Cancer risk due to ingestion of naturally occurring radionuclides through drinking water: A systematic review

Naturally occurring radionuclides can, in rare cases, reach high levels in drinking water of specific areas, leading to meaningful radiation exposures upon ingestion. Increased cancer risk is the only well-established health impact of exposure to low-dose radiation. Multiple ecological studies have shown conflicting results about cancer risk in areas of high levels of radionuclides in water. However, such studies have methodological limitations and are generally not highly informative. Studies with individual-level data are scarce and have not established a clear association either. To further investigate this issue, we conducted a systematic review of the literature on cancer risk associated with radionuclide ingestion through drinking water, aiming to summarize and evaluate the current epidemiological evidence. Published studies have examined the effects of uranium, radium, and radon, while potential risks of polonium, radioactive lead, and thorium remain unknown. Existing research is heterogeneous regarding the cancer types assessed and faces methodological challenges, including limitations in exposure assessment, dosimetric uncertainties, low statistical power, and inadequate control of confounding factors. Due to lack of high-quality evidence, it is not possible to confirm or rule out an increased cancer risk among highly exposed populations. While risk assessment based on extrapolation from high dose studies does not suggest an excess cancer risk beyond background variation, such assessment involves considerable uncertainties. Future studies should focus on populations with high radionuclide exposure to provide sufficient exposure contrast, prioritizing radionuclides with high-dose coefficients and cancers in tissues with the highest radionuclide deposition.