Radiation hormesis

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Radiation hormesis is the theory that ionizing radiation is benign at low levels of exposure, and that doses at the level of natural background radiation can be beneficial. This is in contrast to the linear no threshold model which posits that the negative health effects of ionizing radiation are proportional to the dose. A number of studies support the theory that low doses of radiation are benign, but other studies disagree. The disagreement arises partly because very low doses of radiation have relatively small impacts on individual health outcomes. It is therefore difficult to detect the 'signal' of decreased or increased morbidity and mortality due to low-level radiation exposure in the 'noise' of other effects.

1 Rationale
2 Evidence for and against
3 Mechanism
- 3.1 Preexposure to radiation with regards to acute health effects and other good effects
- 3.2 Cadmium poisoning as a model
4 See also
5 External links
6 References

[edit] Rationale

The theory is explained by the hypothesis that genes that repair damage due to radiation are activated and reduce damage from other causes, which would otherwise be imperfectly repaired. There is some evidence that radiation levels of 100 mSev/year may actually be positive or at least neutral to health. Indeed there have been claims that humans live in a subclinical deficiency of ionising radiation.^[1]

[edit] Evidence for and against

Evidence for:

Studies of airline crews are exposed to higher levels of cosmic radiation due to altitude but show no overall increase in cancer in spite of higher exposure
Incidence of cancer is found to be low in high-lying areas which are less protected by the atmosphere against cosmic radiation
Genes that protect against radiation damage have been found to be activated in people exposed to radiation in these areas
Ramsar has naturally very high radiation (260 mSv) due to its geology but is found to have no increased cancer risk^[2]
no chromosomal damage was detectable in animals with high radiation counts living around Chernobyl
lower than expected increases in cancers have been found from Chernobyl^[3]
In Taiwan, apartments built with radioactive rebar contaminated with cobalt 60 gave doses of an average of 400 mSv/year to the occupants. Mortality and cancers were considerably lower than in reference populations.^[4]

Evidence against:

Pilots are more prone to brain, rectal and prostate cancers whilst flight crews are twice as susceptible to breast cancer, but are healthier overall than the general public (possibly because they are healthier when selected for the job due to health screening).^[5] However there is a contrary suggestion and evidence that breast cancer in flight crews may be caused by jet lag^[6].
high-lying areas have reduced oxygen levels (oxygen is slightly carcinogenic); once this and other effects are accounted for there is actually an increase in cancer incidence with altitude which seems to be attributable to radiation.^[7]

[edit] Mechanism

[edit] Preexposure to radiation with regards to acute health effects and other good effects

Some studies have suggested that preexposure to radiation exerts a protective effect upon cells. Azzam, E.I., Radiation Research, 1994, 138(1), S28-S31. In mice it has been shown that a 200 mGy X-ray dose protects mice against both further X-ray exposure and ozone gas. Y Miyachi, The British Journal of Radiology, 2000, 73, 298-304.

It has been shown that preexposure to radiation (50 to 100 mGy) results in four hours time in a small reduction of the ability of an 8 Gy dose to damage DNA in intact cells due to a shift in the cell cycle (Cramers P; Atanasova P; Vrolijk H; Darroudi F; van Zeeland AA; Huiskamp R; Mullenders LH; Kleinjans JC [1])

Also some studies have shown that moderate internal exposure to plutonium results in a reduction of the risk of getting cancer, Kendall GM et al. Mortality and occupational exposure to radiation; First analysis of the National Registry for Radiation Workers. Brit Med Jour 1992; 304: 220. Other studied have suggested that a small dose of radiation may be good for you.[2]

[edit] Cadmium poisoning as a model

It is known that many toxic metals can induce oxidative stress in tissue which may result in free radical induced damage. Also it is known that prior exposure to a small dose of cadmium can mitigate the effects of a second larger dose, this suggests that the first lower dose of the poison stimulates the DNA repair processes in the exposed tissue.

Wahba, Z. Z., L. Hernandez, H. J. Issaq and M. P. Waalkes. 1990. Involvement of sulfhydryl metabolism in tolerance to cadmium in testicular cells. Toxicol. Appl. Pharmacol. 104:157-166.

Waalkes, M. P. and A. Perantoni. 1986. Isolation of a novel metal-binding protein from rat testes: characterization and distinction from metallothionein. J. Biol. Chem. 261:13079-13103.

Waalkes, M. P., S. Rehm, C. W. Riggs et al. 1988. Cadmium carcinogenesis in male Wistar (Crl:(WI)BR) rats: dose-response analysis of tumor induction in the prostate and testes, and at the injection site. Cancer Res. 48:4656-4663.

Rugstad, H. E. and T. Norseth. 1975. Cadmium resistance and content of cadmium-binding protein in cultured human cells. Nature 257:136-137.

[edit] See also

[edit] External links

Abstract of a study showing that radiation increases the rate of natural antioxidant (glutathione) production. The study found that this does not act as a radiation protection, but seems to protect against other cellular insults such as oxidation.
History of the idea by a supporter of the concept.