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   Low doses of ionizing radiation are widely believed to produce effects similar to those observed at high doses,only the incidence varies with dose.Furthermore it is assumed that effects other than those observed at high doses will not occur at low doses. In recent decades a hypothesis which has been gaining acceptance states that at low doses ionizing radiation may be beneficial to humans. This is called the "Hormesis" effect.
   Numerous studies support the notion that minute doses of ionizing radiation benefit animal growth,development,fecundity,health and longevity (1).Also many review articles indicate that minute doses of ionizing radiation can stimulate plant growth (2)(3)(4)(5).Results of experiments that pointed out in this direction were attributed to a process named "Hormesis". Hormesis is any physiological effect that occurs at low doses and which can not be anticipated by extrapolating from toxic effects noted to high doses. Hormetic effects are normally beneficial but may co-exist with toxic effects. hormesis may be characterized as a process whereby low doses of an otherwise harmful agent may result in stimulatory or otherwise beneficial effects.
   The word originates from the Greek word "Hormaein" which means to excite, and refers to a much broader spectrum of phenomena including many toxicological observations.
    According to Stebbing the term was probably first coined in a publication dating from 1942 describing the growth stimulation of fungi by a naturally occuring antibiotic, which at higher concentrations supressed fungal growth. There are many examples in nature of processes that follow a hormetic model. A striking example is vitamins, widely accepted to be very beneficial at certain doses , but known to be toxic at higher doses. Hormetic models of ionizing radiation suggest that it behaves in a similar way(6).
    In 1984 Olivieri et al reported that lymphocytes exposed to a low dose of incoming radiation had fewer chromatid aberrations induced by a subsequent high dose exposure, compared to lymphocytes not receiving a low dose (7). They termed this phenomenon an adaptive response (AR) to ionizing radiation and proposed that the low dose induced a chromosomal repair activity.This survey was followed by a series of investigations which suggested some of the conditions required for elicting such an inducible process.It was found that the exposure of lymphocytes to an x-ray dose as low as 0.5 cGy could significantly reduce the cytogenetic damage induced by subsequent challenge with a high dose of 1.5 Gy x-rays (8).

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Prof. Z. Jaworowski , Australas. Phys. Eng. Sci. Med.,Vol 20,No 3, 1997.
 

1- The first large scale radioepidemiological study at low doses was carried out for the white population in the United States in 1976 by Frigerio & Stowe. In 50 states , the population was divided into three categories, according to average natural radiation dose.In the states with average annual radiation dose of 2.1 mSv (population 5,735,000) the mortality rate per (100,000 persons) due to all malignancies was 126.3, in states with 1.7 mSv dose (population 16,897,000) was 132.2 and the highest mortality rate of 146.8 was found in states with the lowest dose of 1.18 mSv (population 59,683,000).Of 14 states at background above 1.4 mSv/year , in 12 states the malignancy mortality rates were very significantly (p<0.01) below the U.S. average ,one insignificantly lower and one slightly higher.

2- In china a meticulous study measured the radon level for one year in the houses of several hundred women with lung cancers and in houses of a similar number of healthy women.The results demonstrated at a 95% confidence level that women who lived in high-level radon houses (more than 350 Bq/m3) had a 30% lower lung cancer risk than those living in low level radon houses (4-70 Bq/m3).This result is opposite to the no-threshold priciple estimate, according to which the lung cancer risk in the high radon houses should be 80% higher than the normal risk.(2).

3- Similarly in one region of Japan with an average indoor radon level of 35 Bq/m3 , the lung cancer incidence was 51% of that in a low-level radon region (11 Bq/m3) , and mortality caused by all types of cancer was 37% lower (3).