A wide variety of reactions occur in response to irradiation in the different organs and tissues of the body.
Some reactions occur quickly, while others occur slowly: killing of cells in affected tissues may be detectable within minutes after exposure, whereas degenerative changes such as scarring and tissue breakdown may not appear until months or years afterward.
Dividing cells are more radiosensitive than nondividing cells.
Radiation injury tends to appear soonest in those organs and tissues in which cells proliferate rapidly: the skin, the lining of the gastrointestinal tract, and the bone marrow, where progenitor cells multiply continually in order to replace the mature cells that are constantly being lost through normal aging.
The early effects of radiation result largely from the destruction of the progenitor cells and the consequent interference with the replacement of the mature cells.
The damaging effects of radiation on an organ are generally limited to that part of the organ directly exposed.
Accordingly, irradiation of only a part of an organ generally causes less impairment in the function of the organ than does irradiation of the whole organ.
Radiation can cause injury to the skin, which depends on the dose and conditions of exposure.
The earliest reaction of the skin is transitory reddening of the exposed area, which may appear within hours after a dose of 6 Gy or more.
This erythematous reaction lasts only a few hours and is followed two to four weeks later by one or more waves of deeper and more prolonged reddening in the same area.
A larger dose may cause blistering and ulceration of the skin and loss of hair, followed by abnormal pigmentation months or years later.
The blood-forming cells of the bone marrow are among the most radiosensitive cells in the body.
If a large percentage of such cells are killed with intensive irradiation of the whole body, the normal replacement of circulating blood cells is impaired: the blood cell count may become depressed and, ultimately, infection, hemorrhage, or both may ensue.
A dose below 0.5–1 Sv generally causes only a mild, transitory depletion of blood-forming cells; however, a dose above 8 Sv delivered rapidly to the whole body usually causes a fatal depression of blood-cell formation.
The response of the gastrointestinal tract is comparable in many respects to that of the skin.
Proliferating cells in the mucous membrane of the gastrointestinal tract are easily killed by irradiation, resulting in the denudation and ulceration of the mucous membrane.
If a portion of the small intestine is exposed rapidly to a dose in excess of 10 Gy, as may occur in a radiation accident, a fatal dysentery-like reaction results within a very short period of time.
Mature spermatozoa are relatively resistant to radiation.
Immature sperm-forming cells are among the most radiosensitive cells in the body.
The rapid exposure of both testes to a dose as low as 0.15 Sv may interrupt sperm-production temporarily, and a dose in excess of 4 Sv may be sufficient to cause permanent sterility in a certain percentage of men.
Oocytes in the ovary of intermediate maturity are more radiosensitive than those of greater or lesser maturity.
A dose of 1.5–2.0 Sv delivered rapidly to both ovaries may thus cause only temporary sterility, whereas a dose exceeding 2–3 Sv is likely to cause permanent sterility in an appreciable percentage of women.
Irradiation can cause opacification of the lens, that increases with the dose.
The threshold for a progressive, vision-impairing opacity, or cataract, varies from 5 Sv delivered to the lens in a single exposure to as much as 14 Sv delivered in multiple exposures over a period of months.
CNS wise people do not sense a moderate radiation field.
Small doses of radiation (less than 0.01 Gy) can produce phosphene, a light sensation on the dark-adapted retina: American astronauts observed irregular light flashes and streaks during their flight, which probably resulted from single heavy cosmic-ray particles striking the retina.
The mature brain and nervous system are relatively resistant to radiation injury, but the developing brain is radiosensitive to damage .
The signs and symptoms resulting from intensive irradiation of a large portion of the bone marrow or gastrointestinal tract causes radiation sickness, or the acute radiation syndrome.
Early manifestations of radiation sickness include loss of appetite, nausea, and vomiting within the first few hours after irradiation, followed by a symptom-free interval that lasts until the main phase of the illness.
Symptoms of acute radiation sickness after exposure supralethal dose range.
several hoursno definite symptoms
nausea and vomiting
first weekdiarrhea, vomiting, inflammation of throat
second weekfever, rapid emaciation leading to death for 100 percent of the population
third week loss of hair begins
loss of appetite
general malaise
fever, hemorrhages, pallor leading to rapid emaciation and death for 50 percent of the population
The intestinal form of the radiation illness typically begins two to three days after irradiation, with abdominal pain, fever, and diarrhea.
It progresses rapidly in severity and lead within several days to dehydration, prostration, and a fatal, shocklike state.
The main phase of the hematopoietic form of the radiation exposure illness begins in the second or third week after irradiation, with fever, weakness, infection, and hemorrhage.
Death from overwhelming infection or hemorrhage may ensue four to six weeks after exposure unless corrected by transplantation of compatible unirradiated bone marrow cells.
The higher the dose received, the sooner and more profound are the radiation effects.
Following a single dose of more than 5 Gy to the whole body, survival is improbable.
A dose of 50 Gy or more to the head may cause the cerebral form of the acute radiation syndrome, with immediate and discernible effects on the central nervous system, followed by intermittent stupor and incoherence alternating with hyperexcitability, epileptiform seizures, and death within several days.
When the dose to the whole body is between 6 and 10 Gy, the earliest symptoms are loss of appetite, nausea, and vomiting, followed by prostration, watery and bloody diarrhea, abhorrence of food, and fever.
The bone marrow can be profoundly injured with radiation exposure, and the white blood cell count may decrease within 15–30 days from about 8,000 per cubic millimetre to as low as 200.
These radiation effects results in the body to lose its defenses against microbial infection, and the mucous membranes lining the gastrointestinal tract may become inflamed.
Internal or external bleeding may occur because of thrombocytopenia.
