Biological Radiation Effects

Biological Radiation Effects, effects observed when ionizing radiation interacts with living tissue by transferring energy to molecules of cellular matter. Cellular function may be temporarily or permanently impaired as a result of such interaction, or the cell may be destroyed. The severity of the injury depends on the type of radiation, the absorbed dose, the rate at which the dose was absorbed, and the radiosensitivity of the tissues involved. The effects are the same, whether from a radiation source outside the body or from material within.

The biological effects of a large dose of radiation delivered rapidly differ greatly from those of the same dose delivered slowly. The effects of rapid delivery are due to cell death, and they become apparent within hours, days, or weeks. Protracted exposure is better tolerated because some of the damage is repaired while the exposure continues, even if the total dose is relatively high. If the dose is sufficient to cause acute clinical effects, however, repair is less likely and may be slow even if it does occur. Exposure to doses of radiation too low to destroy cells can induce cellular changes that may be detectable clinically only after some years.

High whole-body doses of radiation produce a characteristic pattern of injury. Doses are measured in grays, 1 gray (Gy) being equal to an amount of radiation that releases 1 joule of energy per kilogram of matter. Doses of more than 40 Gy severely damage the human vascular system, causing cerebral oedema, which leads to profound shock and neurological disturbances; death occurs within 48 hours. Whole-body doses of 10 to 40 Gy cause less severe vascular damage, but they lead to a loss of fluids and electrolytes into the intercellular spaces and the gastrointestinal tract; death occurs within ten days as a result of fluid and electrolyte imbalance, severe bone-marrow damage, and terminal infection. Absorbed doses of 1.5 to 10 Gy cause destruction of human bone marrow, leading to infection and haemorrhage; death, if it occurs, can be expected about four to five weeks after exposure. Currently only the effects of these lower doses can be treated effectively; but if untreated, half the people receiving as little as 3 to 3.25 Gy to the bone marrow will die.

Exposure of small areas of the body—the most frequent kind of radiation accident—leads to localized tissue damage. Damage to the blood vessels in exposed areas causes disturbed organ function and, at higher doses, necrosis (localized tissue death) and gangrene.

Injury from internally deposited radiation sources is not likely to cause acute effects but, rather, delayed phenomena, depending on the target organ and on the half-life, radiation characteristics, and biochemical behaviour of the radiation source. Consequences may include degeneration or destruction of the irradiated tissue and the initiation of cancer.

Non-malignant delayed effects of ionizing radiation are manifested in many organs—particularly bone marrow, kidneys, lungs, and the lens of the eye—by degenerative changes and impaired function; these are largely secondary to radiation-induced damage to blood vessels. The most important late effect of radiation exposure, however, is an increased incidence of cancers and leukaemia of the types that occur naturally in unexposed individuals. Statistically significant increases in leukaemia and of cancers of the thyroid, the lung, and the female breast have been demonstrated unequivocally only in populations exposed to relatively high doses (greater than 1 Gy). Non-specific life-shortening effects suggested by animal experiments have not been demonstrated in humans as yet, however.

The radio-frequency radiation from sources such as power lines, radar, communications networks, and microwave ovens is non-ionizing, and for many years only high doses of such radiation were known to be harmful, causing burns, cataracts, temporary sterility, and other effects. In recent years, however, with the proliferation of such devices, the possible effects of long-term exposure to low levels of non-ionizing radiation began to be a matter of scientific concern. Subtle biological effects have been observed in some instances, but their health significance is thus far not certain in all cases. One authoritative, large-scale study organized by the International Agency for Research on Cancer (IARC) reported in January 2004 that the using of mobile phones does not increase the risk of brain cancer, at least during the first ten years of use. As the study continues it should reveal whether any risks increase with mobile phone-use of more than ten years.