Radiation hormesis

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.Cite journal
doi = 10.1038/421691a
issn = 0028-0836
volume = 421
issue = 6924
pages = 691–692
last = Calabrese
first = Edward J
coauthors = Linda A Baldwin
title = Toxicology rethinks its central belief
journal = Nature
accessdate = 2008-04-01
date = 2003-02-13
url = http://www.nature.com/nature/journal/v421/n6924/full/421691a.html] [Cite journal
volume = 78
pages = 3–7
last = Feinendegen
first = L.E.
title = Evidence for beneficial low level radiation effects and radiation hormesis
journal = British Journal of Radiology
doi = 10.1259/bjr/63353075
date = 2005
pmid = 15673519] Cite journal
doi = 10.1126/science.302.5644.376
volume = 302
issue = 5644
pages = 376–379
last = Kaiser
first = Jocelyn
title = HORMESIS: Sipping From a Poisoned Chalice
journal = Science
accessdate = 2008-03-31
date = 2003-10-17
url = http://www.sciencemag.org/cgi/content/summary/302/5644/376
pmid = 14563981] [Cite journal
volume = 106
issue = 1
pages = 277–283
last = Wolff
first = S.
title = The adaptive response in radiobiology: evolving insights and implications
journal = Environmental Health Perspectives
url = http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=9539019
date = 1998-02
doi = 10.2307/3433927
pmid = 9539019]

The Académie des Sciences - Académie nationale de Médecine (French Academy of Sciences - National Academy of Medicine) in their 2005 report concerning the effects of low level radiation, acknowledged that many laboratory studies have observed radiation hormesis.Cite journal
doi = 10.1016/j.taap.2004.02.007
volume = 197
issue = 2
pages = 125–136
last = Calabrese
first = Edward J.
title = Hormesis: from marginalization to mainstream: A case for hormesis as the default dose-response model in risk assessment
journal = Toxicology and Applied Pharmacology
accessdate = 2008-04-01
date = 2004-06-01
url = http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6WXH-4C1T2FR-3-3&_cdi=7159&_user=10&_orig=search&_coverDate=06%2F01%2F2004&_sk=998029997&view=c&wchp=dGLbVzW-zSkWb&md5=ae4625e58b2c144985bb1f4204f22ca9&ie=/sdarticle.pdf] Cite journal
doi = 10.1504/IJLR.2003.003488
volume = 1
issue = 11
pages = 120–131
last = Duport
first = P.
title = A database of cancer induction by low-dose radiation in mammals: overview and initial observations
series = International Journal of Low Radiation
accessdate = 2008-04-01
date = 2003-09-11
url = http://www.ie.uottawa.ca/Shared/ADatabaseofCancerInduction.pdf
journal = International Journal of Low Radiation] However, they cautioned that it is not yet known if radiation hormesis occurs outside the laboratory, in humans.Cite paper
author = Aurengo "et al."
title = "Dose-effect relationships and estimation of the carcinogenic effects of low doses of ionizing radiation."
publisher = Académie des Sciences & Académie nationale de Médecine
date = 2005-30-03
url =http://www.radscihealth.org/rsh/Papers/FrenchAcadsFinal07_04_05.pdf
accessdate = 2008-03-27] The major 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.

Rejection by Leading Nuclear Bodies

Radiation hormesis has been rejected by both the United States National Research Council (part of the National Academy of Sciences) [http://books.nap.edu/catalog/11340.html Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2] and the National Council on Radiation Protection and Measurements (a body commissioned by the United States Congress). [ [http://www.ncrppublications.org/index.cfm?fm=Product.AddToCart&pid=6714063164 NCRP Store - Add Product to Cart ] ] In addition, the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) wrote in its most recent report: [ UNSCEAR 2000 REPORT Vol. II: Sources and Effects of Ionizing Radiation: Annex G: Biological effects at low radiation doses. page 160, paragraph 541. Available online at [http://www.unscear.org/docs/reports/annexg.pdf] . ]

Until the [...] uncertainties on low-dose response are resolved, the Committee believes that an increase in the riskof 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 [http://www8.nationalacademies.org/onpinews/newsitem.aspx?RecordID=11340] . See the National Academies Press book [http://books.nap.edu/catalog/11340.html 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" [http://books.nap.edu/openbook/030909156X/html/315.html] .

:"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)" [http://books.nap.edu/openbook/030909156X/html/333.html] .

See also Hormesis under "Non-acceptance".

