It's actually naturally-occuring cyanide in the green potatoes - you find it in uncooked dried beans as well. Very bad for you, but we either cook it till it breaks down (eg boil dried beans hard for 10 minutes before simmering, like your mother taught you), or don't eat the food at all (eg green potatoes, apple seeds etc). Arsenic is a heavy metal, rather than an organic compound, and would only be found in foods that were either grown in contaminated soil, or if the food were contaminated in processing. Not likely, and definitely not something to worry about. Sounds like one of those web legend (asbestos in tampons, that sort of thing). (although they did use arsenic to flavour things a century or so ago, but due to various interesting neurological effects, this fell out of favour....)