Inflated bag forms a barrier between you and the hard parts of the car, cushioning you from the impact for the fraction of a second that a crash actually lasts.

Johannesburg - The recent Takata recall has highlighted the huge role that airbags play in road safety - but how much do you know about them, how they work and what happens when they go off?

Airbags, originally known as Supplementary Restraint Systems (now you know where the SRS logo on your steering wheel comes from!) have been around since the 1970s, initially as an option on Ford and Chevrolet cars, and have since become mandatory on new vehicles in most countries - including, since 1998, South Africa.

They're usually installed in the steering wheel, dashboards and sides of a car, and are controlled by sensors that measure impact, acceleration and pressure.

When the sensors detect an impact on the car, either from the front or the side, they set off an explosive charge inside a small steel chamber. That almost instantaneously fills the bag with the gases produced by the explosion - we're talking thousandths of a second here.

The inflated bag forms a barrier between you and the hard parts of the car - such as the steering wheel or dashboard - cushioning you from the impact for the fraction of a second that a crash actually lasts. The bag is not airtight - your body’s inertia would cause it to burst if it was - so it deflates quite quickly (usually within about half a second) after the initial inflation, letting you down gently.

ONE-TIME THING

For obvious reasons this is a one-time thing; once it's been deployed, the airbag and its built-in inflator cartridge have to be replaced as a unit by a qualified technician.

It's that cartidge that's at the root of the problem with Takata airbags, and the cause of the world's biggest recall yet. In some of their airbags, fitted to cars of a dozen makes all over the world, the explosive cartridge is a little too powerful.

Under rare and unusual circumstances (typically, high humidity can have something to do with it) the explosion can cause the steel chamber to break up, turning it into flying shrapnel, resulting in serious injury or even death.

All this takes place in the middle of a car crash, so it's very difficult to figure out exactly how it happened - or, sometimes, whether it has. The bottom line is that every suspect airbag will have to be replaced and, in the fullness of time, they will be.

Meanwhile, tens of million of airbags made by other makers, in tens of millions of cars worldwide, will go on doing their job, which is to sit and wait in eternal readiness for the moment of crisis and then to react, instantly and accurately - which they do.

They need little or no maintenance because they're made to last the life of the car - and they have a warning mechanism that lights up an icon on your car's dashboard if the inflator or its sensors stop working. If that light stays one for more than a few seconds at time, take your car to the dealership; the system may need repair or even replacement.

DEADLY TO CHILDREN

But the most important thing about airbags is that they can be deadly to children, especially if the child is sitting in the front seat. It's always safest for children under 12 to sit in the back - always buckled up, on a booster seat if they're less than eight years old, or in child-safety seat for infants and toddlers.

Many parents are advised to secure their infant's safety seat on the front seat, facing to the rear; never do that unless your car allows you to deactivate the front passenger's airbag. If you do you will be placing a small, fragile person just centimetres away from an explosive device powerful enough to blow the seat - and its occupant - into the back of the car.

Never let anybody - of any age - ride in your car, front or rear, without buckling up. Seatbelts and airbags are designed to work together; either on its own can cause serious injury in a collision.

Together, however, they reduce the risk of fatal injury an a car crash by more than 60 percent.

Follow IOL Motoring on Facebook

Dear IOL reader, we're closing comments