Harmful biological effects of ionizing radiation

Ionizing radiation causes reversible physiological effects in its early stages, but, when prolonged, biological damage is produced as a result of metabolic processes.

Biological radiation damage may be acute or long-term. Acute damage occurs soon (within days or weeks) after exposure to high doses of radiation delivered over a short period of time, and may range from slight and temporary reddening of the skin to dramatic and often lethal syndromes which involve the major body systems. One fatal outcome may be determined by disturbances in the central nervous system, causing cessation of heart activity and paralysis of breathing. Pathological acute effects arise after exposure to doses hundreds of times higher than those likely to be received from environmental contamination, except in major accidents or nuclear warfare.

Continuing after-effects are characteristic long-term exposure to ionizing radiation, since the chain of biochemical and physiological reactions initiated with absorption of radiant energy continues for a long time. Long-term biological damage may be genetic or somatic. Genetic damage affects the germ cells of the irradiated individual, is transmitted to his or her descendants and may not appear for generations, eventually to result in hereditary diseases of various degrees of seriousness. Gene mutations may either be changes in single genes or gross chromosome anomalies which are due to loss, duplication, or rearrangement of chromosomes. Most of the known defects associated with such chromosome anomalies of the sex cells are so severe as to preclude survival of the embryos or reproduction of the children affected. Chronic, long-term exposure to non-lethal doses of ionizing radiation can also cause disturbance of sexual function and of sex-cell production to the point of complete sterility in irradiated organisms.

Somatic damage appears clinically in the irradiated individuals only years after exposure and consists largely of an increased frequency of malignancies (or cancers, mostly leukaemias and tumours of the thyroid and bone), generally decreased longevity, varying according to exposure. All forms of cell damage caused by radiation are indistinguishable from those that occur spontaneously in the general population.

While experiments have made it possible to describe the mutational effects of irradiation, they do not provide adequate evidence (that could be applied to man) regarding the manner or rate with which induced gene mutations would be eliminated from the population, or the proportion of mutations that would have serious consequences. It is not, therefore, possible to assess how many crippled or mentally defective individuals descended from irradiated persons would appear in any generation, and the total number summed over all generations is also highly uncertain. It is reasonably certain that a population which had been irradiated at an intensity sufficient to kill even a few percent of its members would suffer important long-term consequences.

The units of activity of a radioactive substance refers to how many atomic disintegrations are happening each second as it decays. However it tells very little about the biological effects of the substance, because it does not indicate whether the radiation it produces, when the atoms disintegrate, is in the form of alpha, beta or gamma radiation. Each of these affects the body differently.

Irradiated cells disrupt tissues, especially the mucous membranes of the stomach and intestines which leads to disturbance of digestion and absorption, emaciation, body poisoning with the products of cell decomposition (toxaemia), and the penetration of bacteria living in the intestines into the blood (bacteraemia). The lymphatic system is damaged, which leads to a decrease in the leukocytes in the peripheral blood and to a decline in its defensive properties. Manufacture of antibodies falls off, which further weakens the body's defensive forces. The number of red blood cells decreases, which causes disruption of the respiratory function of the blood.