Polyester is typically used in modern pillows today and is known to be petroleum based. It holds onto heat and is not as breathable. To be safer there are other fillers out there than can provide better air quality while being anti-bacterial and anti-microbial. There is wool (keeping humidity levels low while encouraging better breathable air and being fire resistant/bacteria resistant), cotton (anti-microbial), buckwheat (organic chemical based), and hemp (required less water and less pesticides, making it safer and does not trap heat or moisture).