Int. J. Environ. Res. Public Health 2009, 6 192 hypothetical critical group in Muslyumovo (equivalent to Group 3, termed as herdsmen) was found to potentially receive more than 1 mSv yr-1, the corresponding group in Brodokalmak receiving three times less dose. However, contributions of external doses to the total doses were less than the previous assessments [24-25] being 47 % and 40 % for residents in Muslyumovo and Brodokalmak, respectively [41]. 3. Health and well-being of riverside residents. The registry of the exposed population serves as the basis for conducting epidemiological cohort studies. The registry of mortality contains information from death certificates retrieved from the State Registrar’s Office archives for the entire population in administrative districts across which the Techa River flows. Altogether, the registry contains information from more than 70,000 death certificates. By cross-matching these certificates with TRC data it has been possible to select more than 6000 death certificates for people who had been exposed along the Techa River and died between 1950 and 1982. These data have made it possible to study possible health effects caused by the contamination in the Techa River [e.g., 27-28]. Epidemiological studies of Techa River residents also offer interesting new insight such as the protracted exposure at low dose rates, a wide range of doses, inclusion of both males and females, ethnicity variation and the long-term follow-up documentation [26]. 3.1. Potential Health Effects There is circumstantial evidence to indicate that DNA in chromosomes is the principal target for the biological effects of radiation [e.g., 42-44] and that a connection exists between biological damage and radiation energy absorbed, or dose. Two types of damage can occur when a cell is irradiated. Cell death is usually observed soon after irradiation at high doses. The cell can also survive being irradiated and sometimes will reproduce itself in an altered form due to the radiation energy absorbed. Such a transformation of the cell can result in the formation of cancer cells and genetic damage. The effects of such cell transformation may take years to materialize and are therefore sometimes called “late effects” of radiation. Several studies of health effects in different populations exposed to ionising radiation have been performed. Population groups studied include survivors after the detonation of atomic bombs over Japan in the Second World War, residents near nuclear plants and nuclear weapons production facilities and individuals exposed to fallout from nuclear weapons testing [e.g., 45]. Studies of survivors after the nuclear bombs detonated in Japan have shown a significant dose dependent increase in the risk of cancers in the stomach, colon, lung, breast, ovary, urinary bladder and thyroid gland. Among persons who were under the age of 20 years at the time of the bomb there was also an increased risk of tumours in neural tissue, excluding the brain [46]. Leukemia was one of the earliest effects seen and remains one of the most striking findings from the follow-up study of the atomic bomb survivors: a significant dependence between radiation dose and leukemia was established. An increased risk for lymphoma was also observed among men. The excess in leukemia mortality has declined with time, while excess deaths still increase with time for cancers other than leukemia [46].