The seat belt is a car's primary restraint device, designed to keep your body within the confines of the protective seat frame during sudden acceleration or deceleration resulting from an impact.
Pretensioners in the seat belt system complement this process by tightening the belt once an impending collision is recognised and thereby preventing your body from flailing around the car, or partially exiting a window.
The airbag is a supplementary restrain system (typically identified by the SRS logo) that is designed to complement the seat belt operation and prevent or at least limit a variety of potential injuries occurring in an accident.
In a frontal collision, the occupants are thrust forward with immense force and speed, and the airbag is deployed to prevent them from hitting solid objects such as the steering wheel and dashboard.
In addition, their neck will be exposed to immense pressure as the head accelerates forward, even stretching the neck several centimetres in the process. The airbag limits the extent of this movement and contributes to a significant reduction in severe neck injuries.
During a side impact, the occupant's head and torso are likely to smash into the car's hard A, B or C-pillars, as well as the doors and windows with extreme force, and the side or curtain airbags prevent this by cushioning the impact to a large extent.
The airbag system relies on sophisticated crash sensors (or accelerometers) strategically placed around the vehicle (such as in the front cross-member of the chassis) that primarily measure deceleration.
If the sudden change in speed is severe enough, and roughly equal to driving into a solid wall at around 24 km/h or faster, the sensor signals the unit's inflator that ignites a solid propellant to inflate the airbag with nitrogen gas in a mere 0,05 sec - and at a relative speed of 322 km/h!
Multi-stage airbags refer to a varying level of the airbag deployment relative to the force involved - a softer and slower release for a relatively minor collision and maximum application for a heavy impact.
Yes, the 'explosion' makes a noise, yes, hot gases are involved, and yes, you could incur minor burns when an airbag deploys, and this could well complement the severe bruising from the seat belt - but it's better than being dead! And the ramifications if you're not wearing a belt will be severe, or life-threatening as your body is exposed to the full brunt of the airbag deployment, at over 300 km/h!
Roughly a second after the release, the gases begin escaping through tiny holes in the bag to facilitate movement, and then you're likely to find the interior of your car covered in powder which is used to keep the airbag pliable when its tightly packed away.
As effective as the airbag is, it needs to be used properly. The driver should sit with his or her chest at least 25 cm from the centre of the steering wheel at all times in order to ensure maximum efficiency and a decent margin of safety - any closer and you're very likely to get injured.
Children and airbags don't mix well. Keep the kids buckled in at the rear in appropriate child or booster seats, and never place a rear-facing child seat in the front where there's a passenger airbag, unless it's completely deactivated.
In terms of maintenance, unless an airbag warning light appears on the dashboard, no attention is required typically for between 10 and 14 years when the unit should be examined, or replaced if necessary, by the servicing dealer.