We're told that air bags prevent injuries, but we're also warned that they can sometimes cause them. This seems like a lot of controversy over what is basically a big bag of nothing. A Moment of Science looks at what's in an air bag.

An air bag has three basic parts: the bag, the sensor which releases the bag, and the inflation system. An air bag isn't really inflated with air: it's inflated with nitrogen gas. In most air bags, the sensor triggers chemical reactions that produce nitrogen gas from chemical tablets stored in the unit. The gas whooshes into the bag and inflates it in only a few milliseconds.

Here lies the problem: the speed of the deployment can be dangerous. If you sit less than ten inches from the steering wheel, the bag may hit you as it inflates, potentially causing major injuries especially to smaller adults and children. Newer air bags improve on each of the three parts of the system. Some reshape the bag or tether it so that it stays farther away from the driver or passenger.

Others simply use fewer chemical tablets so that less gas is produced. So-called "smart" air bags have sensors in the passenger compartment that tailor the inflation for the severity of the crash and for the people in the seats, whether large or small, close or farther away.

What about the danger of smothering? Air bags have tiny pores that let them deflate in less than a second so they will not stay pressed against the driver or passenger.