Vehicle air bags tend to be made out of fabric, like nylon or polyamide. They are manufactured in such a way that there are tiny vents in the actual make up of the material, and these allow the gas inside of the bag to slowly escape. When blown up, it is like a soft cushion rather than a solid object. Once it has been used it comes back to the outside pressure within a few minutes. The inner section of the bag contains a small sodium azide pellet, which in turn results in nitrogen gas being formed, causing the bag to inflate. The gas is harmless and activates within seconds.
For more info, see Small Car Safety: Are Small Cars Less Safe Than Larger Ones