Nuclear accidents, such as the one at the Chernobyl reactor in 1986, sometimes release radioactive isotopes into the atmosphere where they can be carried long distances by the prevailing winds. Even low levels of contamination can present problems if the isotopes involved can be concentrated in their passage through the food chain.
For example, the isotopes strontium-90, caesium-137, iodine-129 and iodine-131 are extremely dangerous as they can be absorbed from contaminated soil and water by grasses, eaten by cattle and introduced into the human diet via meat and milk.
Once in the human body, these isotopes can become localised, increasing further the risk of cancer. For example,