Health effects from exposure to ionizing radiation

The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including:

• Type of radiation involved. All kinds of ionizing radiation can produce health effects. The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have. Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues.  
• Size of dose received. The higher the dose of radiation received, the higher the likelihood of health effects.
• Rate the dose is received. Tissue can receive larger dosages over a period of time. If the dosage occurs over a number of days or weeks, the results are often not as serious if a similar dose was received in a matter of minutes.
• Part of the body exposed. Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso. 
• The age of the individual. As a person ages, cell division slows and the body is less sensitive to the effects of ionizing radiation. Once cell division has slowed, the effects of radiation are somewhat less damaging than when cells were rapidly dividing.
• Biological differences. Some individuals are more sensitive to the effects of radiation than others. Studies have not been able to conclusively determine the differences.
• Stochastic Effects
  Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects. Stochastic effects often show up years after exposure. As the dose to an individual increases, the probability that cancer or a genetic effect will occur also increases. However, at no time, even for high doses, is it certain that cancer or genetic damage will result. Similarly, for stochastic effects, there is no threshold dose below which it is relatively certain that an adverse effect cannot occur. In addition, because stochastic effects can occur in individuals that have not been exposed to radiation above background levels, it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure.
  While it cannot be determined conclusively, it often possible to estimate the probability that radiation exposure will cause a stochastic effect. As mentioned previously, it is estimated that the probability of having a cancer in the US rises from 20% for non radiation workers to 21% for persons who work regularly with radiation. The probability for genetic defects is even less likely to increase for workers exposed to radiation. Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur.
  Radiation-induced hereditary effects have not been observed in human populations, yet they have been demonstrated in animals. If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation, hereditary effects could occur in the progeny of the individual. Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and, during certain periods in early pregnancy, may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high.
   
  Cancer
  Cancer is any malignant growth or tumor caused by abnormal and uncontrolled cell division.  Cancer may spread to other parts of the body through the lymphatic system or the blood stream. The carcinogenic effects of doses of 100 rads (1 Gy) or more of gamma radiation delivered at high dose rates are well documented, consistent and definitive.
  Although any organ or tissue may develop a tumor after overexposure to radiation, certain organs and tissues seem to be more sensitive in this respect than others. Radiation-induced cancer is observed most frequently in the hemopoietic system, in the thyroid, in the bone, and in the skin.  In all these cases, the tumor induction time in man is relatively long - on the order of 5 to 20 years after exposure.
  Carcinoma of the skin was the first type of malignancy that was associated with exposure to x-rays. Early x-ray workers, including physicists and physicians, had a much higher incidence of skin cancer than could be expected from random occurrences of this disease. Well over 100 cases of radiation induced skin cancer are documented in the literature. As early as 1900, a physician who had been using x-rays in his practice described the irritating effects of x-rays. He recorded that erythema and itching progressed to hyper-pigmentation, ulceration, neoplasia, and finally death from metastatic carcinoma. The entire disease process spanned a period of 9 years. Cancer of the fingers was an occupational disease common among dentists before the carcinogenic properties of x-rays were well understood. Dentists would hold the dental x-ray film in the mouths of patients while x-raying their teeth.
  
