Your baby will sleep soundly on the Simmons Nap Time extra firm crib mattress. The core is made from 100% recycled polyester from water bottles for a sleep surface that is uniform. The polyester fibers are heat bonded vertically producing thousands of tiny air pockets that let air flow freely through the mattress so it breathes. And unlike polyurethane foam, polyester doesn’t off-gas and release VOC’s (volatile organic compounds) into the baby’s sleep environment. The PEVA cover is phthalate free and completely waterproof. It’s strong and durable and won’t crack with use over time.