The development of early symptoms, frequently accompanied by delirium or coma, presage death.
Complete loss of hair within 10 days has been taken as an indication of a lethally severe exposure.
In the dose range of 1.5–5.0 Gy, survival is possible, thougand the symptoms appear as described above but in milder form and generally following some delay: nausea, vomiting, and malaise may begin on the first day and then disappear, and a latent period of relative well-being follows.
Pancytopenia Anemia sets in gradually, and three weeks, internal hemorrhages may occur in almost any part of the body, but particularly in mucous membranes.
Susceptibility to infection remains high, and some loss of hair occurs.
Lassitude, emaciation, and fever may persist for many weeks before recovery or death occurs.
Moderate doses of radiation can depress the immunologic defense mechanisms, resulting in enhanced sensitivity to bacterial toxins, greatly decreased fixation of antigens, and reduced efficiency of antibody formation.
Antibiotics are of limited effectiveness in combating postirradiation infections.
Below a dose of 1.5 Gy, an irradiated person is generally able to survive whole-body irradiation.
With a dose under 1 Gy, the symptoms may be so mild that the exposed person is able to continue a normal life, in spite of measurable depression of his bone marrow.
Some suffer subjective discomfort from doses as low as 0.3 Gy.
The tissues of the embryo are highly radiosensitive.
The types and frequencies of radiation effects depend on the stage of development of the embryo or fetus at the time it is exposed.
When radiation exposure occurs while an organ is forming, malformation of the organ may result.
Exposure earlier in embryonic life is more likely to kill the embryo than cause a congenital malformation, whereas exposure at a later stage is more likely to produce a functional abnormality in the offspring than a lethal effect or a malformation.
A wide variety of radiation-induced malformations have been observed in experimentally irradiated rodents.
Functional abnormalities produced in laboratory animals by prenatal irradiation include abnormal reflexes, restlessness, and hyperactivity, impaired learning ability, and susceptibility to externally induced seizures.
The abnormalities induced by radiation are similar to those that can be caused by certain virus infections, neurotropic drugs, pesticides, and mutagens.
Abnormalities of the nervous system, occurs in 1–2 percent of human infants, and were found with greater frequency among children born to women who were pregnant and residing in cities at the time of the atomic explosions.
The incidence of reduced head size and mental retardation in such children was increased by about 40 percent per Gy when exposure occurred between the eighth and 15th week of gestation.
The risk that a given dose will produce a particular malformation is smaller if the dose is spread out over many days or weeks than if it is received during the few hours of the critical period itself.
Effects on the incidence of cancer
Individuals exposed to radiation show dose-dependent increases in the incidence of certain types of cancer.
The induced cancers have not appeared until years after exposure.
The incidence of cancer has not been increased detectably by doses of less than 0.01 Sv.
The overall incidence of leukemia other than the chronic lymphatic type has increase roughly in proportion to dose during the first 25 years after irradiation.
Different types of leukemia, however, vary in the magnitude of the radiation-induced increase for a given dose, the age at which irradiation occurs, and the time after exposure.
The total excess of all types besides chronic lymphatic leukemia, averaged over all ages, amounts to approximately one to three additional cases of leukemia per year per 10,000 persons at risk per sievert to the bone marrow.
Cancer of the female breast is increased in incidence in proportion to the radiation dose.
The magnitude of the increase for a given dose appears to be essentially the same in women whose breasts were irradiated in a single, brief exposure as in those who were irradiated over a period of years.
Although the susceptibility of breast cancer induced by radiation decreases sharply with age at the time of irradiation.
The excess of breast cancer averaged over all ages amounts to three to six cases per 10,000 women per sievert each year.
The carcinogenic effects can be produced by a relatively small dose of radiation is noted by the increase in the incidence of thyroid tumors to result from a dose of 0.06–2.0 Gy of X rays delivered to the thyroid gland during infancy or childhood, and by the association between prenatal diagnostic X irradiation and childhood leukemia.
Exposure to as little as 10–50 mGy of X radiation during intrauterine development may increase the subsequent risk of leukemia in the exposed child by as much as 40–50 percent.
An approximate 0.6–1.8 cases per 1,000 persons per sievert per year when the whole body is exposed to radiation, beginning two to 10 years after irradiation, corresponding to a cumulative lifetime excess of roughly 20–100 additional cases of cancer per 1,000 persons per sievert, or to an 8–40 percent per sievert increase in the natural lifetime risk of cancer.
Estimated lifetime cancer risks attributed to low-level irradiation
site irradiated cancers per 10,000 person-Sv
bone marrow (leukemia) 15–20
thyroid 25–120
breast 40–200
lung 25–140
stomach 5–60
liver 5–60
colon 5–60
bone 5–60
esophagus 5–60
small intestine 5–30
urinary bladder 5–30
pancreas 5–30
lymphatic tissue 5–30
skin 10–20
The above-cited risk estimates imply that no more than 1–3 percent of all cancers in the general population result from natural background ionizing radiation.
Up to 20 percent of lung cancers in nonsmokers may be attributable to inhalation of radon and other naturally occurring radionuclides present in air.
Mortality from diseases other than cancer has not been increased detectably by irradiation among atomic-bomb survivors.
Bone-marrow cells administered soon after irradiation may enable an individual to survive an otherwise lethal dose of X rays.
The transplantation of bone-marrow cells has been helpful in preventing radiation deaths among the victims of reactor accidents.
Efforts are made to avoid unnecessary exposure to ionizing radiation in medicine, science, and industry.
Limits have been placed on the amounts of radioactivity and on the radiation doses that the different tissues of the body are permitted to accumulate in radiation workers or members of the public at large.