  Leukemia
  Leukemia is a cancer of the early blood-forming cells. Usually, the leukemia is a cancer of the white blood cells, but leukemia can involve other blood cell types as well. Leukemia starts in the bone marrow and then spreads to the blood. From there it can go to the lymph nodes, spleen, liver, central nervous system (the brain and spinal cord), testes (testicles), or other organs. Leukemia is among the most likely forms of malignancy resulting from overexposure to total body radiation. Chronic lymphocytic leukemia does not appear to be related to radiation exposure.
  Radiologists and other physicians who used x-rays in their practice before strict health physics practices were common showed a significantly higher rate of leukemia than did their colleagues who did not use radiation. Among American radiologists, the doses associated with the increased rate of leukemia were on the order of 100 rads (1 Gy) per year. With the increased practice of health physics, the difference in leukemia rate between radiologists and other physicians has been continually decreasing.
  Among the survivors of the nuclear bombings of Japan, there was a significantly greater incidence of leukemia among those who had been within 1500 meters of the hypocenter than among those who had been more than 1500 meters from ground zero at the time of the bombing. An increase in leukemia among the survivors was first seen about three years after the bombings, and the leukemia rate continued to increase until it peaked about four years later. Since this time, the rate has been steadily decreasing.
  The questions regarding the leukemogenicity of low radiation doses and of the existence of a non-zero threshold dose for leukemia induction remain unanswered, and are the subject of controversy. On the basis of a few limited studies, it was inferred that as little as 1-5 rads (10-50 mGy) of x-rays could lead to leukemia. Other studies imply that a threshold dose for radiogenic leukemia is significantly higher. However, it is reasonable to infer that low level radiation at doses associated with most diagnostic x-ray procedures, with occupational exposure within the recommended limits, and with natural radiation is a very weak leukemogen, and that the attributive risk of leukemia from low level radiation is probably very small.
  
  Genetic Effects
  Genetic information necessary for the production and functioning of a new organism is contained in the chromosomes of the germ cells - the sperm and the ovum. The normal human somatic cell contains 46 of these chromosomes; mature sperm and ovum each carry 23 chromosomes. When an ovum is fertilized by a sperm, the resulting cell, called a zygote, contains a full complement of 46 chromosomes. During the 9-month gestation period, the fertilized egg, by successive cellular division and differentiation, develops into a new individual. In the course of the cellular divisions, the chromosomes are exactly duplicated, so that cells in the body contain the same genetic information. The units of information in the chromosomes are called genes. Each gene is an enormously complex macromolecule called deoxyribonucleic acid (DNA), in which the genetic information is coded according to the sequence of certain molecular and sub-assemblies called bases. The DNA molecule consists of two long chains in a spiral double helix. The two long intertwined strands are held together by the bases, which form cross-links between the long strands in the same manner as the treads in a step-ladder.
  The genetic information can be altered by many different chemical and physical agents called mutagens, which disrupt the sequence of bases in a DNA molecule. If this information content of a somatic cell is scrambled, then its descendants may show some sort of an abnormality. If the information that is jumbled is in a germ cell that subsequently is fertilized, then the new individual may carry a genetic defect, or a mutation. Such a mutation is often called a point mutation, since it results from damage to one point on a gene. Most geneticists believe that the majority of such mutations in man are undesirable or harmful.
  In addition to point mutations, genetic damage can arise through chromosomal aberrations. Certain chemical and physical agents can cause chromosomes to break. In most of these breaks, the fragments reunite, and the only result may be a point mutation at the site of the original break. In a small fraction of breaks, however, the broken pieces do not reunite. When this happens, one of the broken fragments may be lost when the cell divides, and the daughter cell does not receive the genetic information contained in the lost fragment. The other possibility following chromosomal breakage, especially if two or more chromosomes are broken, is the interchange of the fragments among the broken chromosomes, and the production of aberrant chromosomes. Cells with such aberrant chromosomes usually have impaired reproductive capacity as well as other abnormalities.
  Studies suggest that the existence of a threshold dose for the genetic effects of radiation is unlikely. However, they also show that the genetic effects of radiation are inversely dependent on dose rate over the range of 800 mrad/min (8 mGy/min) to 90 rads/min (0.9 Gy/min). The dose rate dependence clearly implies a repair mechanism that is overwhelmed at the high dose rate. Geneticists estimate that there are 320 chances per million of a "spontaneous" mutation in a dominant gene trait of a person. The radiation dose that would eventually lead to a doubling of the mutation rate is estimated to be in the range of 50-250 rads (0.5-2.5 Gy).
  
