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An air bag, A.K.A. a Supplementary Restraint System (SRS), an Air Cushion Restraint System (ACRS), or the Supplemental Inflatable Restraint (SIR) (reflecting the air bag system's intended role as a supplement to conventional restraints such as seatbelts) is a flexible membrane or envelope. Air bags are most commonly used for cushioning, in particular for rapid inflation in the case of an automobile collision. Air bags are designed to complement conventional restraints such as seatbelts and seatbelt pre-tentioners, not replace them. The number of lives saved by airbags is hard to pin down. One study, cited below, puts the number at just under 400 per year (6,000 total), and another study indicates that air bags reduce fatalities by 8% when seatbelts are worn. Airbags also greatly increase the efficiency of seat-belts, in some cases up to 50%.

History
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Invention
John W. Hetrick of Newport, Pennsylvania, USA invented the air bag in 1952 and patented his device the following year. Hetrick came up with the idea to help protect his own family using expertise from his naval engineering days. Throughout the years, the saving of many lives have been attributed to Hetrick's invention and highlighted in television shows such as Nova on PBS, or his winning of awards such as the Golden Gear award. There have been devices similar to airbags for airplanes as early as the 1940s, with the first patents filed for those devices in 1958. Early air bag system origins traces back to air filled bladders. These systems were large and bulky, and primarily consisted of compressed or heated air, compressed nitrogen gas (N2), freon, carbon dioxide (CO2), or a mixture of water and potassium (KH2O).

An American inventor Allen K. Breed then developed a key component for automotive use - the ball-in-tube sensor for crash detection. He marketed this innovation first in 1967 to Chrysler. A similar "Auto-Ceptor" crash-restraint, developed by Eaton, Yale & Towne Inc. for Ford was soon offered as an automatic safety system in the USA while the Italian Eaton-Livia company offered a variant with localized air cushions.

First automotive applications: an alternative to the seatbelt
The device was briefly available in the United States in the mid-1970s.

During this era, drivers were infrequent users of seat belts and a means of automatically offering seat belt-like levels of occupant protection to unbelted occupants in a head-on collision was felt to be a valuable innovation.

Ford built an experimental fleet of cars with airbags in 1971. General Motors followed with a fleet of 1,000 experimental vehicles in 1973, and these Chevrolet cars equipped with dual air bags were sold to the public through GM dealers two years later. GM called this the Air Cushion Restraint System. In the seventies GM cars had two-stage deployment similar to newer air bags.

Before these Chevrolets were sold, air bags were made available to the public in November 1973 when General Motors began offering dual air bags as an extra-cost option on several 1974 model full-size cars made by the Buick, Cadillac and Oldsmobile divisions. This system was known as the Air Cushion Restraint System.

The 1970s fleet of 10,000 airbag-equipped GM experienced seven fatalities. One is now suspected to have been caused by the airbag. The crash severity was only moderate and at the time a heart attack was suspected. The victim was cremated without autopsy.

The standard shoulder belts were removed on these cars, as they were designed to replace seat belts.

The market did not appear to reward this innovation. Air bags were first implemented by GM and Ford in low-volume production (in approximately 12,000 automobiles in the 1973–76 era), then abandoned by GM and Ford. The passenger side air bag on 1970s cars was located in the lower part of the dashpad and it also acted as a knee restraint. The lower part of the dash on the driver side was also different on cars with air bags as it was padded.

The development of air bags coincided with an international interest in vehicle road safety legislation. Some safety experts cautioned that mandating a particular technical solution, rather than a general occupancy safety standard, could rapidly become dated and might not be the most cost-effective approach. Motor manufacturers would tool up for a particular standard which could not easily be changed. This proved to be the case, and as countries successively mandated seat belt restraints there was less emphasis placed on other designs for several decades.

Rebirth: supplemental restraint
Car designers have moved on from the initial view of the air bag as a seat belt replacement. Automobile air bags are now designed and sold as Supplemental Restraint Systems (SRS).

In 1980, Mercedes-Benz re-introduced the air bag in Germany that it had patented in 1971 as an option on its high end S-Class (W126), which also offered such other exotic options as hydropneumatic suspension. In the Mercedes system, the sensors would tighten the seat belts and then deploy the air bag on impact. The air bag was thus no longer marketed as a means of avoiding seat belts, but as a way to obtain an extra margin of occupant safety.

