How Does an Airbag Inflate Before a Passenger Reaches It?

During a serious collision, an airbag must recognize the crash, inflate, and begin protecting an occupant within a fraction of a second. It cannot wait for compressed air to be pumped into a bag.

Sensors throughout the vehicle measure sudden changes in speed and movement. A control unit evaluates those signals to determine whether the impact is severe enough and whether a particular airbag should deploy. Modern systems may also consider the direction of impact, seat position, belt use, and the presence or size of a passenger.

When deployment is required, an electrical signal activates an igniter inside the airbag module. The igniter starts a rapid chemical process that produces a large quantity of gas. Older inflators commonly used sodium azide to create nitrogen. Modern designs use several other compounds or stored gas systems.

The gas fills the folded fabric bag in less than one twentieth of a second. A cover in the steering wheel, dashboard, seat, or roof area opens along designed seams as the bag expands.

The airbag does not remain firm like a balloon. Small vents allow gas to escape as the passenger presses into it. This controlled collapse lengthens the time over which the body slows, reducing the force of contact with harder parts of the vehicle.

Airbags are designed to work with seat belts, not replace them. A person sitting too close can be injured by the rapidly expanding fabric, which is why children and rear facing child seats require particular precautions.

An airbag protects the occupant through a carefully timed sequence: detect, ignite, inflate, cushion, and collapse, all within the brief interval between the first impact and the body's forward movement.