genetic "coding" in each nucleus is carried by a complex protein called DNA--deoxyribonucleic acid. This DNA is tightly coiled in the forty-six chromosomes, which are stored in the cell nucleus. Surrounding the nucleus is the cytoplasm, the "factory" that carries out the directions of the DNA intelligence center. The cytoplasm in turn is contained by a semipermeable membrane, the cell wall. It is the whole of this cell mechanism--cell wall, cytoplasm, and nucleus--that forms the basis of human life.
When a radioactive particle or ray strikes a cell, one of at least four things can happen:
- It may pass through the cell without doing any damage;
- It may damage the cell, but in a way that the cell can recover and repair itself before it divides;
- It may kill the cell;
- Or, worst of all, it may damage the cell in such a way that the damage is repeated when the cell divides.
Three of those four circumstances can have health effects. The issue of what happens to a cell once it repairs itself, for example, is the subject of scientific debate. Dr. Alice Stewart has compared the radiation-damaged cell to a broken plate. Though the plate can be glued together again, its original integrity will never be the same. Every time it is stressed, it can be more prone to break. The repaired cell may not react to disease or physical injury as well as an undamaged cell; when it reproduces, this defect may be passed on.
Cell killing can also be harmful. Thousands of dead cells are eliminated from the human body every day, and thus the body has a certain tolerance for it when radiation adds to the natural toll. In fact radiation is used in some forms of therapy to kill cancerous cells, to prevent their reproducing. But if enough cells are killed by radiation, it can seriously impair bodily functions or cause blockages in the body's circulatory system.
The prime danger from radiation striking a cell, however, comes from the potential for damage to the DNA coding and the creation of cancerous cells. If the DNA is damaged by a ray or particle, it may reproduce itself in an abnormal manner that is, in essence, the basis of radiation-induced cancer. It is still not fully understood how radiation actually induces cancer or genetic damage in cells. Drs. John Gofman and Arthur Tamplin theorized in the early 1970s that when radiation damages a cell "a massive nonspecific disorganization" and destruction of chemical bonds occurs that is similar to "the effect of a jagged piece of shrapnel passing through a tissue."
Damage can occur to the cell wall, cytoplasm, and nucleus. It is most serious, however, when the DNA or genetic coding in the nucleus is harmed. Dr. Karl Z. Morgan has likened the disorganization by radiation of the cell DNA structure to a madman loose in a vast library, randomly tearing out pages of ancient, irreplaceable manuscripts. Once the DNA is damaged, distorted messages can be transmitted to the cell and passed on through reproduction. Thus thousands of mutated clone cells can reproduce themselves, forming the basis for tumors and a devastated bodily system. By the time a tumor can be seen or felt by the touch, it is composed of several million of these abnormal cells.
There has been considerable debate among radiobiologists about how often a cell must be hit by radiation to mutate into a cancer. Dr. E. B. Lewis in 1957 advanced the idea that it took just one "hit" to produce irreversible cell damage. Others believe it may take two or more. There is little dispute, however, over the fact that the cell is most vulnerable when it is dividing. The human fetus, infants, and young children--whose cells are multiplying most frequently--are thus the most sensitive to radiation damage; blood-forming organs such as the bone marrow are also particularly vulnerable.
Radiation can also damage the body's immune system and cause a general degeneration in the health of the cell structures. Thus radiation may cause illness and premature aging without actually bringing on the more easily isolated diseases of cancer and leukemia.
In recent years controversy has arisen over the particular vulnerability of infants in utero and small children to the ill-effects of radiation. Exposure of the fetus to radiation during all stages of pregnancy increases the chances of developing leukemia and childhood cancers. Because their cells are dividing so rapidly, and because there are relatively so few of them involved in the vital functions of the body in the early stages, embryos are most vulnerable to radiation in the first trimester--particularly in the first two weeks after conception. This period carries the highest risk of radiation-induced abortion and adverse changes in organ development. During this stage of development the tiny fetus can be fifteen times more sensitive to radiation-induced cancer than in its last trimester of development, and up to a thousand or more times more sensitive than an adult. In general it is believed that fetuses in the very early stages of development are most vulnerable to penetrating radiation such as X rays and gamma rays.