In 1987 the Porsche 944 turbo became the first car in the world to have driver and passenger air bags as standard equipment. The Porsche 944 and 944S had this as an available option. This year also saw the first air bag in a Japanese car, the Acura Legend.

Audi was relatively late to offer airbag systems on a broader scale; until the 1994 model year, for example, the 80/90, by far Audi's bread-and-butter model, as well as the 100/200, did not have air bags in their standard versions. Instead, the German automaker until then relied solely on its proprietary Procon-ten belt tensioner restraint system.

Air bags became common in the 1980s, with Chrysler and Ford introducing them in the mid-1980s; the former made them standard equipment across its entire line in 1990.

In Europe, air bags were almost unheard of on family cars until the early 1990s. The first European Ford to feature an air bag was the facelifted Escort MK4 in 1992; within a year, the entire Ford range had at least one air bag as standard. By the mid 1990s, European market leaders such as Vauxhall/Opel, Rover, Peugeot, Renault and Fiat had included air bags as at least optional equipment across their model ranges. By the end of the decade, it was very rare to find a mass market car without an air bag, and some late 1990s products - such as the Volkswagen Golf MK4 - also featured side airbags. The Peugeot 306 was a classical example of how commonplace airbags became on mass market cars during the 1990s. On its launch in early 1993, most of the range did not even have driver air bags as an option. By 1999, however, side airbags were available on several variants.

During the 2000s, side air bags were commonplace on even budget cars such as the smaller-engined versions of the Ford Fiesta and Peugeot 206, and curtain air bags were also becoming regular features on mass market cars. The Toyota Avensis, launched in early 2003, was the first mass market car to be sold in Europe with a total of nine airbags.

How They Work
The design is conceptually simple - accelerometers trigger the ignition of a gas generator propellant to inflate a nylon fabric bag very rapidly, which reduces the deceleration experienced by the passenger as they come to a stop in the crash. The bag has small vent holes to allow the propellant gas to be (relatively) slowly expelled from the bag as the occupant pushes against it.

Many vehicles have several accelerometers and gyroscopes to help sense various impacts and other crash events. The different signals from these sensors are fed into a microprocessor which determines the angle of impact and the severity of the crash. Depending on the result of these calculations, the microprocessor will deploy various restraint devices such as seat belt pre-tensioners and/or air bags. Each restraint device is typically activated with one or more pyrotechnic devices commonly called an initiator or squib. The squib activates with a current pulse between 1 to 3 amperes in less than 2 milliseconds. The squib has a fuse-like conductor inside that heats the pyrotechnic material tightly packed around the conductor. When the conductor becomes hot enough, it ignites the pyrotechnic material which in turn generates hot gas. In a seat belt pre-tensioner, this hot gas is used to drive a piston that pulls the slack out of the seat belt. In an air bag, the hot gas from the squib is used to ignite solid propellent inside the air bag inflator. The burning propellent generates inert gas which rapidly inflates the air bag in approximately 20 to 30 milliseconds. An air bag must inflate quickly in order to be fully inflated by the time the forward-traveling occupant reaches its outer surface. Typically, the decision to deploy an airbag in a frontal crash is made within 15 to 30 milliseconds after the onset of the crash, and both the driver and passenger airbags are fully inflated within approximately 60-80 milliseconds after the first moment of vehicle contact. If an air bag deploys too late or too slowly, the risk of occupant injury from contact with the inflating air bag may increase. Since more distance typically exists between the passenger the instrument panel, the passenger air bag is larger and requires more gas to fill it.

During the crash, the force of the occupant going forward (toward the point of impact) forces the gas out of the air bag through its vent holes, thereby causing it to deflate. It is the deflation of the air bag that provides the cushioning effect to the occupants as they continue to be thrown forward toward the point of impact. The air bag's volume and the size of the vents in the bag are tailored to each vehicle type to dissipate the occupant's energy over time and distribute the deceleration forces across a larger portion of the occupant's body (compared to a seat belt alone).