Studies of Low Level Radiation

Many studies have suggested that pre-exposure to radiation exerts a protective effect upon cells [Azzam, E.I., "Radiation Research", 1994, 138(1), S28-S31] and whole animals [Kensuke Otsuka, Takao Koana, Hiroshi Tauchi and Kazuo Sakai, "Activation of Antioxidative Enzymes Induced by Low-Dose-Rate Whole-Body γ Irradiation: Adaptive Response in Terms of Initial DNA Damage", "Radiation Research", 2006, 166(3), 474-478] . 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. [Y Miyachi, "The British Journal of Radiology", 2000, 73, 298-304.] Another rodent study found that (1 mGy hr^-1) gamma irradiation prevents the development of cancer (induced by chemical means, injection of methylcholanthrene). [Sakai, Kazuo; Iwasaki, Toshiyasu; Hoshi, Yuko; Nomura, Takaharu; Oda, Takeshi; Fujita, Kazuko; Yamada, Takeshi; Tanooka, Hiroshi, "International Congress Series" (2002), 1236 (Radiation and Homoeostasis), 487-490.] Irradiation with gamma rays increases the concentration of glutathione (an antioxidant) found within cells, this is likely to lead to an adaptive response. [Sonia M. de Toledo, Nesrin Asaad, Perumal Venkatachalam, Ling Li, Roger W. Howell, Douglas R. Spitz and Edouard I. Azzam, "Radiation Research", 2006, 166(6), 849-857]

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), [Hahn, F.F. ; Brooks, A.L. ; Mewhinney, J.A., Radiation Research, 1987, 112(2), 391-397] 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] .

In plants radiation hormesis has been observed [Atkinson, G.F., 1898. Report upon some preliminary experiments with Roentgen rays in plants. Science, 7: 7.] However the existence of radiation hormesis in humans has been questioned, it is reasonable to state that for late effects (such as cancer) that the scientific community has not come to an agreement regarding this matter. [http://www-unix.oit.umass.edu/~nrephc/Calabrese%203.pdf]

Studies from the 1950s have shown that the rate of radiation damage to DNA is linear. The more radiation in the environment, the greater the damage. So any observed radiation hormesis would be the result of some incompletely understood repair mechanisms. Mainstream science is divided on the question, so there is no consensus on whether a radiation dose divided into smaller doses reduces the likelihood that the radiation will induce a cancer.

Cancer studies on homeowners exposed to radon have at times reported results consistent with a small hormesis effect [cite journal| title =Case-Control study of lung cancer risk from residential Radon exposure in Worchester County, Massachusetts | journal = Health Physics | volume= 94(3) | pages= 228-241|date= March 2008|author = Thompson, Richard E., Nelson, Donald F., Popkin, Joel H., Popkin, Zenaida] .

In a recent paper [Elmore, E.; Lao, X.-Y.; Kapadia, R.; Redpath, J. L., The effect of dose rate on radiation- induced neoplastic transformation "in vitro" by low doses of low-LET radiation, "Radiation Research", 2006, 166(6), 832-838] 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 stated 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 [C.K. Hill, A. Han, F. Buonaguro and M.M. Elkind, "Multifractionation Of Co-60 Gamma-Rays Reduces Neoplastic Transformation in vitro", Carcinogenesis, 1984, 5, 193] that fractionation of gamma irradiation reduces the likelihood of a neoplastic transformation. It is clear that the findings in these two papers do not agree with the hypothesis of the second event theory. But in a further paper [J. Cao, R.I. Wells and M.M. Elkind, "Enhanced Sensitivity To Neoplastic Transformation By Cs-137 Gamma-Rays Of Cells In The G2-/M-Phase Age Interval", "International Journal of Radiation Biology", 1992, 62, 191] it is reported that for both fast neutron and gamma rays from Cs-137 that preexposure can increase the ability of a second dose to induce a neoplastic transformation.

Ongoing Debate

Proponents of radiation hormesis accept that radiation levels far above natural background are harmful; that intense artificial radiation, for example, is toxic. But they believe that low levels of radiation, comparable to the natural background 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.

No studies have confirmed the Linear no-threshold model (LNT) in the low level regime, and according to the 2005 French Academy of Science-National Academy of Medicine's report concerning the effects of low level radiation (only they rejected LNT), 40% of laboratory studies on cell cultures and animals have observed radiobiological hormesis - "its existence in the laboratory is beyond question and its mechanism of action appears well understood." However, they cautioned that it is not yet known based on laboratory studies if radiation hormesis occurs in humans.

In their 2005 report, the French Academy of Sciences-National Academy of Medicine acknowledged 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 activation of enzymatic DNA repair mechanisms "c." 10 mSv.
* The existence of a cellular signaling system that alerts neighboring cells of cellular damage.
* 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 tumours.

