Challenging the validity of linear no-threshold hypothesis: results obtained by studies on high background radiation areas

Full text: Ramsar, a city in the northern Iran, is among the world's well-known areas with highest levels of natural radiation studied so far. Annual external gamma exposure levels in areas with high levels of natural radiation in Ramsar are up to 130 mGy y-1 and average exposure rates are about 10 mGy y-1 for a population of about 2000 residents. Ramsar residents are also exposed to high levels of alpha particles in the form of ingested radium and radium decay progeny as well as very high radon levels (over 1000 MBq m-3) in their dwellings. In some cases, the inhabitants of these areas receive doses much higher than the current ICRP-60 dose limit of 20 mSv y-1 set for radiation workers. Based on our results obtained to date, no detrimental health effects have been observed and the inhabitants are still living in their unaltered paternal dwellings. Furthermore, lymphocytes of residents in high level background radiation in Ramsar, when subjected to 1.5 Gy of gamma rays, showed fewer chromosome aberrations as compared to the residents in a nearby control area with normal background radiation. These findings indicate that high levels of natural radiation can induce radioadaptive response. Epidemiological studies performed in other well-known high background radiation areas such as Kerala, India and Yangjiang, China have indicated that the natural radiation is not harmful to residents. Furthermore, there is some evidence indicating that cancer mortality rate is significantly lower in the high background areas than in the control areas. It can be concluded that the LNT model is inappropriate for public health measures in high background radiation areas