How Do Air Bags Work?

Mercedes-Benz safety - airbag deployment

You hear them mentioned and you may have seen the videos that show airbags inflating, in slow motion, to cushion dummies thrown against them in crash tests before they hit the hard, unyielding surfaces of the car's interior.

But how does an airbag actually work?

First, sensors in the car instantly detect when it's crashing into something with a force at least as great as that from hitting a solid wall at 15 mph or higher. A device called an accelerometer senses sudden deceleration, and instantly signals a control system that tells the airbag to inflate. (If the front passenger seat is empty, though, that airbag won't be triggered.)

When the signal is received, the inflation system instantly mixes two substances, sodium azide and potassium nitrate. The two combined to produce nitrogen gas that inflates the thin, nylon airbag--within a few hundredths of a second--just as the seat occupant's body is being thrown toward it.

The goal of the airbag is to slow that body's momentum to zero, cushioning the person on first impact and then deflating at a known rate to absorb more of the force.

Airbags have been required for the driver and front passenger in every car since 1998, and every light truck since 1999. Today, virtually every passenger vehicle also has bags mounted in the sides of the seats to protect against side impacts, and also side-curtain airbags that cover the front and rear windows together to ensure that no passengers' heads can go through a side window in a collision or rollover.

There are now also knee airbags, usually on the driver's side, and bags on the inside upper part of seats to prevent one passenger being thrown against another. As of the current model year, a few cars have as many as 11 airbags. In a frontal crash, airbags reduce the overall risk of death for car occupants by about 30 percent.

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