Although the process leading to airbag deployment has become more technical over the years, the airbag safety sensors’ job has remained the same.

In earlier vehicles, these airbag sensors were basic switches that responded to changes in velocity as the vehicle slowed down during the crash. Once two sensors “closed” to confirm a crash was taking place, electrical current was allowed to flow to the airbag modules.

In newer vehicles, electronic sensors measure a vehicle’s deceleration (negative acceleration), process it mathematically through a computer algorithm, and then compare the measured values to the values stored inside it from crash testing. If the measured values indicate the crash is more severe than the stored crash tests, the control module allows electrical current to flow to the airbag modules.

Once the electrical current flows to the airbag modules, it heats up a “squib” within the inflator that has a small filament inside a container of chemically explosive or flammable material. Once the filament gets hot enough, the chemicals begin burning. This sets off a larger reaction of a chemical called sodium azide within the inflator, which rapidly produces nitrogen gas, along with numerous byproducts.

In some vehicles, the sodium azide inflator was replaced with an inflator using pressurized gas, usually a combination of helium and argon. With either type of inflator, the gas from the inflator then fills the fabric airbag that was folded over the inflator.

As the gas fills the airbag, it increases in size, eventually breaking out from behind its plastic cover and inflating to its maximum size. Driver airbags are generally shaped like a round pancake – just larger than the diameter of the steering wheel – and are normally about 12 to 20 inches thick when filled. Passenger airbags are generally about 2 to 3 feet wide, and fill the space between the passenger and the dash or windshield.

Since passenger airbags are usually 2 to 4 times larger than driver airbags, they require a more forceful inflator to fill that larger size in the same amount of time.

For frontal airbags, the process of sensing the crash and inflating the airbags is usually over in less than one-tenth of a second. As the forces of the crash propel the driver/passenger forward into the airbag, it begins to absorb the energy by compressing and letting some of the gas out through the fabric or through specially designed vent holes.

This explains why many people involved in a vehicle accident in which airbags deployed remember the distinct chemical odor of the inflation gas and seeing smoke in the car.

For side airbags and rollover airbags, the process is similar. A sensor in the side structure of the car, or sometimes inside the front door, detects the rapid deceleration from the side or the vehicle beginning to rotate upwards during a rollover crash. Electrical current is then sent to the side airbags or to the rollover airbags (depending on the type of crash), which causes those airbags to deploy. Although the chemicals and gases may be different than for front airbags, the inflation process is very similar.