Vehicle safety systems are designed to protect occupants during accidents, and can be classified as primary, or passive systems and secondary, or active systems.
Primary systems are ready to use in any accident. They include bumper bars, body panels, seatbelts, crumple zones and collapsible steering columns.
A secondary system has to be activated to work and is only necessary in severe accidents. The two most popular types of secondary systems are supplemental restraint system air bags, and seatbelt pre-tensioners.
Seatbelts locate and secure the occupant within the seat and vehicle cabin, and in minor collisions perform their task well.
In a more severe impact inertia causes the occupant to move more and with greater force. This increases the possibility of injury caused by the restraining force exerted by the seatbelt or from the occupant striking interior fittings.
If a vehicle is fitted with an air bag, it deploys during a collision, offering a greater degree of protection from injury.
Air bags provide cushioning against the effects of inertia. The bag deploys towards the occupants approaching body, inflated rapidly by pressurized nitrogen gas. Typically this takes no longer than three hundredths, or 0.03 of a second.
The air bag is not a nice soft pillow, but a strong counter force to react against the inertia of the occupants. It is not designed to be comfortable. It is designed to minimize injury.
Immediately after absorbing the momentum, the air bag deflates having done its job.