The uranium ( 238 U) decay series provides the most important isotopes of elements radium ( 226 Ra), radon ( 222 Rn), and polonium ( 210 Po) with half-lives of about 1600 years, 3.8 days, and 140 days, respectively. Although the chemical structure of radium is very similar to calcium, the fact that it produces a radioactive gas (radon) complicates its handling in the laboratory and natural environment. In this study, we used the average concentrations of naturally occurring radionuclide 226 Ra

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... n drinking water at different parts of Iran to estimate the annual effective dose. In the other part of the study, we measured the concentrations of 210 Po in Iranian cigarettes to estimate the internal intake of this radionuclide and its concentration in the lung tissues of smokers. The results indicate that the average concentration of 226 Ra in Iranian drinking water was below the 100 mBq L -1 recommended by the World Health Organization while the average concentration of 210 Po and 210 Pb in Iranian cigarettes was relatively high in comparison with other cigarettes found on the market.