Front air bags are not designed to deploy in side impacts, rear impacts or rollover crashes. Since air bags deploy only once and deflate quickly after the initial impact, they will not be beneficial during a subsequent collision. Safety belts help reduce the risk of injury in many types of crashes. They help to properly position occupants to maximize the air bag's benefits and they help restrain occupants during the initial and any following collisions.

In vehicles equipped with a rollover sensing system, accelerometers and gyroscopes are used to sense the onset of a rollover event. If a rollover event is determined to be imminent, side-curtain air bags are deployed to help protect the occupant from contact with the side of the vehicle interior, and also to help prevent occupant ejection as the vehicle rolls over.

Functional Considerations
Air bags are typically designed to deploy in frontal and near-frontal collisions, which are comparable to hitting a solid barrier at approximately 13-23 km/h (8-14 mph). Roughly speaking, a 23 km/h (14 mph) barrier collision is equivalent to striking a parked car of similar size across the full front of each vehicle at about 45 km/h (28 mph). This is because the parked car absorbs some of the energy of the crash, and is pushed by the striking vehicle. Unlike crash tests into barriers, real-world crashes typically occur at angles, and the crash forces usually are not evenly distributed across the front of the vehicle. Consequently, the relative speed between a striking and struck vehicle required to deploy the air bag in a real-world crash can be much higher than an equivalent barrier crash.

Because air bag sensors measure deceleration, vehicle speed and damage are not good indicators of whether an air bag should have deployed. Occasionally, air bags can deploy due to the vehicle's undercarriage violently striking a low object protruding above the roadway surface. Despite the lack of visible front-end damage, high deceleration forces may occur in this type of crash, resulting in the deployment of the air bag.

Nearly all air bags are designed to automatically deploy in the event of a vehicle fire when temperatures reach 150-200 °C (300-400 °F). This safety feature, often termed auto-ignition, helps to ensure that such temperatures do not cause an explosion of the entire air bag module.

From the onset of the crash, the entire deployment and inflation process is faster than the blink of an eye (about 0.2 seconds). Air bags deploy in about 0.05 seconds. Because a vehicle changes speed so fast in a crash, air bags must inflate rapidly if they are to help reduce the risk of the occupant hitting the vehicle's interior.

Sensor
The air bag sensor is a MEMS accelerometer, which is a small integrated circuit chip with integrated micromechanical elements. The microscopic mechanical element moves in response to rapid deceleration, and this motion causes a change in capacitance, which is detected by the electronics on the chip, which then sends a signal to fire the airbag. The most common MEMS accelerometer in use is the ADXL-50 by Analog Devices, but there are other MEMS manufacturers as well.

There was some work initially in mercury switches but they did not work very well. Before MEMS, the primary system used to deploy air bags was called a "rolamite". A rolamite is a mechanical device, consisting of a roller suspended within a tensioned band. As a result of the particular geometry and material properties used, the roller is free to translate with very little friction or hysteresis. This device was developed at Sandia National Laboratories. The rolamite and similar macro-mechanical devices were used in air bags until the mid-1990s when they were universally replaced with MEMS.

Today, air bag triggering algorithms are becoming much more complex. They try to reduce unnecessary deployments (for example, at low speed, no shocks should trigger the air bag to help reduce damage to the car interior in conditions where the seat belt would be an adequate safety device) and to adapt the deployment speed to the crash conditions. The algorithms are considered as very valuable intellectual property. Experimental algorithms may take into account such factors as the weight of the occupant, the seat location, seatbelt use, and even attempt to determine if a baby seat is present.

When there is a moderate to severe frontal crash that requires the frontal air bag to deploy, a signal is sent to the inflator unit within the air bag module. An igniter starts a rapid chemical reaction generating primarily nitrogen gas (N2) to fill the air bag making it deploy through the module cover. Some air bag technologies use compressed nitrogen or argon gas with a pyrotechnic operated valve ("hybrid gas generator"), while other technologies use various energetic propellants. Propellants containing sodium azide (NaN3) were very common in early inflator designs. However, propellants containing sodium azide were widely phased out during the 1990s in pursuit of more efficient, less expensive and less toxic alternatives.

