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How Do Air Bags Work and Why Can They Be Dangerous?

How can a system designed to protect drivers and passengers cause them harm? Here's how air bags work.
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This week's nationwide recall by federal safety regulators on vehicles that contain air bags by the Japanese supplier Takata has consumers on high alert as they try to understand how a safety restraint system that is supposed to protect them could cause bodily harm.

In some cases, drivers died after a Takata air bag inflater ruptured and sprayed metal shrapnel into the car. Regulators are asking for more information on the propellant being used in Takata air bags, to find out if ammonium nitrate—a common compound used in fertilizer—is one of them. The concern is that exposure to moisture in humid regions can cause the propellant to degrade. This can make it burn too strongly when the air bag is deployed, rupturing the inflater and sending metal fragments into an automobile's interior.

Air bags are controlled by the laws of motion and are activated and fired through a carefully controlled explosion. They are triggered by high velocity and open up at more than 200 miles per hour—much faster than a car crash.

These restraint systems are designed to help seat belts protect passengers. Here's the mechanics of how they work:

  1. When a car hits something, it starts to decelerate very rapidly, and an accelerometer—electronic chip that measures acceleration or force—detects the change of speed. If the deceleration is great enough, the accelerometer triggers the air bag circuit. Normal braking doesn't generate enough force to do this.
  2. The air bag circuit passes an electric current through a heating element, which then ignites a chemical explosive. Older air bags used sodium azide as their explosive to generate nitrogen gas; new ones use different chemicals.
  3. As the explosive burns, it generates a massive amount of gas (typically either nitrogen or argon) that floods into a nylon bag packed behind the steering wheel.
  4. It takes only 40 milliseconds to fully inflate the air bag, and as it expands, it blows the plastic cover off the steering wheel and inflates in front of the driver. The bag is coated with a chalky substance, such as talcum powder, to help it unwrap smoothly.
  5. The driver or passenger is moved forward because of the impact and pushes against the bag. This makes the bag deflate as the gas it contains escapes through small holes around its edges. By the time the car stops, the bag should be completely deflated.


-- Lori Ioannou, CNBC