The specially designed cover is waterproof and breathable. It repels water like vinyl without the off-gassing and vinyl odor. But unlike vinyl it breathes so that baby's sleep surface never gets too hot. It's durable and tear resistant too. The firm heat bonded polyester core is 2 sided with a firmer side for infants less than 12 months and a more comfortable side for toddlers after 12 months. The firm mattress core is made from 100% recycled polyester fibres that are heat bonded vertically allowing the core to breathe and air to flow through it so that baby's sleep environment always stays at a comfortable temperature.