Edited sentence: Aluminum foil is made from extremely thin aluminium metal which needs to be protected from oxygen in the air or it will oxidize to a white powdery compound (aluminium oxide).For this reason, cooking food which makes contact with aluminum foil is not recommended.Once the plastic layer is gone, the aluminum is free to oxidize and so when a baked potato in unwrapped, the foil is generally a dull grey to powdery white.Only use it as a vapor barrier where it does not come in direct contact with food, particularly fatty or moist items.The problem with cooking food in this material is that the plastic dissolves in the fats and carbohydrates in your food and contributes monomers of plastics to your diet.These have been shown to be carcinogenic and are banned or controlled substances in most foods.This protection is generally afforded by coating both sides of the aluminum with plastic polymers which are kept flexible because of the plasticizers they contain.While some aluminum metal is harmless, high levels may contribute to the development of Alzheimer's disease with prolonged exposure