Beginning on July 1st, 2007 all mattresses must meet flame-resistance standards set by the federal law. This regulation requires the mattresses to be significantly more resistant to an open flame. Manufacturers use various techniques and different materials to comply with the law. They apply flame-retardant chemicals like polybrominated diphenyl ether or PBDE to outer mattress layers. Or use fire resistant fiber barriers utilizing materials like Kevlar. Some manufacturers use wool as a natural fire retardant material or rayon and cloth made of hydrated silica and baking soda.