Radioactive Pollution

The amount of injury caused by a radioactive isotope depends on its physical half-life, and on how quickly it is absorbed and then excreted by an organism. Most studies of the harmful effects of radiation have been performed on single-celled organisms. Obviously, the situation is more complex in humans and other multicellular organisms, because a single cell damaged by radiation may indirectly affect other cells in the individual. The most sensitive regions of the human body appear to be those which have many actively dividing cells, such as the skin, gonads, intestine, and tissues that grow blood cells (spleen, bone marrow, lymph organs).

Radioactivity is toxic because it forms ions when it reacts with biological molecules. These ions can form free radicals, which damage proteins, membranes, and nucleic acids. Radioactivity can damage DNA (deoxyribonucleic acid) by destroying individual bases (particularly thymine), by breaking single strands, by breaking double strands, by cross-linking different DNA strands, and by cross-linking DNA and proteins. Damage to DNA can lead to cancers, birth defects, and even death.

However, cells have biochemical repair systems which can reverse some of the damaging biological effects of low-level exposures to radioactivity. This allows the body to better tolerate radiation that is delivered at a low dose rate, such as over a longer period of time. In fact, all humans are exposed to radiation in extremely small doses throughout their life. The biological effects of such small doses over such a long time are almost impossible to measure, and are essentially unknown at present. There is, however, a theoretical possibility that the small amount of radioactivity released into the environment by normally operating nuclear power plants, and by previous atmospheric testing of nuclear weapons, has slightly increased the incidence of certain cancers in human populations. However, scientists have not been able to conclusively show that such an effect has actually occurred.

Currently, there is disagreement among scientists about whether there is a threshold dose for radiation damage to organisms. In other words, is there a dose of radiation below which there are no harmful biological effects? Some scientists maintain that there is no such threshold, and that radiation at any dose carries a finite risk of causing some biological damage. Furthermore, the damage caused by very low doses of radiation may be cumulative, or additive to the damage caused by other harmful agents to which humans are exposed. Other scientists maintain that there is a threshold dose for radiation damage. They believe that biological repair systems, which are presumably present in all cells, can fix the biological damage caused by extremely low doses of radiation. Thus, these scientists claim that the extremely low doses of radiation to which humans are commonly exposed are not harmful.

One of the most informative studies of the harmful effects of radiation is a long-term investigation of the survivors of the 1945 atomic blasts at Hiroshima and Nagasaki by James Neel and his colleagues. The survivors of these explosions had abnormally high rates of cancer, leukemia, and other diseases. However, there seemed to be no detectable effect on the occurrence of genetic defects in children of the survivors. The radiation dose needed to cause heritable defects in humans is higher than biologists originally expected.

Radioactive pollution is an important environmental problem. It could become much worse if extreme vigilance is not utilized in the handling and use of radioactive materials, and in the design and operation of nuclear power plants.