Originally Posted by elgarak
A quick google says it's polonium:
"Radioactive materials, like polonium-210 and lead-210 are found naturally in the soil and air. They are also found in the high-phosphate fertilizers that farmers use on their crops. Polonium-210 and lead-210 get into and onto tobacco leaves and remain there even after the tobacco has been processed."
In any case, doesn't the same apply to any vegetable material entering the body? Lungs clear themselves out constantly, as do the bowels (though not constantly in the latter case