Cette page porte sur la radioactivité et ce qu’il est intéressant d’en savoir, en terme de connaissances de base. Le texte est toutefois en anglais, mais dès que je trouverai un équivalent français de la même qualité, je l’ajouterai ici. La page se divise ainsi:
– Radiation injuries: words to know
– For more information
These radiologists are measuring radioactivity levels in the soil near the Chernobyl nuclear plant, Ukrain.
Radiation injuries are damage to the body caused by ionizing radiation. Ionizing radiation (IR) is given off by the sun, X-ray machines, and radioactive elements.
Map of United States natural radioactivity.
source: United States Geological Survey
The word radiation comes from a Latin term that means “ray of light”. It is used in a general sense to cover all forms of energy that travel through space from one place to another as “rays”. Some forms of radiation are relatively harmless, like radio waves. Some forms of radiation carry a tremendous amount of energy and cause damage when they come into contact with other materials.
These high energy forms of radiation cause damage to substances by tearing apart the atoms and molecules that make up the substances. This may cause materials to undergo harmful changes. For example, an X ray that passes through water can tear the molecules of water apart. An X ray that passes through a living cell can also damage the cell by tearing apart the chemicals that make up the cell. The cell may be badly injured or killed.
Any form of radiation that can tear atoms and molecules apart is called ionizing radiation (IR). Damage to the body caused by IR is known as radiation injury. Ionizing radiation can come in the form electromagnetic waves or subatomic particles.
Radio and television signals, radar, heat, infrared and ultraviolet radiation, sunlight, starlight, gamma rays, cosmic rays, and X rays are all forms of electromagnetic radiation (ER). All forms of electromagnetic radiation travel in the form of waves at the speed of light (182,282 miles per second, 299,727 kilometers per second). Because ER travels in waves, its energy can be expressed in terms of wavelengths. Types of ER differ with regard to wavelength. The higher the energy wave, the shorter its wavelength. Types of ER also differ from one another with regard to their frequency. The frequency of a wave is the rate at which it vibrates in space.
X rays, gamma rays, and cosmic rays all have very high frequencies and short wavelengths. They vibrate very rapidly—many billions of times per second—in space. Radio and television signals and radar all have very low frequencies and long wavelengths. They vibrate quite slowly in space.
Waves that vibrate rapidly (have high frequencies) are carry more energy and can cause damage to substances by tearing apart the atoms and molecules that make up the substances.
Man and young boy operated for thyroid cancer after Chernobyl nuclear accident.
photo: Gerd Ludwig
source: National Geographic
Tissue found in the center of bones from which all types of blood cells are formed.
Electromagnetic radiation (ER):
Radiation that travels as waves at the speed of light.
The rate at which a wave vibrates in space.
A unit used to measure the amount of damage done to tissue by ionizing radiation.
Ionizing radiation (IR):
Any form of radiation that can break apart atoms and molecules and cause damage to materials.
An older unit used to measure the amount of damage done to tissue by ionizing radiation, now replaced by the gray.
Energy transmitted in the form of electromagnetic waves or subatomic particles.
An element that gives off some form of radiation and breaks down into a different element or a different form of the same element.
An older unit used to measure the amount of damage done to tissue by ionizing radiation, now replaced by the sievert.
A unit used to measure the amount of damage done to tissue by ionizing radiation.
Radioactive elements also give off forms of radiation similar to electro-magnetic radiation, but it is given off in sprays of subatomic particles. These particles may be produced intentionally in machines know as particle accelerators (atom-smashers) or they may be given off spontaneously by naturally occurring radioactive materials such as uranium 235 and radium 226. These forms of radiation can also cause damage to atoms and molecules.
There are two units used to measure the damage done to tissue by ionizing radiation. Those units were once called the rad and the rem. They have now been given new names, the gray (Gy) and the sievert (Sv). These units are very similar to, but not exactly the same as, each other.
The damage IR causes to a body can range from very mild to very severe. The damage depends on a number of factors, including the kind of radiation, how close the person is to the source of radiation, and how long the person was exposed to the radiation. In mild cases, a radiation injury may be no more serious than a mild sunburn. In the most serious cases, radiation injury can cause death within a matter of hours.
Humans are exposed to ionizing radiation from a variety of sources. These sources fall into four general categories: natural, intentional, accidental, and therapeutic. Natural sources include sunlight and cosmic radiation. Sunlight includes not only visible light, which has relatively few health effects, and radiation of higher frequency, such as ultraviolet radiation. Just stepping outdoors exposes a person to IR in sunlight.
Cosmic rays are similar to sunlight in that they are always present around us. They are not visible, but they do contain ionizing radiation. Exposure to natural sources of IR account for a very small fraction of radiation injuries.
Intentional exposure to IR is rare. It occurs when nuclear weapons (hydrogen and atomic bombs) are used as weapons of war. This has occurred only twice in history, when the United States dropped atomic bombs on Hiroshima and Nagasaki, Japan, at the end of World War II. Many thousands of people were killed or injured by these attacks. They are the only people ever to have been injured by intentional exposure to IR.
Accidental exposure occurs when a person is exposed to IR by mistake. For example, radioactive elements are sometimes spilled in a research laboratory. Workers in the lab may be exposed to the IR from those elements.
