| Air bags are typically designed to deploy in frontal | | | | alternatives. |
| and near-frontal collisions, which are comparable to | | | | From the onset of the crash, the entire |
| hitting a solid barrier at approximately 8 to 14 | | | | deployment and inflation process is faster than |
| miles per hour (mi/h) (13 to 23 km/h). Roughly | | | | the blink of an eye. Airbags deploy in 15 |
| speaking, a 14 mi/h (23 km/h) barrier collision is | | | | milliseconds (0.015 seconds) for high speed |
| equivalent to striking a parked car of similar size | | | | crashes and in 25 milliseconds for low speed |
| across the full front of each vehicle at about 28 | | | | crashes (0.025 seconds). Because a vehicle |
| mi/h (45 km/h). This is because the parked car | | | | changes speed so fast in a crash, air bags must |
| absorbs some of the energy of the crash, and is | | | | inflate rapidly if they are to help reduce the risk |
| pushed by the striking vehicle. Unlike crash tests | | | | of the occupant hitting the vehicle's interior. |
| into barriers, real-world crashes typically occur at | | | | Once an air bag deploys, deflation begins |
| angles, and the crash forces usually are not | | | | immediately as the gas escapes through vents in |
| evenly distributed across the front of the vehicle. | | | | the fabric. Deployment is frequently accompanied |
| Consequently, the relative speed between a | | | | by the release of dust-like particles in the vehicle's |
| striking and struck vehicle required to deploy the | | | | interior. Most of this dust consists of cornstarch |
| air bag in a real-world crash can be much higher | | | | or talcum powder, which are used to lubricate the |
| than an equivalent barrier crash. | | | | air bag during deployment. In older designs, small |
| Because air bag sensors measure deceleration, | | | | amounts of sodium hydroxide may initially be |
| vehicle speed and damage are not good indicators | | | | present. This chemical can cause minor irritation to |
| of whether an air bag should have deployed. | | | | the eyes and/or open wounds; however, with |
| Occasionally, air bags can deploy due to the | | | | exposure to air, it quickly turns into sodium |
| vehicle's undercarriage violently striking a low | | | | bicarbonate (baking soda). Depending on the type |
| object protruding above the roadway surface. | | | | of air bag system, potassium chloride (a table salt |
| Despite the lack of visible front-end damage, high | | | | substitute) may also be present. |
| deceleration forces may occur in this type of | | | | For most people, the only effect the dust may |
| crash, resulting in the deployment of the air bag. | | | | produce is some minor irritation of the throat and |
| The airbag sensor is a MEMS accelerometer, | | | | eyes. Generally, minor irritations only occur when |
| which is a small integrated circuit chip with | | | | the occupant remains in the vehicle for many |
| integrated micromechanical elements. The | | | | minutes with the windows closed and no |
| microscopic mechanical element moves in | | | | ventilation. However, some people with asthma |
| response to rapid deceleration, and this motion | | | | may develop an asthmatic attack from inhaling |
| causes a change in capacitance, which is detected | | | | the dust. With the onset of symptoms, |
| by the electronics on the chip, which then sends a | | | | asthmatics should treat themselves as advised by |
| signal to fire the airbag. The most common MEMS | | | | their doctor, then immediately seek medical |
| accelerometer in use is the ADXL-50 by Analog | | | | treatment. |
| Devices, but there are other MEMS | | | | Once deployed, the air bag cannot be reused and |
| manufacturers as well. | | | | should be replaced by an authorized service |
| There was some work initially in mercury | | | | department. The vehicle can be driven after |
| switches but they did not work very well. Before | | | | deployment, but there will be no supplemental |
| MEMS, the primary system used todeploy airbags | | | | restraint system until a replacement has been |
| was called a "rolamite". A rolamite is a mechanical | | | | installed. |
| device, consisting of a roller suspended within a | | | | Air bags must inflate very rapidly to be effective, |
| tensioned band. As a result of the particular | | | | and therefore come out of the steering wheel |
| geometry and material properties used, the roller | | | | hub or instrument panel with considerable force, |
| is free to translate with very little friction or | | | | generally at a speed over 180 mi/h (290 km/h). |
| hysteresis. This device was developed at Sandia | | | | Because of this initial force, contact with a |
| National Laboratories. The rolamite and similar | | | | deploying air bag may cause injury. These air bag |
| macro-mechanical devices were used in air bags | | | | contact injuries, when they occur, are typically |
| until the mid-1990s when they were universally | | | | very minor abrasions or burns. The sound of air |
| replaced with MEMS. | | | | bag deployment is very loud, in the range of 165 |
| Most air bags are designed to automatically deploy | | | | to 175 decibels for 0.1 second. Hearing damage |
| in the event of a vehicle fire when temperatures | | | | can result in some cases. |
| reach 300 to 400 degrees Fahrenheit (150 to 200 | | | | More serious injuries are rare; however, serious or |
| °C). This safety feature helps to ensure that | | | | even fatal injuries can occur when someone is |
| such temperatures do not cause an explosion of | | | | very close to, or in direct contact with an air bag |
| the inflator unit within the air bag module. | | | | module when the air bag deploys. Such injuries |
| Today, airbag triggering algorithms are becoming | | | | may be sustained by unconscious drivers who are |
| much more complex. They try to reduce useless | | | | slumped over the steering wheel, unrestrained or |
| deployments (for example, at low speed, no | | | | improperly restrained occupants who slide forward |
| shocks should trigger the airbag to help reduce | | | | in the seat during pre-crash braking, and even |
| damage to the car interior in conditions where the | | | | properly restrained drivers who sit very close to |
| seat belt would be an adequate safety device) | | | | the steering wheel. Objects must never be |
| and to adapt the deployment speed to the crash | | | | attached to an air bag module or placed loose on |
| conditions. The algorithms are considered as very | | | | or near an air bag module, since they can be |
| valuable intellectual property. Experimental | | | | propelled with great force by a deploying air bag, |
| algorithms may take into account such factors as | | | | potentially causing serious injuries. |
| the weight of the occupant, the seat location, | | | | An unrestrained or improperly restrained occupant |
| seatbelt use, and even attempt to determine if a | | | | can be seriously injured or killed by a deploying air |
| baby seat is present. | | | | bag. The National Highway Traffic Safety |
| When there is a moderate to severe frontal | | | | Administration (NHTSA) recommends drivers sit |
| crash that requires the frontal air bag to deploy, a | | | | with at least 10 inches (254 mm) between the |
| signal is sent to the inflator unit within the air bag | | | | center of their breastbone and the center of the |
| module. An igniter starts a rapid chemical reaction | | | | steering wheel. Children under 12 should always be |
| generating primarily nitrogen gas (N2) to fill the air | | | | properly restrained in a rear seat. A rear-facing |
| bag making it deploy through the module cover. | | | | infant restraint must never be put in the front |
| Some air bag technologies use compressed | | | | seat of a vehicle with a front passenger air bag. |
| nitrogen gas while other technologies use various | | | | A rear-facing infant restraint places an infant's |
| energetic propellants. Propellants containing sodium | | | | head close to the air bag module, which can cause |
| azide (NaN3) were very common in early inflator | | | | severe head injuries or death if the air bag |
| designs. However, propellants containing sodium | | | | deploys. Modern cars include a switch to turn off |
| azide were widely phased out during the 1990s in | | | | the airbag system of the passenger seat, in which |
| pursuit of less expensive and less toxic | | | | case a child-supporting seat must be installed. |