The simple chemical makeup of EPS is carbon, hydrogen, and oxygen – elements found in wood and other organic materials.  EPS foam products do NOT contain formaldehyde. Both EPS and XPS can be used for insulation and some gas escapes from both types of foam. In EPS foam that gas is air since EPS is made with steam. In XPS that gas is primarily Tetrafluoroethane a dangerous chlorofluorocarbon also used as a refrigerant. (http://en.wikipedia.org/wiki/Polystyrene)