Int. J. Environ. Res. Public Health 2009, 6 187 reconstruction of external dose requires information about population behavior, such as time spent indoors and outdoors, what areas are frequented and how often. Here, one should bear in mind that it is difficult to accurately reconstruct behavioral patterns that have occurred 45 to 50 years previously. Several studies have tried to model behavioral factors, where the most important factor was deemed to be time spent on the shoreline. More details can be found in Degteva et al., [30]. This approach can give an estimated average annual absorbed dose from external sources for different age groups in each village, but can not provide information on variations in individual doses. 2.3. Internal and External Doses Received from 1950 Onwards Drinking water from the Techa River was the primary pathway for 90Sr incorporation in 1950-1951. Later, locally produced milk and fish were the main sources. Internal dose reconstruction has been based on individual measurements of 90Sr body contents. 90Sr concentrations in the body and tooth enamel were clearly dependent on age. Maximum body contents, and thereby the largest internal doses, were observed in people who were 13-15 years old during 1950-1951; the period with the largest releases. Dietary intake of 90Sr in the 1950s and 1960s has been reconstructed from in vivo 90Sr measurements in teeth and on a model of its accumulation in the enamel [37]. The most important factors determining the intake of 90Sr were changes in the radionuclide concentration in the river water, due to dilution, provision of water supply from non-contaminated sources, dietary habits, usage of floodplain lands for pasturing and fodder and the timing and degree of implementation of protective measures. In Muslyumovo, 90Sr intake after 1955 was much less than compared to the period 1950- 1955. Estimated mean daily intake of 90Sr for adult residents were as high as 5,700 Bq day-1 in 1950, about 1,200 Bq day-1 in 1951 and 1952, decreasing to about 20 Bq day-1 in 1956. Estimated daily intakes of 90Sr for adults had decreased further to about 2-3 Bq day-1 in the 1970s [16]. Correspondingly, the average 90Sr content in the skeleton of adult residents from the upper and mid- Techa River region had decreased from about 100 kBq in 1952 to about 8 kBq in 1990. The maximum body burdens of 90Sr corresponded with distance from the point of release in the same manner as did the concentration of 90Sr in river water, indicating that most of the 90Sr was ingested with river water during this period [35]. Highest annual doses were found in bone surface cells, but also the red bone marrow was exposed to significant radiation doses, with over half ETRC members having internal RBM doses of between 0.1 and 0.5 Gy [30]. Since these doses are primarily due to 90Sr, earlier calculations have not been significantly altered. Kossenko et al., [38] estimated RBM doses of 0-0.049 Gy (21.3 %); 0.05-0.09 Gy (5.2 %); 0.1-0.24 Gy (22.3 %); 0.25-0.49 Gy (17.7 %); 0.5-1.0 Gy (20.3 %) and >1.0 Gy (13.1 %) in the studied population. The highest individual RBM doses were estimated to be 3-4 Gy and riverside residents accumulated 80 % to 95 % of committed lifetime dose within the first ten years of contamination. Internal doses calculated for the GI tract have increased when calculating using the new TRDS-2000 system due the inclusion of short-lived radionuclides [27]. Calculations of external dose for permanent riverside residents during the main releases presented in Degteva et al., [30] were significantly lower than earlier assessments published in 1994. The reasons given for this were the adoption of two assumptions during previous calculations: that exposure rates decreased at the same rate downstream as the concentration of beta emitting radionuclides in river