In all stages, they are vulnerable to emitting isotopes ingested by the mother. For example, if a pregnant mother inhales or ingests radioiodine, it can be carried through the placenta to the fetus, where it can lodge in the fetal thyroid and where its gamma and beta emissions can cause serious damage to the developing organ. Once the fetal thyroid is damaged, changes in the hormonal balance of the body may result in serious--possibly fatal--consequences for the development of the child through pregnancy, early childhood, and beyond. Such effects include underweight and premature birth, poorly developed lungs causing an inability to breathe upon delivery, mental retardation, and general ill-health.
Other emitters can lodge in other fetal organs. For example, yttrium-90, a decay product of strontium 90, can gravitate toward the pituitary gland. Overall, fetal irradiation during the second and third trimester has been linked to microcephaly (small head size), stunted growth and mental retardation, central nervous system defects, and behavioral changes. Exposure of the fetus to radiation during all stages of pregnancy increases the chances of developing leukemia and childhood cancers.
Young children also undergo more rapid cell division than adults, as do children in puberty. This rapid growth makes them very susceptible to radiation damage. Also at high risk are the elderly and chronically ill. These groups have weakened immune systems because of less active red bone marrow. Healthy immune systems can often isolate and remove damaged cells before malignancies develop. Older people generally have less vigorous immune systems; they have also generally experienced more radiation from both natural and human-made sources than young people, and thus may be more susceptible to additional exposure.
Women are also considered to be twice as sensitive to radiation as men because of their predominance in contracting breast and thyroid cancers.
Cancers shown to be initiated by radiation include leukemia, and cancers of the pancreas, lung, large intestine, thyroid, liver, and breast. Life-shortening anemia and other blood abnormalities, benign tumors, cataracts, and lowered fertility are other random effects attributed to radiation exposure.
The health effects of radiation with the greatest long-term implications are those centered on damage to the genes. Radiation is known to increase genetic mutations that can be passed on from generation to generation. Natural background radiation contributes some genetic mutations, and has been labeled by some as a factor in the evolutionary process. Some inherited mutations change a plant or animal so that it is better equipped to live in its surroundings.
But problems arise with artificially produced mutations. No mutation randomly produced by human-made radiation has been known to be beneficial. And mutations may not surface for generations. In 1972 the National Academy of Sciences Advisory Committee on the Biological Effects of Ionizing Radiation (BEIR committee) stated that "the spectrum of radiation-caused genetic disease is almost as wide as the spectrum from all causes." They added that "a genetic death may be the death of an embryo that no one ever knows about, or it may be the failure to reproduce. On the other hand, it may be a lingering and painful death in early adult life that causes great distress."
Based on the BEIR committee's assumptions of genetic risk from ionizing radiation, the risks to future generations can multiply enormously through time. If a single exposed radiation worker produces two children, who in turn have two children each, and so on through the generations, by the twentieth generation there may be as many as 2,097,152 human beings put at risk from the single exposed worker.