Examples of studies that observed radioadaptive and hormetic effects include experiments on cells [Cite journal
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first = E.I.
coauthors = G.P. Raaphorst, R.E.J. Mitchel
year = 1994
title = Radiation-Induced Adaptive Response for Protection against Micronucleus Formation and Neoplastic Transformation in C3H 10T1/2 Mouse Embryo Cells
journal = Radiation Research
volume = 138
issue = 1
pages = S28–S31
url = http://links.jstor.org/sici?sici=0033-7587(199404)138%3A1%3CS28%3ARARFPA%3E2.0.CO%3B2-9
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last = Azzam
first = E.I.
coauthors = S.M. de Toledo, G.P. Raaphorst, R.E.J. Mitchel
year = 1996.
title = Low-Dose Ionizing Radiation Decreases the Frequency of Neoplastic Transformation to a Level below the Spontaneous Rate in C3H 10T1/2 Cells
journal = Radiation Research
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last = Ko
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coauthors = X-Y Liao, S Molloi, E Elmore, J L Redpath
title = Neoplastic transformation in vitro after exposure to low doses of mammographic-energy X rays: quantitative and mechanistic aspects
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pages = 117–122
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coauthors = LHF Mullenders, F Darroudi, AT Natarajan
title = Adaptive response to DNA and chromosomal damage induced by X-rays in human blood lymphocytes
journal = Mutagenesis
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volume = 250
issue = 1-2
pages = 299–306
last = Wolff
first = S
coauthors = R Jostes, F T Cross, T E Hui, V Afzal, J K Wiencke
title = Adaptive response of human lymphocytes for the repair of radon-induced chromosomal damage
journal = Mutation research
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last = Wolff
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title = Indications of repair of radon-induced chromosome damage in human lymphocytes: an adaptive response induced by low doses of X-rays
journal = Environmental Health Perspectives
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url = http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1521171
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last = Miyachi
first = Y
title = Acute mild hypothermia caused by a low dose of X-irradiation induces a protective effect against mid-lethal doses of X-rays, and a low level concentration of ozone may act as a radiomimetic
journal = British Journal of Radiology
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volume = 48
issue = 2
pages = 113–20
last = Pathak
first = Chander Mohan
coauthors = Pramod Kumar Avti, Surender Kumar, Krishan Lal Khanduja, Suresh Chander Sharma
title = Whole body exposure to low-dose gamma radiation promotes kidney antioxidant status in Balb/c mice
journal = Journal of radiation research
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date = 2007-03
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Another question is the effect of prolonged exposure to low level radiation on populations of people. Most epidemiological studies have upheld LNT and by default reject radiation hormesis, but epidemiological studies are very difficult do carry out due to compounding factors. For example, a town may have a lower cancer rate, not because of slightly elevated background radiation but because it has more new houses with young families who have a lower cancer rate. The few epidemiological studies that appear to refute LNT and suggest radiation hormesis include, most notably:

* Bernard Cohen's 1995 study of lung cancer rates vs. average radon concentration in homes for 1,601 U.S. counties, that questioned the validity LNT. [Cite journal
issn = 00179078
volume = 68
issue = 2
pages = 157–74
last = Cohen
first = B L
title = Test of the linear-no threshold theory of radiation carcinogenesis for inhaled radon decay products
journal = Health physics
date = 1995-02
url = http://www.health-physics.com/pt/re/healthphys/abstract.00004032-199502000-00002.htm;jsessionid=HnqK0WJNsyMT1Shpv55kn8PNZ68gpXTLfTxwyhsw29PL55N8t79y!1675702673!181195628!8091!-1]
* Thompson et al. (2008) 7 year long case-controlled study of residential radon exposure in Worcester County, Massachusetts, that found an apparent 60% "reduction" in lung cancer risk amongst people exposed to low levels (0-150 Bq/m³) of radon gas; levels typically encountered in 90% of American homes. [Cite journal
issn = 00179078
volume = 94
issue = 3
pages = 228–41
last = Thompson
first = Richard E
coauthors = Donald F Nelson, Joel H Popkin, Zenaida Popkin
title = Case-control study of lung cancer risk from residential radon exposure in Worcester county, Massachusetts
journal = Health physics
url = http://www.health-physics.com/pt/re/healthphys/abstract.00004032-200803000-00002.htm
date = 2008-03
doi = 10.1097/01.HP.0000288561.53790.5f
doi_brokendate = 2008-07-04]

