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The deadly dangers of air bags

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.

More than 14 million vehicles from 11 automakers have been recalled worldwide since 2008 over concerns about the Takata-made air bags, including Ford, Honda, Chrysler, Mazda and BMW, mostly from models made in 2008 or earlier. Today a Senate committee hearing will take place, with representatives of Takata, Honda and Chrysler set to appear.


Air bag deployment
fStop Images | Caspar Benson | Getty Images

The recall comes after a long series of ruptured air bags injured drivers and passengers in several states, especially those in humid regions.

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.

Read MoreTakata, auto executives to face Senate over deadly air bag scandal

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.

Read MoreFeds call for recall of 'millions of vehicles' over Takata air bags

  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.

Many newer vehicles may contain multiple air bag modules in various side and frontal locations, and sensors may deploy one or more air bags in an impact zone, based on the severity of impact. But air bags typically are designed to inflate in moderate to severe frontal crashes.

There are a lot of people credited with inventing air bags. But in the U.S., John W. Hetrick of Newport, Pennsylvania, came up with the idea after an accident swerved his car off the road, almost throwing his daughter through the windshield in 1952. He was granted a patent for the product on Aug. 18, 1953. Many other inventors have built on the idea since then, developing various ways to trigger the explosion of gas inside an air bag just before the impact of a crash.