__________________________________________________________________ Sensitivity of Various Tissues to Cancer Induction by Radiation __________________________________________________________________ Relative Sensitivity Spontaneous to Radiation Site or Type Incidence Induction of Cancer of Cancer of Cancer Remarks __________________________________________________________________ Major radiation-induced cancers Female breast Very high High Puberty increases sensitivity Thyroid Low Very high, Low mortality rate especially females Lung (bronchus) Very high Moderate Quantitative effect of smoking uncertain Leukemia Moderate Very high Especially myeloid leukemia Alimentary tract High Moderate Occurs especially in colon to low Minor radiation-induced cancers Pharynx Low Moderate - Liver and biliary Low Moderate - tract Pancreas Moderate Moderate - __________________________________________________________________ Lymphomas Moderate Moderate Lymphosarcoma and multiple myeloma, but not Hodgkin's disease Kidney and bladder Moderate Low - Brain and nervous Low Low - system Salivary glands Very low Low - Bone Very low Low - Skin High Low Low mortality. High dose necessary? Sites or tissues in which magnitude of radiation-induced cancer is uncertain Larynx Moderate Low - Nasal sinuses Very low Low - Parathyroid Very low Low - Ovary Moderate Low - Connective tissues Very low Low - Sites or tissues in which radiation-induced cancer has not been observed Prostate Very high Absent? - Uterus and cervix Very high Absent? - Testis Low Absent? - Mesentery and Very low Absent? - mesothelium Chronic lymphatic Low Absent? - leukemia __________________________________________________________________
Source: 1980 BEIR Report
High- and Low-Level Radiation
Growing controversy has focused on what levels of radiation exposure are capable of doing the most harm. It has long been assumed that the most serious harm came from high-level exposures, such as those produced by the flash of the explosions at Hiroshima and Nagasaki, or those endured by scientists killed at the Los Alamos Laboratory while experimenting with primitive fission reactions. One of the most serious effects of high-level exposure to the body is the destruction of the red bone marrow. Once this occurs, a person's ability to resist infection is seriously compromised and can lead to chronic illness and early death. Other high-dose effects include skin burns, cataracts, loss of hair, loss of appetite, nausea, vomiting, sterility, and fatigue.
It now appears that constant exposure to small doses of radiation may also be extremely dangerous. A 1972 study by Dr. Abram Petkau found that prolonged exposures of low-dose radiation could do more damage to cell membranes than short flashes of intense doses. This insight, along with studies of fetal irradiation over long periods of time, has lent weight to a body of evidence indicating that such doses may be causing unexpected disease among far more people than previously believed.
Some of the units used to measure radiation are curies, rads, and rems.
The curie (Ci) is so named to honor Marie Curie, who discovered radium. A curie refers to the amount of radioactivity in a gram of radium: twenty-seven billion disintegrations per second. There are many billion curies of radioactivity in an atomic reactor. The curie is often broken down into smaller units, with one curie equaling one thousand millicuries (mCi), one million microcuries (uCi), or one trillion pico-curies (pCi). The curie is a measurement of gross radioactivity and does not refer to biological damage.
The rad (radiation absorbed dose) measures the amount of radiation absorbed by body tissues. Rads usually describe doses from both external penetrating radiation and from radionuclides contained within the body, but do not measure specific biological damage. A rad to the hand, for example, is not considered as dangerous as a rad distributed over the whole body.
The rem (radiation equivalent man) is currently considered the most appropriate for measuring biological damage from radiation. It reflects the fact that some forms of radiation create more damage in a given exposure than others. The rem is calculated by multiplying the rad dose by modifying factors calculated on considerations of ionization and radiosensitivity of the tissues involved.
In terms of measuring X and gamma rays, the rad and the rem are the same relative to their biological damage potential. But alpha- and beta-emitters do much more biological damage when they are taken inside the body and lodged in sensitive tissues than when there is exposure just from outside the body. The rem measurement factors in these differences.
Because the units are too large for many uses, the prefix milli (m) is often used with roentgens, rads, and rems to signify smaller quantities. One rad or rem equals one thousand millirads (mrad) or one thousand millirems (mrems).
When radiation doses are measured for large populations, the unit person rem is used. This is calculated by multiplying the total number of people exposed times their average dose in rems. Or it can be the actual sum of all the doses they receive. For example, ten thousand person rems is a dose received by five thousand people exposed to two rems each; or by ten thousand people exposed to one rem each. According to Dr. John Gofman, at least one death will result for every 300 person-rem dose. The nuclear industry says some 2000 person rems escaped at Three Mile Island.
Fission and Fusion
Radioactive rays and fallout from atomic weapons and power plants are created in the nuclear fission process, adding to global radiation levels. In fission a heavy radioactive element--usually uranium 235 or plutonium 239--is struck with a slow-moving neutron. The neutron trips a reaction within the fissionable nuclei that causes them to split apart, releasing large quantities of energy, radioactive particles, and large numbers of fission by-products--radioactive isotopes of different elements.
