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Radiation hormesis (also called Radiation homeostasis) is the hypothesis that chronic low doses of ionizing radiation are beneficial, stimulating repair mechanisms that protect against disease. The Académie des Sciences - Académie nationale de Médecine (French Academy of Sciences - National Academy of Medicine) stated in their 2005 report concerning the effects of low level radiation that many laboratory studies have observed radiation hormesis. However, they cautioned that it is not yet known if radiation hormesis occurs outside the laboratory, in humans.

Consensus reports by the United States National Research Council and the National Council on Radiation Protection and Measurements and the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) have upheld the Linear no-threshold model (LNT), concluding that radiation is dangerous no matter how low the exposure and discounting the existence of radiation hormesis in humans.

Studies of Low Level Radiation

Studies have suggested that pre-exposure to radiation exerts a protective effect upon cells and whole animals. One such study was conducted on mice and found that a 200 mGy X-ray dose protects mice against both further X-ray exposure and ozone gas. Another rodent study found that (1 mGy hr) gamma irradiation prevents the development of cancer (induced by chemical means, injection of methylcholanthrene). Irradiation with gamma rays increases the concentration of glutathione, an antioxidant found in cells.

While it is clear that a large single exposure to plutonium dioxide powder is able to cause a fatal lung cancer in monkeys (and thus it is likely that PuO₂ powder is carcinogenic in humans), some studies have shown that moderate internal exposure to plutonium results in a reduction of the risk of getting cancer,.

Cancer studies on homeowners exposed to radon have at times reported results consistent with a small hormesis effect.

In a 2006 paper a dose of 1 Gy was delivered to the cells (at constant rate from a radioactive source) over a series of lengths of time. These were between 8.77 and 87.7 hours, the abstract states for a dose delivered over 35 hours or more (low dose rate) no transformation of the cells occurred. Also for the 1 Gy dose delivered over 8.77 to 18.3 hours that the biological effect (neoplastic transformation) was about 1.5 times smaller than that which that had been observed using a single high dose rate of X-ray photons of similar energy. Likewise it has been reported that fractionation of gamma irradiation reduces the likelihood of a neoplastic transformation. Pre-exposure to fast neutrons and gamma rays from Cs-137 is reported to increase the ability of a second dose to induce a neoplastic transformation.

Rejection by Leading Nuclear Bodies

Radiation hormesis has been rejected by both the United States National Research Council and the National Council on Radiation Protection and Measurements. In addition, the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) wrote in its most recent report:

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".

Ongoing Debate

Proponents of radiation hormesis accept that high radiation levels are harmful; that intense artificial radiation, for example, is toxic. But they believe that low levels of radiation, comparable to the natural background level of radiation are not harmful. The subject of radiation hormesis has captured the attention of scientists and public alike in recent years, perhaps because of its counter-intuitive properties. Opinion pieces on chemical and radiobiological hormesis appeared in the journals Nature and Science in 2003.

While most major studies have upheld the Linear no-threshold model (LNT) and rejected the existence of radiation hormesis in humans, according to the 2005 French Academy of Sciences-National Academy of Medicine's report concerning the effects of low level radiation, acknowledge that 40% of laboratory studies on cell cultures and animals observed radiobiological hormesis, and they state:

"...its existence in the laboratory is beyond question and its mechanism of action appears well understood."

In their 2005 report, the French Academy of Sciences-National Academy of Medicine outlined the growing body of research that illustrates that the human body is not a passive accumulator of radiation damage but it actively repairs the damage caused via number of different processes, including:

• Mechanisms that mitigate reactive oxygen species generated by ionising radiation and oxidative stress.
• Apoptosis of radiation damaged cells that may undergo tumorigenesis is initiated at only few mSv.
• Cell death during meiosis of radiation damaged cells that were unsuccessfully repaired.
• The existence of a cellular signaling system that alerts neighboring cells of cellular damage.
• The activation of enzymatic DNA repair mechanisms around 10 mSv.
• Modern DNA microarray studies which show that numerous genes are activated at radiation doses well below the level that mutagenesis is detected.
• Radiation induced tumorigenesis may have a threshold related to damage density, as revealed by experiments that employ blocking grids to thinly distribute radiation.
• A large increase in tumours in immunosuppressed individuals illustrates that the immune system efficiently destroys aberrant cells and nascent tumors.

However, French Academy of Sciences-National Academy of Medicine cautioned that it is not known, based on laboratory studies, if radiation hormesis occurs in the general population and they rejected its use in the assessment of radiation risk. Instead, they rejected both Radiation Hormesis and the popular LNT model, and instead proposed that below a certain threshold, low to moderate levels of natural radiation poses no risk to human health.

Most epidemiological studies on populations of people have upheld LNT and therefore reject radiation hormesis. Epidemiological studies at such low levels of radiation are very difficult do carry out due to confounding factors. For example, cultural factors such as smoking rates could cause huge problems in estimating excess cancer rates. Radon gas in homes is the largest source of radiation dose for most individuals and it is generally advised that the dose be kept below 150 Bq/m^3 (4 pCi/L). However, a recent retrospective case-control study of lung cancer risk showed substantial risk reduction between 50 and 123 Bq per cubic meter relative to a group at zero to 25 Bq per cubic meter. However, a single study cannot be regarded as definitive unless later studies reproduce the same results. Additionally, all other studies into the effects of domestic radon exposure have failed to detect a hormetic effect; including for example the respected "Iowa Radon Lung Cancer Study" of Field et al. (2000), which also used sophisticated radon exposure dosimetry.

Given the uncertain effects of low level radiation, there is a pressing need for quality research in this area. An expert panel convened at the 2006 Ultra-Low-Level Radiation Effects Summit at Carlsbad, New Mexico, proposed the construction of an Ultra-Low-Level Radiation laboratory. The laboratory, if built, will investigate the effects of almost no radiation on laboratory animals and cell cultures, and it will compare these groups to control groups exposed to natural radiation levels. The expert panel believes that the Ultra-Low-Level Radiation laboratory is the only experiment that can explore with authority and confidence the effects of low level radiation; that it can confirm or discard the various radiobiological effects proposed at low radiation levels e.g. LNT, threshold and radiation hormesis.

In popular treatments of radiation hormesis, a study of apartment buildings in Taiwan accidentally contaminated with Cobalt-60 has received prominent attention. This study has no scientific value, because it compares the irradiated population with the much older general population of Taiwan. A subsequent study by Hwang et al. (2006) found a significant exposure-dependent increase in cancer in the irradiated population, particularly leukemia in men and thyroid cancer in women, though this trend is only detected amongst those who were first exposed before the age of 30.

See also

• Hormesis
• Dose fractionation
• Electromagnetic therapy
• Linear no-threshold model
• Petkau effect
• Radioresistance

External links

• Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2
• Radiation Hormesis Overview by T. D. Luckey, who wrote a book on the subject (Luckey, T. D. (1991). Radiation Hormesis. Boca Raton, FL: CRC Press. ISBN 0-8493-6159-1)

References

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• Radiobiology
• Radiation health effects