Short-term exposure to high levels of natural external gamma radiation does not induce survival adaptive response

In some areas of Ramsar, a city in northern Iran, residents receive a much higher annual radiation exposure than is permitted for radiation workers. Induction of adaptive response in residents of Ramsar has been reported previously. In this study induction of such a response in short term exposure to high background levels of gamma radiation is investigated. Fifty male NMRI mice were randomly divided into four groups of 10-17 animals and 53 Wistar rats were randomly divided into five groups of 10-12 animals were studied. Animals in the 1st group were kept for 7 days in an outdoor area with normal background radiation while the 2nd, 3rd, 4th and 5th (in case of rats) groups were kept in 3 different outdoor areas with naturally elevated levels of gamma radiation. Animals were then exposed to a lethal dose of 8 Gy gamma radiation. For mice, 30 days after exposure to lethal dose, the survival fraction for the control group was 40% while the 2nd, 3rd, and 4th groups had survival rates of 20%, 33.30%, and 35.20%, respectively. For rats, 30 days after exposure to the lethal dose, the survival fraction for the control group was 40% while the 2nd, 3rd, 4th and 5th groups had survival rates of 20%, 41.6%, 60.0% and 35.7%, respectively. Results indicate that shortterm exposure to extremely high levels of natural gamma radiation (up to 196 times higher than the normal background) do not lead to induction of survival adaptive response.