Gas Generator
The azide-containing pyrotechnic gas generators contain a substantial amount of the propellant. The driver-side air bag may contain a canister which is 5.0 cm (2") in diameter, 3.8 cm (1.5") long, and contains about 50 grams of sodium azide. The passenger side container is 15 cm (6") long and contains 200 grams of sodium azide. The incomplete combustion of the charge due to rapid cooling leads to production of carbon monoxide (CO) and nitrogen(II) oxide as reaction byproducts.

The alternative propellants may incorporate, for example, a combination of nitroguanidine, phase-stabilized ammonium nitrate (NH4NO3) or other nonmetallic oxidizer, and a nitrogen-rich fuel different than azide (eg. tetrazoles, triazoles, and their salts). The burn rate modifiers in the mixture may be an alkaline metal nitrate (NO3-) or nitrite (NO2-), dicyanamide or its salts, sodium borohydride (NaBH4), etc. The coolants and slag formers may be eg. clay, silica, alumina, glass, etc. Other alternatives are eg. nitrocellulose based propellants (which have high gas yield but bad storage stability, and their oxygen balance requires secondary oxidation of the reaction products to avoid buildup of carbon monoxide), or high-oxygen nitrogen-free organic compounds with inorganic oxidizers (e.g., di or tricarboxylic acids with chlorates (ClO3-) or perchlorates (HClO4) and eventually metallic oxides; the nitrogen-free formulation avoids formation of toxic nitrogen oxides).

Additional Components to Improve Performance and Safety
Many advanced air bag technologies are being developed to tailor air bag deployment to the severity of the crash, the size and posture of the vehicle occupant, belt usage and how close that person is to the air bag module. Many of these systems use multi-stage inflators that deploy less forcefully in stages in moderate crashes than in very severe crashes. Occupant sensing devices let the air bag diagnostic unit know if someone is occupying a seat in front of an air bag, whether the person is an adult or a child, whether a seat belt or child restraint is being used and whether the person is forward in the seat and close to the air bag module. Based on this information and crash severity information, the air bag is deployed at either a high force level, a less forceful level or not at all.

Adaptive air bag systems may utilize multi-stage air bags to adjust the pressure within the air bag. The greater the pressure within the air bag, the more force the air bag will exert on the occupants as they come in contact with it. These adjustments allow the system to deploy the air bag with a moderate force for most collisions; reserving the maximum force air bag only for severest of collisions. Additional sensors to determine the location, weight or relative size of the occupants may also be used. Information regarding the occupants and the severity of the crash are used by the electronic control unit to determine whether air bags should be suppressed or deployed, and if so, at various output levels.

Air bags in certain car models deploy twice, for two crashes; it first deploys and deflates, and then re-inflates upon a subsequent collision.

Side-Impact Air Bags
Side-impact air bags are a category of air bag usually located in the roof rail above the door and inflate next to the windows, or are mounted inside the door panel.

These are specifically designed to reduce the risk of head injury and/or help keep the head and upper body inside the vehicle. Some vehicles are now being equipped with a different types of designs to help reduce injury and ejection from the vehicle in rollover crashes. Side-impact air bags have been said to reduce brain injury or fatalities by up to 45% in a side impact with an SUV. These air bags come in various forms (e.g., tubular, curtain, door-mounted) depending on the needs of the application.

The Swedish company Autoliv AB, was granted a patent on side air bags, and they were first offered as an option on the 1995 model year Volvo 850, and as standard equipment on all Volvo cars made after 1995. The first head protection air bags were included as standard equipment on the 1997 Volvo S70.

In 1998 the Volvo S80 was first given curtain airbags to protect both front and rear passengers. They were then were made standard equipment on all new Volvo cars from 1998. The Volvo Curtain Airbag design is now considered to be superior by most car manufacturers and is the one mostly used; in some cars it has been designed to stay inflated for rollover crashes.

The Volvo XC90 and some other SUVs and MPVs have a long inflatable curtain air bag that protects all 3 rows of seats.

In 1998 the BMW 7-series and E39 5-series were fitted with a smaller driver's head protection, called an "air tube". However, if the driver is taller or shorter than usual the tube may be found wanting; the air tube system was also not designed to protect rear seat passengers (unlike the Volvo design). It was suspected that this system was provided merely in order to help score well on NCAP testing in Europe, which only tests front seat passengers for head protection.