Significantly, both studies reportedly found a very similar exposure/risk relationship curve that appears to match the predictions of radiation hormesis. [Raabe, Otto G. "Radon hormesis suggested by a careful case-controlledstudy" [http://lists.radlab.nl/pipermail/radsafe/2008-February/009450.html] Retrieved on 2008-03-30] Donald Nelson, co-author of Thompson et al. (2008), indicated that the hormetic effect was detected because of the studies application of improved radon exposure dosimetry. Physorg. Exposure to low levels of radon appears to reduce the risk of lung cancer, new study finds. [http://www.physorg.com/news125672761.html] Retrieved on 2008-03-30] However, a single study cannot be regarded as definitive unless later studies using the same methods of Thompson et al. (2008) 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. [Cite journal
volume = 151
issue = 11
pages = 1091–1102
last = Field
first = R. William
coauthors = Daniel J. Steck, Brian J. Smith, Christine P. Brus, Eileen L. Fisher, John S. Neuberger, Charles E. Platz, Robert A. Robinson, Robert F. Woolson, Charles F. Lynch
title = Residential Radon Gas Exposure and Lung Cancer: The Iowa Radon Lung Cancer Study
journal = Am. J. Epidemiol.
accessdate = 2008-04-03
date = 2000-06-01
url = http://aje.oxfordjournals.org/cgi/content/abstract/151/11/1091
pmid = 10873134 studies ]

Given the uncertain effects of low level radiation, there is a pressing need for quality research in this area. ["Ultra-Low-Level Radiation Effects Summit." January 2006. ORION International Technologies, Inc. (ORION) and sponsored by the U.S. Department of Energy’s Waste Isolation Pilot Plant (WIPP) 03 Apr. 2008. [http://www.orionint.com/projects/ullre.cfm] ] 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. ["Ultra-Low-Level Radiation Effects Summit Report." January 2006. ORION International Technologies, Inc. (ORION) and sponsored by the U.S. Department of Energy’s Waste Isolation Pilot Plant (WIPP) 03 Apr. 2008. [http://www.orionint.com/ullre/report-2006.pdf] ] 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. ["Ultra-Low-Level Radiation Effects Summit - Report Summary." January 2006. ORION International Technologies, Inc. (ORION) and sponsored by the U.S. Department of Energy’s Waste Isolation Pilot Plant (WIPP) 03 Apr. 2008. [http://www.orionint.com/ullre/summary-2006.pdf] ] 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.

Cadmium poisoning is cited as a similar 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.; 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.] [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.] [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.] [Rugstad, H. E.; Norseth, T. (1975): "Cadmium resistance and content of cadmium-binding protein in cultured human cells." Nature, 257:136-137.]

ee also

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

External links

* [http://library.lanl.gov/cgi-bin/getfile?23-03.pdf" Radiation and Risk: A Hard Look at the Data" by M.E. Schillaci of Los Alamos National Laboratory] - A non-academic magazine article that discusses the hazards of radiation and radiological protection schemes.
* [http://books.nap.edu/catalog/11340.html?onpi_newsdoc062905 Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2]
* [http://lowdose.tricity.wsu.edu/2001mtg/abstracts/bailey.htm 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.
* [http://www.fortfreedom.org/s10.htm History of the idea] by a supporter of the concept.
* [http://www.radpro.com/641luckey.pdf 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)
* [http://www.scientificexploration.org/jse/articles/pdf/17.3_kauffman.pdf Radiation Hormesis: Demonstrated, Deconstructed, Denied, Dismissed, and Some Implications for Public Policy] by Joel M. Kauffman: "evidence is presented that chronic doses up to 100 times those of normal ambient (including medical) exposures are beneficial..."
* [http://www.sciencedaily.com/releases/2008/03/080325122807.htm Exposure To Low Levels Of Radon Appears To Reduce The Risk Of Lung Cancer] From Sciencedaily.com
* [http://www.wpi.edu/News/Releases/20078/radonstudy.html News release from the Worcester Polytechnic Institute] describing the study of Thompson et al. (2008) that found low levels of radon gas appears to reduce the risk of Lung Cancer.
* [http://www.phyast.pitt.edu/%7Eblc/Cancer_risk.pdf Cancer Risk from Low-Level Radiation] , a 2002 paper by Professor Bernard L. Cohen, examines the origins of the Linear-No-Threshold model and the results of various studies on the effects of low dose radiation. With the exception of atomic bomb survivors, the cancer rates are substantially reduced below baseline rates at low doses. Five biological processes which can lead to hormesis are mentioned.

References