Gum was originally made from tree sap called chicle, a natural rubber, but when scientists created synthetic rubber, polyethylene and polyvinyl acetate began to replace the natural rubber in most gum. Not only are you chewing on plastic, but you may also be chewing on toxic plastic — polyvinyl acetate is manufactured using vinyl acetate, a chemical shown to cause tumors in lab rats. While it is possible to recycle your gum, it may be best to skip it — and its plastic packaging — altogether.