Radioresistance is defined as the ability of some organisms to survive in environments where there is a high level of radioactivity. There may be naturally occurring radiation from uranium ores or man-made radiation from nuclear bombs or nuclear accidents. After the Chernobyl accident in Ukraine, scientists were surprised to find that many species survived when the assumption was that the high level of radiation should have killed most of them.
Research has shown that repeated exposure to small doses of radiation may stimulate biological changes that confer some resistance to larger doses of radiation. Biological self-repair mechanisms have time to repair damage if the doses of radiation are small and spread out over time. If a culture of cells is irradiated, set aside for a time and then exposed to another dose of radiation, fewer cells will die from the second exposure. Studies have shown this response in yeast, bacteria, protozoa, algae, plants and insects. Tests of cultures of mammal cells and human cells have also shown this effect. Suggested mechanisms are enhanced DNA repair, increased expression of some genes and increased levels of certain proteins in the nucleus and in the rest of the cell.
It has been suggested that radioresistance can be inherited genetically from parent organisms. Experiments with the common fruit fly confirmed this possibility. Some cancers are resistant to radiation treatment. It is unclear whether this resistance is an original property of the cancer cells themselves or may be induced by repeated exposures to radiation. When a cancer is irradiated, some of the cells will be killed. When those which survive reproduce, they may pass along any enhanced radiation resistance that they have to their offspring. The next time the cancer is exposed to radiation, more of the cells will survive and so on.
One way to compare the radioresistance of different organisms is with a comparison of what is called the median lethal dose. The median lethal dose is similar to the half-life of a radioactive material. When applied to radiation damage, It is the amount of radiation measured in Grays that will kill half of the organisms in a group in a specified period of time. Here is a list of the amount radiation that will kill half of the corresponding group in thirty days.
Dog – 3.5 Grays
Human – 4.5 Grays
Rat – 7.5 Grays
Rabbit – 8 Grays
Goldfish – 20 Grays
German cockroach- 64 Grays
Shellfish- 200 Grays
As the chart indicates, generally, the higher up the evolutionary ladder, the more susceptible to radiation the organisms are. Greater complexity involves greater vulnerability to radiation damage.
There is a theory called radiation hormesis that suggests that small doses of radiation just above the background level of radiation can stimulate the activation of biological protective biological mechanisms that could protect against some diseases. This theory is rejected by the U.S. National Council on Radiation Protection and Measurements and the United Nations Scientific Committee on the Effects of Atomic Radiation. These organizations hold to the linear no-threshold model which states that any exposure to any level of radiation can cause biological damage.
