The article "Radioactive Smoke: A Dangerous Isotope Lurks in Cigarettes" in the January 2011 issue of Scientific American details how tobacco plants accumulate a radioactive isotope of polonium, which can create deadly "hot spots" in smokers' lungs. The tobacco industry has known about the danger for decades but has not taken measures to remove it.
Here are some recently declassified documents (all PDF) from the tobacco industry:
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