  Cataracts
  A cataract is a clouding of the normally clear lens of the eye. A much higher incidence of cataracts was reported among physicists in cyclotron laboratories whose eyes had been exposed intermittently for long periods of time to relatively low radiation fields, as well as among atomic bomb survivors whose eyes had been exposed to a single high radiation dose. This shows that both chronic and acute overexposure of the eyes can lead to cataracts. Radiation may injure the cornea, conjunctiva, iris, and the lens of the eye. In the case of the lens, the principal site of damage is the proliferating cells of the anterior epithelium. This results in abnormal lens fibers, which eventually disintegrate to form an opaque area, or cataract, that prevents light from reaching the retina.
  The cataractogenic dose to the lens is on the order of 500 rad of beta or gamma radiation. No radiogenic cataracts resulting from occupational exposure to x-rays have been reported. From patients who suffered irradiation of the eye in the course of x-ray therapy and developed cataracts as a consequence, the cataractogenic threshold is estimated at about 200 rad. In cases either of occupationally or therapeutically induced radiation cataracts, a long latent period, on the order of several years, usually elapsed between the exposure and the appearance of the lens opacity. The cataractogenic dose has been found, in laboratory experiments with animals, to be a function of age; young animals are more sensitive than old animals.
  
  Nonstochastic (Acute) Effects
  Unlike stochastic effects, nonstochastic effects are characterized by a threshold dose below which they do not occur. In other words, nonstochastic effects have a clear relationship between the exposure and the effect. In addition, the magnitude of the effect is directly proportional to the size of the dose. Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time. These effects will often be evident within hours or days. Examples of nonstochastic effects include erythema (skin reddening), skin and tissue burns, cataract formation, sterility, radiation sickness and death. Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (i.e. acute vs. chronic exposure).
  There are a number of cases of radiation burns occurring to the hands or fingers. These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter. Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1,768 R/s. Contact with the source for two seconds would expose the hand of an individual to 3,536 rems, and this does not consider any additional whole body dosage received when approaching the source.
  More on Specific Nonstochastic Effects
  Hemopoietic Syndrome
  The hemopoietic syndrome encompasses the medical conditions that affect the blood. Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy). This disease is characterized by depression or ablation of the bone marrow, and the physiological consequences of this damage. The onset of the disease is rather sudden, and is heralded by nausea and vomiting within several hours after the overexposure occurred. Malaise and fatigue are felt by the victim, but the degree of malaise does not seem to be correlated with the size of the dose. Loss of hair (epilation), which is almost always seen, appears between the second and third week after the exposure. Death may occur within one to two months after exposure. The chief effects to be noted, of course, are in the bone marrow and in the blood. Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs. In this case, however, spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow. An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow.
  Gastrointestinal Syndrome
  The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines. This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater, and is a consequence of the desquamation of the intestinal epithelium. All the signs and symptoms of hemopoietic syndrome are seen, with the addition of severe nausea, vomiting, and diarrhea which begin very soon after exposure. Death within one to two weeks after exposure is the most likely outcome.
  Central Nervous System
  A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system, as well as all the other organ systems in the body. Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days. The rapidity of the onset of unconsciousness is directly related to the dose received. In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy), the victim was ataxic and disoriented within 30 seconds. In 10 minutes, he was unconscious and in shock. Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident.
  Other Acute Effects
  Several other immediate effects of acute overexposure should be noted. Because of its physical location, the skin is subject to more radiation exposure, especially in the case of low energy x-rays and beta rays, than most other tissues. An exposure of about 300 R (77 mC/kg) of low energy (in the diagnostic range) x-rays results in erythema. Higher doses may cause changes in pigmentation, loss of hair, blistering, cell death, and ulceration. Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century.
  The reproductive organs are particularly radiosensitive. A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men. For women, a 300 rad (3 Gy) dose to the ovaries produces temporary sterility. Higher doses increase the period of temporary sterility. In women, temporary sterility is evidenced by a cessation of menstruation for a period of one month or more, depending on the dose. Irregularities in the menstrual cycle, which suggest functional changes in the reproductive organs, may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization.
  The eyes too, are relatively radiosensitive. A local dose of several hundred rads can result in acute conjunctivitis.