When there is a sufficient quantity of fissionable material present--called a critical mass--a chain reaction occurs. Here atoms are struck by particles and energy from other fissioning atoms, leading to more releases and collisions until a self-generating explosive situation is created. In a bomb it is this explosive power that is used to inflict damage. In a reactor the power is modified by "control rods"--usually made of boron--which absorb some of the particles and energy and allow reactor operators to manipulate the speed of the chain reaction. The energy from the fissioning core is then converted to steam, by the circulation of water through it, which is then used to turn turbines to generate electricity.
In the course of the reaction, nuclear fission creates a wide array of radioactive by-products emitting gamma rays, alpha, beta, and neutron radiation--plus several hundred radioisotopes created by the fission process. These include fission products such as cobalt 60, strontium 90, iodine 131, xenon 133, cesium 137, and plutonium 239.
The other principal human-made source of atomic energy is nuclear fusion, which is in a sense the opposite of fission. In fusion, light atoms such as hydrogen are brought together under conditions of enormous heat and pressure. Hydrogen atoms then "fuse" together into helium. But in the process additional mass is lost--the helium atom weighs less than the hydrogen atoms that created it--and this excessive mass is released as energy.
Fusion is the process by which the sun creates heat and light. It is also the basis of the hydrogen bomb. The federal government is actively pursuing the use of fusion energy to produce electricity. But the process is not entirely "clean." Radioactive tritium is one by-product of fusion; so are large quantities of neutrons which could render a fusion-reactor building highly radioactive after a short period of time.
In nuclear weaponry a fission explosion is required to create the conditions under which a fusion--hydrogen--explosion can take place. Thus the plutonium "triggers" built at Rocky Flats serve as the basis of hydrogen weaponry. This weaponry is sometimes called "thermonuclear" because of the huge amounts of heat involved.
Other Sources of Ionizing Radiation
It is generally acknowledged that the amount of radioactive bomb debris now lacing the air flow of the earth's atmosphere is in the tens of tons. Scientists now estimate that everyone living in the Northern Hemisphere carries some fallout debris--including plutonium--in their bodies. New York City residents, for example, eat plutonium in their bread every day.
It is also true that each of us is exposed every day to certain quantities of background radiation that are naturally produced. Some of this comes from cosmic radiation from outer space. Two forms of this radiation are speeding protons and neutrons, which enter the earth's atmosphere and collide with the air we breathe. Carbon 14, which can cause long-term biological damage including genetic mutations, is created when a cosmic neutron collides with nitrogen in the atmosphere. People who live in higher altitudes generally receive more cosmic radiation than those in the lowlands because there is less protective atmosphere for shielding.
Radiation exposures also result from the natural radioactivity in many of the earth's minerals. There are some extreme examples: thorium-bearing sands in Kerala, India, and soils in Brazil measure as much as twenty times above average background levels. Isotopes present in the body, such as potassium 40 and radium 226, also contribute to background levels.
Overall, background radiation levels in the United States are estimated to range from 100 to 150 millirem per year. These amounts are not harmless; background radiation is generally acknowledged to cause thousands of cancer deaths every year and even more genetic mutations in the United States, and far more globally.
Human-made radiation can add to that toll. Fallout from nuclear weapons testing, atomic reactor emissions, the mining and milling of uranium, the creation and storage of nuclear wastes, the transportation and use of radioactive materials in industry, and the exposure of millions of people to medical X rays all have their costs in terms of human health.
In recent years knowledge that radiation tends to concentrate through the food chain far more intensively than previously believed has contributed to growing fears of human-made ionizing radiation. That, in turn, has coupled with a basic acknowledgment on the part of the global medical community that the human body is far more sensitive to radiation than previously believed. As the 1980 edition of the Encyclopaedia Britannica notes, "it can be concluded that there is no `safe' level of radiation exposure, and no dose set so low that the risk is zero."
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