100% cotton batting has scrim. Scrim is a very thin layer of poly over the batting. Too little amount to be listed on the package. Scrim is what will catch on fire. The batting has to say NO SCRIM or it will have scrim. Low wattage microwaves will not get hot enough to cause the scrim to flash so that's why some can use it and never have a problem. There is no excuse for any fabric shop owner/manager to not know this information.