People in some areas of Ramsar, a city in northern Iran, receive an annual radiation absorbed dose from background radiation that is up to 260 mSv y−1, substantially higher than the 20 mSv y−1 that is permitted for radiation workers. Inhabitants of Ramsar have lived for many generations in these high background areas. Cytogenetic studies show no significant differences between people in the high background compared to people in normal background areas. An in vitro challenge dose of 1.5 Gy of gamma rays was administered to the lymphocytes, which showed significantly reduced frequency for chromosome aberrations of people living in high background compared to those in normal background areas in and near Ramsar. Specifically, inhabitants of high background radiation areas had about 56% the average number of induced chromosomal abnormalities of normal background radiation area inhabitants following this exposure. This suggests that adaptive response might be induced by chronic exposure to natural background radiation as opposed to acute exposure to higher (tens of mGy) levels of radiation in the laboratory. There were no differences in laboratory tests of the immune systems, and no noted differences in hematological alterations between these two groups of people.
*National Radiation Protection Department, Iranian Nuclear Regulatory Authority, P.O. Box 14155-4494, Tehran, Iran; †Biophysics Department, Tarbiat Modares University, Tehran, Iran; ‡Medical Physics Department, School of Medicine, Rafsanjan University of Medical Sciences, Rafsanjan, Iran; §Departments of Medical Physics, Radiology, and Physics, University of Wisconsin, Madison, WI; ∥Department of Radiation Medicine, Georgetown University, LL Bles Building, 3800 Reservoir Road, Washington, DC 20007-2197; ¶Department of Environmental Medicine, University of Rochester, 601 Elmwood Ave Box HPH, Rochester, NY 14642.
(Manuscript received 5 January 2001;
revised manuscript received 23 April 2001, accepted 5 August 2001)
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