BPA is a chemical found in some plastics. It is poisonous and can be released when the bottle is heated (milk heating, sterilising etc). You will find it in most hard, clear plastic baby bottles. I cant remember the plastic numbers that are bad - I think baby bottles are often a 7 - although most wont put it on their bottles. Bottles with a 5 are good (look inside the recycling symbol on the bottom on the bottle).