By Jerry Cuttler, DSc, PEng
All organisms have very remarkable protection capabilities. While most people still think that living cells are structurally stable, scientists discovered in the 1980s that cells undergo a very high rate of naturally-occurring physical and chemical damage caused by the heat, oxygen and other chemicals in their bodies. Approximately 10,000 measurable endogenous DNA changes occur during every hour, in each cell of a mammal.
The reason that organisms can survive and even thrive in their surroundings, with many external challenges, is because they have many very powerful protection systems that are activated by the expression of more than 150 different genes. These systems prevent, repair, replace and remove cell and tissue damage, and they also resist all threats to the organism, from internal or external sources. Some of these systems act immediately against cell and tissue damage, and other protection systems adapt themselves to changes in the local environment.
How do x-rays and nuclear radiation affect organisms? About 100 (or fewer) measurable DNA changes occur per centigray (cGy) of radiation per cell. If we compare the amount of natural (endogenous) DNA damage that occurs in one year, which is ~ 8766 h/y x 10,000 = 90 million changes per cell, with the damage caused per cell by natural background radiation, which is about 0.24 cGy/year x 100 = 24 changes, we can see that the cell damage from background radiation is relatively negligible.
However, radiation has an important effect on the adaptive protection systems of all organisms. A low radiation dose causes stresses, which turn on genes at specific radiation dose thresholds. These genes increase the activities of protection systems, which produce more positive effects than would otherwise be observed. A high dose overstresses organisms, and genes are turned off at specific radiation dose levels. This inhibits the activities of protection systems. The result is the observation of harmful effects.
The threshold dose at which a gene turns on and the dose at which it turns off is different for each gene. The dose thresholds and the amounts of benefit and harm observed is characteristic for each organism. Overall, the response of an organism to an acute radiation dose or an increase in dose rate is generally described by a dose-response function that changes from low-dose benefit into high-dose harm, at a specific threshold called the NOAEL, no observed adverse effects level. The two figures below illustrate the NOAEL for a short-term radiation dose, ~ 50 rem (0.5 Gy), and for a lifetime dose rate, ~ 0.7 Gy per year.
Billen D. 1990. Commentary: Spontaneous DNA Damage and Its Significance for the “Negligible Dose” Controversy in Radiation Protection. Radiation Research 124: 242-245. Available at: http://radiationeffects.org/wp-content/uploads/2014/08/Billen-1990_SpontaneousDNAdamage- RadProt.pdf
Cuttler JM. 2014. Remedy for Radiation Fear—Discard the Politicized Science. Dose-Response 12(2): 170-184. Available at: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4036393/