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

ABSTRACT

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 1[st] group were kept for 7 days in an outdoor area with normal background radiation while the 2[nd], 3[rd], 4[th] and 5[th] [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 2[nd], 3[rd], and 4[th] 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 2[nd], 3[rd], 4[th] and 5[th] groups had survival rates of 20%, 41.6%, 60.0% and 35.7%, respectively. Results indicate that short term 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