# 7.2 Sources of radiation in the environment and in drinking water

Radioactive materials occur naturally in the environment (e.g. uranium, thorium and potassium). Some radioactive compounds arise from human activities (e.g. from medical or industrial uses of radioactivity) and some natural sources of radiation are concentrated by mining and other industrial activities.

By far the largest proportion of human exposure to radiation comes from natural sources – from external sources of radiation, including cosmic radiation, or from ingestion or inhalation of radioactive materials. A very low proportion of the total human exposure comes from drinking water. Elevated levels of radioactivity in drinking water can be the result of:

* naturally occurring concentrations of radioactive material (e.g. radionuclides of the thorium and uranium series in drinking water sources). Refer to Information Sheet 2.2 for details of the radionuclides in the uranium and thorium decay series.
* technological processes involving naturally occurring radioactive materials (e.g. the mining and processing of mineral sands or phosphate fertilizer production), where there is contact with the water supply.
* manufactured radionuclides, which might enter drinking water supplies from routine discharges or emergency situations.


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