Accidental exposure to IR has caused a number of radiation injuries and deaths. Between 1945 and 1987, there were 285 nuclear reactor accidents worldwide. More than fifteen hundred people were injured and sixty-four were killed in these accidents.
Therapeutic exposure to IR occurs during various medical procedures. Radioactive elements and ionizing radiation have many valuable applications in diagnosing and treating disorders. But those treatments can have harmful as well as beneficial effects on patients. The rate of radiation injuries due to this cause probably cannot be measured. Many people who may have been injured by a radiation treatment probably died of the condition for which they were being treated.
France after Chernobyl.
source: Institut de Protection et de Sûreté Nucléaire (IPSN)
Radiation causes damage because it destroys chemicals in a cell. The cell loses its ability to function normally and dies.
Cells in tissues that are growing rapidly are more sensitive to radiation. For example, bone marrow cells in the center part of a bone are the fastest-growing cells in the body. They are the most sensitive of all body cells to IR. The cells of a fetus are also growing very rapidly. They are also at high risk for damage from IR.
The sensitivity of various types of cells is shown below. The dose given in each case is the lowest amount of radiation that cells in the tissue can absorb without being damaged:
* Fetus: 2 Gy
* Bone marrow: 2 Gy
* Ovaries: 2–3 Gy
* Lens of the eye: 5 Gy
* A child’s bone: 20 Gy
* An adult’s bone: 60 Gy
* A child’s muscle: 20–30 Gy
* An adult’s muscle: 100 or more Gy
A great deal of research was conducted on people who survived the atomic bomb explosions in Japan in 1945. From that research, we know what effect large doses of IR have on people. Those effects include:
* 1–2 Sv: Vomiting, loss of appetite, and generalized discomfort. These symptoms usually disappear in a short time.
* 2–6 Sv: Good chance for survival, provided the patient is given blood transfusions and antibiotics.
* 6–10 Sv: Massive destruction of bone marrow. If bone marrow is destroyed, the body cannot produce new blood cells. The patient usually dies in less than two months from infection or uncontrolled bleeding.
* 10–20 Sv: Destruction of intestinal tissue, causing serious digestive problems. The patient usually dies within three months from vomiting, diarrhea, infection, and starvation.
* More than 20 Sv: Massive damage to the nervous system and the circulatory (heart and blood vessels) system. The patient usually dies within a few days.
The most severe symptoms are very rare. They have been seen only in atomic bomb blasts and the most serious nuclear power plant accidents.
Far more commonly, doctors see symptoms of exposure to much lower levels of radiation. These symptoms most often appear in the form of cancer. Cancers (see cancer entry) develop when the number of cells damaged by IR gradually increases over time. Cells begin to grow out of control and spread throughout the body. Ionizing radiation is believed to be responsible for about 3 percent of all human cancers. The most common forms of cancer caused by IR are leukemia and cancers of the thyroid, brain, bone, breast, skin, stomach, and lungs (see breast cancer, leukemia, lung cancer, and skin cancer entries).
Patients who have received more than about 10 Sv of radiation are unlikely to survive. No treatment is available for people in this group.
Patients who receive very low doses of IR are most likely to develop some form of cancer. When the cancer has developed, it is treated by the techniques usually used for cancers, such as chemotherapy, radiation, and surgery.
Patients who are exposed to about 1 to 6 Sv can benefit from medical treatment. One step usually involves the use of antibiotics to protect the patient against infection. The patient may also require a blood transfusion. In some cases, superficial damage to the skin can be treated with surgery. The damaged portion of skin is removed and replaced with a skin graft.
There is much current interest in helpful chemicals called “free radical scavengers”. It is not yet known how they work, but studies strongly suggest that diets full of free radical scavengers are beneficial. Free radical scavengers are also called antioxidants and include beta-carotene, vitamins E and C, and selenium. Beta-carotene is present in yellow and orange fruits and vegetables. Vitamin C is found in citrus fruits such as oranges.
Traditional Chinese medicine, acupuncture, and herbal medicines may help in recovery from radiation injuries.
The prognosis for radiation injuries depends strongly on the amount of IR received by the patient. People who have been exposed to more than 10 Sv stand little or no chance of survival. People who have received a dose of 1 to 10 Sv may survive, provided they receive prompt treatment with antibiotics and blood transfusions, where needed. People who develop cancers as the result of low exposure to radiation have the same prognosis as those who develop the same cancers for other reasons.
There is no way to protect against radiation injuries caused by natural radiation. Some natural radiation reaches us even if we never leave our homes. Injuries caused by intentional exposure can be prevented, of course, by avoiding the use of nuclear weapons, such as atomic and hydrogen bombs.
Accidental exposure to radiation is difficult to avoid. Facilities where radiation is present, as in nuclear power plants, have developed safety measures to protect workers against exposure to IR. In most cases, these measures are very effective. However, it is impossible to prevent all accidents. When those accidents occur, some workers are likely to be exposed to IR and develop radiation injuries.
Exposure to IR during therapeutic procedures can always be avoided. A person can choose not to have the procedure, thereby avoiding exposure to the radiation. But in the vast majority of cases, the potential benefits of the procedure are greater than the potential risks. People choose to be treated with radiation because it is likely to help them get better or live longer. The careful use of equipment to protect healthy parts of the body is probably the best guarantee against radiation injuries due to therapeutic procedures.
Murphy, Jack, et al. Nuclear Medicine. New York: Chelsea House Publishers, 1993.
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