The invention of the [air bag](https://www.yourmechanic.com/question/how-do-air bags-work) fundamentally changed vehicle safety. The fact is that vehicles with air bags are safer. Why? Because an air bag helps reduce the force of a collision, resulting in less damage to a vehicle's interior, including drivers and passengers!
Air bags work via a four-step process that is triggered when sensors detect an impact of sufficient force to necessitate air bag deployment. From there, a chemical canister is detonated and its contents eject pellets into the air bag, producing nitrogen gas that inflates each individual air bag. The inflated air bag helps soften a potential blow to the driver's head, as well as lessens the point of impact on passengers. After the impact happens and the air bags deploy, the air bags then slowly deflate. This means you must replace an air bag after it deploys because it can't be used again.
The [location of the air bags](https://www.yourmechanic.com/question/where-are-the-air bags-located) in a vehicle vary according to its model and make. Most older models do not have air bags, though a few cars in the early 1970s, such as the Oldsmobile Toronado, utilized this potential life-saving technology. Many of today's vehicles use air bags as an added safety feature, and they have even found their way onto some motorcycles, such as the Honda Goldwing.
Part 1 of 4: An impact triggers the air bag system
Air bag inflation is triggered when one of the vehicle sensors detects an impact of sufficient force to warrant the deployment of one of the vehicle's air bags. Sensor position depends mainly on how many air bags a vehicle has and where they are located.
Area 1: Front side air bag sensors. A majority of [air bag sensors](https://www.yourmechanic.com/article/what-does-the-air bag-warning-light-mean-by-spencer-cates) are located at the front of the vehicle.
The most common crash sensor is the electromechanical sensor, which contains a gold ball secured by a magnet within a tube. If an impact is hard enough to jolt the ball to the end of the tube, where it hits a switch, it will cause the frontal air bag system to deploy.
Area 2: Side impact air bag sensors. Side air bag sensors are typically found at the midpoint and rear of a vehicle.
Another type of air bag sensor is the Rolamite design. In this sensor, a small metal roller rolls forward upon impact, tripping a switch and causing the air bags to deploy.
Area 3: Rollover air bag sensors. In some vehicles, if a vehicle suffers a rollover, overhead air bags are deployed to protect vehicle occupants.
A new type of sensor, called a solid state crash sensor, uses a piezoelectric crystal or micro-machined accelerometer chip to produce an electronic signal upon impact, triggering air bag deployment.
Part 2 of 4: The chemical canister is detonated
Regardless of the sensor type used, once the sensor is triggered, the air bag system detonates a chemical canister to aid in the deployment of the air bag. The sensor sends a signal to the air bag Control Unit (ACU), a type of Engine Control Unit (ECU), that determines the severity and direction of the impact. If the impact is severe enough, the ACU sends a signal to the inflator, which fires, releasing a chemical agent into the air bag.
Step 1: Signal sent to ACU. The ACU helps determine if the impact was severe enough to necessitate air bag deployment.
This process is done in a split second, taking roughly 1/20th of a second to decide whether to deploy the air bags or not and then send the signal.
Step 2: Signal sent from ACU to inflator. Once the ACU determines that the impact was of a sufficient force to require air bag deployment, a signal is sent to the inflator of the air bag that lies in the direction of the point of impact.
The inflator ignites the price, a device within the inflator that propels the chemical pellets into the air bag.
Step 3: Chemical is released. As the chemical pellets are released, they explode within the air bag to produce the inflating agent.
Most often the pellets are composed of sodium azide, which produces a nitrogen gas. Some older air bags, contain other chemical agents, in addition to the sodium azide, that help in the production of the nitrogen gas needed for proper inflation.
Part 3 of 4: The gas inflates the air bag
After the sodium azide pellets have been released into the air bag, they explode, producing the nitrogen gas necessary to fully inflate the air bag. This process takes roughly 1/20th of a second, rushing out to meet the driver's or passenger's body part, usually the head, that it is meant to protect.
Step 1: The nitrogen gas is released. As the gas is released, the bag expands out of its container.
Usually this is a compartment in the center of the steering wheel, the dash, the doors, or overhead.
Step 2: The air bag inflates. As the air bag inflates, it rushes to meet the body of the vehicle occupant.
As stated above, this process takes around 1/20th of a second, meaning the whole process from impact to full inflation takes approximately .41 seconds.
Step 3: The air bag reaches full volume. The primary purpose of an air bag is to absorb the energy of a vehicle impact or rollover.
When used properly, an air bag is a life-saving device that help prevent serious injury in moderate to severe vehicle impacts.
- Warning: air bags are dangerous if used with young children in the front passenger seat. Any child 13 years old or younger should ride in the back seat of a vehicle. The height of the child's head in relation to the air bag can cause severe injury or even death, even if the child is properly restrained using the vehicle's seat belts. If you must place your older child in the front seat, move the seat all the way back, and make sure they are properly restrained using the seat belt in order to reduce the chance of injury from air bag deployment.
Part 4 of 4: The air bag deflates
Once the air bag has deployed, the air bag almost immediately starts to deflate. In essence, as your head or body part impacts the air bag, it should already be deflating. This helps prevent a rebound from the air bag, which could cause whiplash or other injury.
Step 1: The air bag starts to deflate. The air bags in your vehicle have tiny holes, or vents, to help with the release of the nitrogen gas.
In addition to the release of gas, the air bag can also release a white, powder-like substance, This substance is made up of cornstarch, chalk, and talcum powder, which help lubricate the air bag while it deploys.
Step 2: The air bag totally deflates. After deflation, the air bag remains outside of its chamber and is no longer usable.
In addition, the cornstarch, chalk, and talcum powder released can cause cosmetic damage to the dashboard and upholstery.
Step 3: Replace the air bag. Once an air bag has deployed, you need to replace it in addition to whatever area it was located.
For example, if the steering wheel air bag deployed, you need to replace the steering wheel boss, or the center part of the steering wheel around which the wheel attaches.
The same goes for any dashboard, door, or overhead panel from which an air bag deploys.
Air bags are used in motor vehicles to save lives and prevent injury. Making sure your air bag system works properly can keep both you and your passengers safe. If you have any questions about the functioning of your vehicle's air bag system, Ask a Mechanic for more information.
This article originally appeared on YourMechanic.com as How to Understand How Car Air Bags Work and was authored by Cheryl Knight.