Air bags have been used in automobiles since the 1980s. The trend gathered momentum in the early and mid 1990s, during which major car manufacturers repeatedly boasted putting airbags in their new models for the driver. Beginning in 1998, driver- and passenger-side air bags have been required by law for all new cars in the United States. The air bag, however, was not born in a design room at the Ford Motor Company. The first inflatable device patented to help prevent injuries from crashes actually appeared in airplanes during World War II.
Studies have shown that air bags help reduce the risk of death from a direct frontal crash by about 30 percent. Newer, yet not so widely used, are seat- and door-mounted side air bags, which are credited with reducing death and injury rates from side collisions.
An air bag works by stopping the continuing motion of a body within a car, while the car itself has stopped instantly. In theory, this seems to be a rather simple task. The frailty of the human body, however, makes this task one of precision sensitivity.
The first problem with which the air bag must contend is the relatively small space between the passenger and the steering wheel (or dashboard). A body moving at 100, or even 40, kilometers per hour only has about 0.3 meters to come to a stop. The second problem is the fraction of a second that the body has to come to this stop. The final problem is absorbing or cradling the force of the body, rather than stopping it abruptly.
Sensors cause a bag to deploy during a crash if the force is equal to or greater than the force of running into a brick wall at about 17 kph. A mechanical switch inside the car closes due to a shift in mass and an electrical contact is made.
The critical window of prevention is only about 1/25 of a second. The bag must burst from its shell at about 340 kph in order to be effective in a crash. This is accomplished by the reaction of sodium azide and potassium nitrate, which produces hot nitrogen gas to inflate the bag. To solve the problem of cradling, rather than outright stopping, the air bag is made of porous nylon, which begins to deflate 1 second after deployment. This deflation not only causes a cradling effect, but also prevents the passenger from being trapped in that small space between the seat and the dashboard.