Air bags are connected to sensors that detect sudden deceleration. When activated, the sensor sends an electrical signal that ignites a chemical propellant, and when ignited, this propellent produces nitrogen gas, which inflates the air bag. This process occurs very quickly in less than one-twentieth of a second faster than the blink of an eye. Most air bags have internal tether straps that shape the fabric and limit the movement of the bag. Vents in the rear allow the bag to deflate slowly to cushion the head as it moves forward into the deploying air bag.
Sensors deploy air bags only when deceleration exceeds a minimum threshold.
If the change in speed due to an impact is lower than the threshold, the air bag will not inflate. In high-speed crashes, the seat belt may not be able to prevent the driver s head from striking the steering wheel or the passenger s head from hitting the dashboard. Frontal air bags protect the head and upper body in frontal crashes and are not designed to open in rear-end collisions, side impacts, or rollovers. Air bags may deploy in undercarriage impacts and angled collisions, including side impacts, where there is sufficient longitudinal deceleration. Smoke, some of which is caused by residual combustion products, may be present in the vehicle following the inflation of an air bag.