Radiation hormesis is the hypothesis 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 (LNT) model which posits that the negative health effects of ionizing radiation are proportional to the dose. Although a few papers have been published supporting the theory of radiation hormesis, scientific consensus has now developed against the theory.
Proponents advance the hypothesis that those genes that repair damage due to radiation are activated and reduce damage from other causes, which would otherwise be imperfectly repaired.
While most authorities agree that the LNT model is most appropriate, a small number of studies have proposed radiobiological hormesis, suggesting that radiation levels of 100 mSv/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. proposing that chronic exposure to low level radiation protects cells from radiation damage by promoting DNA repair machinery, resulting in improved fitness. Evidence supporting the hypothesis is drawn from in vitro tests (on samples of cells) and in some epidemiological studies.
These studies include:
Three studies found that chronic-exposure to low level radiation may exert a protective effect upon cells.
A study of cosmic radiation and DNA damage in cosmonauts, found that cosmonauts who flew on two or more missions had a level of DNA damage equal to people who never flew in space. The effects of microgravity or acquired resistance to cosmic radiation i.e. hormesis, was suggested.
One study found that a 200 mGy X-ray dose protects mice against both further X-ray exposure and ozone gas.
One study that found increased antioxidant defense activity in the kidneys of mice exposed to a full body exposure low level gamma radiation.
One study found 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.
One study found that moderate internal exposure to plutonium results in a reduction of the risk of cancer.
An article in the "Townsend Letter: The Examiner of Alternative Medicine" suggested that a small dose of radiation may be beneficial.
However, none showed this benefit to last for more than 24 hours, making the usefulness questionable.
Another question is the effect of prolonged exposure to radiation on health. Again, the vast preponderance of studies have upheld the linear no threshold theory, but epidemiological studies are very difficult, as an example, people with longer life-spans are more likely to get cancer, so those who are occupationally exposed have better health on average because they had a job--they are thus more likely to get cancer regardless of their exposure. Three examples include:
Ramsar has very high radiation from naturally occurring radium-226 (with an observed maximum of 260 mSv/year) due to its geology  but is found to have no increased cancer risk.
One study found that pilots, who are exposed to elevated levels of cosmic radiation, are more prone to brain, rectal and prostate cancers whilst flight crews are twice as susceptible to breast cancer, despite being healthier overall than the general public.
Professor Wade Allison, Department of Physics and Keble College, Oxford, notes in The safety of nuclear radiation; a careful re-examination for a world facing climate change:
“Nuclear radiation damages the DNA structure that records the form in which life propagates...However, the relation between radiation dose and damaged chromosome count is found to be non-linear as would be expected. Independent of their detail the existence of these repair mechanisms is at variance with the LNT characteristic.”
Cadmium poisoning is cited 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. 
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.
This is a reference to the fact that very low doses of radiation have only marginal 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.
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 . See the National Academies 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 .
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) . See also Hormesis under "Non-acceptance".
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^ http://books.nap.edu/catalog/11340.html Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2