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.

Radiation hormesis has been rejected by both the United States National Research Council (part of the National Academy of Sciences)^[1] and the National Council on Radiation Protection and Measurements (a body commissioned by the U.S. Congress).^[2] In addition, the United Nations Scientific Committee on the Effects of Ionizing Radiation (UNSCEAR) wrote in its most recent report ^[3]

Until the [...] uncertainties on low-dose response are resolved, the Committee believes that an increase in the risk of tumour induction proportionate to the radiation dose is consistent with developing knowledge and that it remains, accordingly, the most scientifically defensible approximation of low-dose response. However, a strictly linear dose response should not be expected in all circumstances.

Contents 1 Rationale 2 Evidence for and against 3 Rejecting Radiation Hormesis 3.1 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.^[4]

[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.^{[citation needed]}
• Incidence of cancer is found to be low in high-lying areas which are less protected by the atmosphere against cosmic radiation.^{[citation needed]}
• Genes that protect against radiation damage have been found to be activated in people exposed to radiation in these areas.^{[citation needed]}
• Ramsar has naturally very high radiation (with an observed maximum of 260 mSv/year) due to its geology but is found to have no increased cancer risk.^[5]
• Lower than expected increases in cancers have been found from Chernobyl.^[6]
• No chromosomal damage was detectable in animals with high radiation counts living around the Chernobyl.^{[citation needed]}
• Some studies have suggested that pre-exposure to radiation exerts a protective effect upon cells.^[7]
• It has been shown that a 200 mGy X-ray dose protects mice against both further X-ray exposure and ozone gas.^[8]
• It has been shown that preexposure to radiation (50 to 100 mGy for four hours) results 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.^[9]
• Some studies have shown that moderate internal exposure to plutonium results in a reduction of the risk of cancer.^[10]
• Other studies have suggested that a small dose of radiation may be beneficial.^[11]

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).^[12] However there is a contrary suggestion and evidence that breast cancer in flight crews may be caused by jet lag.^[13]

[edit] Rejecting Radiation Hormesis

The notion of radiation hormesis has been rejected by the National Research Council's (part of the National Academy of Sciences) 16 year long study on the Biological Effects of Ionizing Radiation. "The scientific research base shows that there is no threshold of exposure below which low levels of ionizing radiation can be demonstrated to be harmless or beneficial. The health risks – particularly the development of solid cancers in organs – rise proportionally with exposure" says Richard R. Monson, associate dean for professional education and professor of epidemiology, Harvard School of Public Health, Boston [4]. See the National Acadamies Press book Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2.

The possibility that low doses of radiation may have beneficial effects (a phenomenon often referred to as “hormesis”) has been the subject of considerable debate. Evidence for hormetic effects was reviewed, with emphasis on material published since the 1990 BEIR V study on the health effects of exposure to low levels of ionizing radiation. Although examples of apparent stimulatory or protective effects can be found in cellular and animal biology, the preponderance of available experimental information does not support the contention that low levels of ionizing radiation have a beneficial effect. The mechanism of any such possible effect remains obscure. At this time, the assumption that any stimulatory hormetic effects from low doses of ionizing radiation will have a significant health benefit to humans that exceeds potential detrimental effects from radiation exposure at the same dose is unwarranted [5].

In chronic low-dose experiments with dogs (75 mGy/d for the duration of life), vital hematopoietic progenitors showed increased radioresistance along with renewed proliferative capacity (Seed and Kaspar 1992). Under the same conditions, a subset of animals showed an increased repair capacity as judged by the unscheduled DNA synthesis assay (Seed and Meyers 1993). Although one might interpret these observations as an adaptive effect at the cellular level, the exposed animal population experienced a high incidence of myeloid leukemia and related myeloproliferative disorders. The authors concluded that “the acquisition of radioresistance and associated repair functions under the strong selective and mutagenic pressure of chronic radiation is tied temporally and causally to leukemogenic transformation by the radiation exposure” (Seed and Kaspar 1992) [6]. See also Hormesis under "Non-acceptance".

[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. ^{[14][15][16][17]}

[edit] See also

• Hormesis
• Electromagnetic Therapy

[edit] External links

• Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2
• 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.

[edit] References

1. ^ http://books.nap.edu/catalog/11340.html Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2
2. ^ http://www.ncrppublications.org/index.cfm?fm=Product.AddToCart&pid=6714063164
3. ^ UNSCEAR 2000 REPORT Vol. II: Soucres and Effects of Ionizing Radiation: Annex G: Biological effects at low radiation doses. page 160, paragraph 541. Available online at [1].
4. ^ Luckey T (1999). "Nurture with ionizing radiation: a provocative hypothesis.". Nutr Cancer 34 (1): 1-11. PMID 10453435. 
5. ^ http://www.ecolo.org/documents/documents_in_english/ramsar-natural-radioactivity/ramsar.html
6. ^ http://www.tcsdaily.com/article.aspx?id=091905D
7. ^ Azzam, E. I.: Radiation Research, 1994, 138(1), S28-S31
8. ^ Miyachi, Y.: The British Journal of Radiology, 2000, 73, 298-304
9. ^ Cramers, P.; Atanasova, P.; Vrolijk, H.; Darroudi, F.; van Zeeland, A. A.; Huiskamp, R.; Mullenders, L. H.; Kleinjans, J.C.: [2])
10. ^ Kendall, G.M. et al.: Mortality and occupational exposure to radiation; First analysis of the National Registry for Radiation Workers. British Medical Journal, 1992; 304: 220.
11. ^ [3]
12. ^ http://news.bbc.co.uk/1/hi/health/380274.stm
13. ^ http://news.bbc.co.uk/1/hi/health/154933.stm
14. ^ Wahba, Z. Z.; Hernandez, L.; Issaq, H. J.; Waalkes, M. P. (1990): Involvement of sulfhydryl metabolism in tolerance to cadmium in testicular cells. Toxicology and Applied Pharmacology, 104:157-166.
15. ^ Waalkes, M. P.; Perantoni, A. (1986): Isolation of a novel metal-binding protein from rat testes: characterization and distinction from metallothionein. Journal of Biological Chemistry, 261:13079-13103.
16. ^ Waalkes, M. P.; Rehm, S.; Riggs, C.W.; 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 Research, 48:4656-4663.
17. ^ Rugstad, H. E.; Norseth, T. (1975): Cadmium resistance and content of cadmium-binding protein in cultured human cells. Nature, 257:136-137.