100% cotton batting has scrim. The scrim is what catches on fire in some microwaves. If the cotton batting does not say no scrim it has it. It's a very thin layer of poly over the cotton, not enough to list on the package but enough to catch fire. There is scrim free cotton batting available now just for these bags. A low wattage microwave usually doesn't get hot enough to cause a fire but the get hot in a second microwaves will get hot fast.