"There is no such thing as safe microwaveable plastic," vom Saal says. Studies exposing plastic to heat in a microwave caused higher levels of Bisphenol A to be released into food. The studies also found BPA was released when plastic containing BPA was exposed to heat from dishwashers and hot food.
Some are microwave-safe and some are not. Steam-bag vegetables are safe and specifically designed for use in microwave. They do not contain BPA or phthalates. Do not reuse steam bags for other purposes and always follow directions by the manufacturer.