Hello and thank you for your order and your opinion. Our undersheets, in order to be waterproof, as indicated in the product sheet, have indeed a part in Organic Cotton and the other, in polyurethane. Otherwise, unfortunately, the underpad would not be waterproof. For the polyester, it is indeed a choice also on our part because this material is naturally fireproof, so rather than chemically treat the product, we prefer to put a small percentage of polyester. We remain of course available for any questions. Have a nice day. Camille. Customer service.