used dryer sheets are no more flammable than cotton fabric. If they would go poof at any high temperature then they would not work in a dryer. Try burning a scrap of cotton and a scrap of dryer sheet, the difference is that the cotton burns to an ash, the sheet to a black glop as it is a synthetic. If you burn a scrap of polyester fabric and a dryer sheet they act the same. If you are hanging out close to an open flame of course be careful, because any fabric will go poof if you put a flame to it.