You should not use polyester in the microwave - both melting and a fumes issues. And, check the labeling - some are named cotton but when you look at contents, you will see 10% poly. We make and sell dozens per month and always include a tag saying 'for reheating only, please use only 3 - 5 minutes'. We still have some folks who want a refund after using for 10 minutes and having them get hot! One lady left the pin in and it had 2 burn marks where the pin had been. She said she had removed the pin (?!?) and wanted a refund and yes, we gave it to her and told her to keep the cozy.