Level of Improved Protection
Air bags supplement the safety belt by reducing the chance that the occupant's head and upper body will strike some part of the vehicle's interior. They also help reduce the risk of serious injury by distributing crash forces more evenly across the occupant's body. Curtain air bags help to keep all parts of the occupant inside the vehicle.

One recent study concluded that as many as 6,000 lives have been saved as a result of air bags.

Level of Increased Injury Risk
Once an air bag deploys, deflation begins immediately as the gas escapes through vent(s) in the fabric (or, as it's sometimes called, the cushion) and cools. Deployment is frequently accompanied by the release of dust-like particles and gases in the vehicle's interior (called effluent). Most of this dust consists of cornstarch or talcum powder, which are used to lubricate the air bag during deployment. Newer designs produce effluent primarily consisting of harmless talcum powder/cornstarch and nitrogen gas (about 80% of the air we breathe is nitrogen). In older designs using an azide-based propellant (usually NaN3), varying amounts of sodium hydroxide nearly always are initially present. In small amounts this chemical can cause minor irritation to the eyes and/or open wounds; however, with exposure to air, it quickly turns into sodium bicarbonate (baking soda). However, this transformation is not 100% complete and invariably leaves residual amounts of hydroxide ion from NaOH. Depending on the type of air bag system, potassium chloride (a table salt substitute) may also be present.

For most people, the only effect the dust may produce is some minor irritation of the throat and eyes. Generally, minor irritations only occur when the occupant remains in the vehicle for many minutes with the windows closed and no ventilation. However, some people with asthma may develop an asthmatic attack from inhaling the dust. With the onset of symptoms, asthmatics should treat themselves as advised by their doctor, then immediately seek medical treatment.

Air bags involve the extremely rapid deployment of a large cushion. While air bags can protect a person under the right circumstances, they can also injure or kill. To protect occupants not wearing seat belts, U.S. air bag designs trigger much more quickly than air bags designed in other countries. As seat belt use in the U.S. climbed in the late 1980s and early 1990s, auto manufactures were able to adjust their designs. Today all air bag control units recognize if a belt is used and set the trigger time accordingly.

Newer air bag designs trigger at a lesser speed; nonetheless, passengers must remain at least 25 centimeters (10 in) from the panel from which frontal air bags deploy to avoid injury from the bag in a crash. While driving, a driver must be seated so that the center of the chest remains 25 centimeters (10 in) from the center of the steering wheel hub. The design of side air bags means occupants of a vehicle must not lean against the inside of the car window or doors, the pillars or place objects between themselves and the side of the vehicle. Despite many cars still featuring hooks on passenger assist grips, these can not be used when side thorax, and in particular, curtain air bags are fitted.

Injuries such as abrasion of the skin, hearing damage (from the sound during deployment), head injuries, eye damage for spectacle wearers and breaking the nose, fingers, hands or arms can occur as the air bag deploys.

In 1990, the first automotive fatality attributed to an air bag was reported, with deaths peaking in 1997 at 53 in the United States. TRW produced the first gas-inflated air bag in 1994, with sensors and low-inflation-force bags becoming common soon afterwards. Dual-depth (also known as dual-stage) air bags appeared on passenger cars in 2005. By that time, deaths related to air bags had declined, with no adults deaths and two child deaths attributed to airbags that year. Injuries remain fairly common in accidents with an air bag deployment.

Air bags must inflate very rapidly to be effective, and therefore come out of the steering wheel hub or instrument panel with considerable force, generally at a speed of about 97.8 m/s (220 mph). Because of this initial force, contact with a deploying air bag may cause injury. These air bag contact injuries, when they occur, are typically very minor abrasions or burns. The sound of air bag deployment is very loud, in the range of 165-175 dB for 0.1 second. Hearing damage can result in some cases.

More serious injuries are rare; however, serious or even fatal injuries can occur when someone is very close to, or in direct contact with an air bag module when the air bag deploys. Such injuries may be sustained by unconscious drivers who are slumped over the steering wheel, unrestrained or improperly restrained occupants who slide forward in the seat during pre-crash braking, and even properly restrained drivers who sit very close to the steering wheel. Objects must never be attached to an air bag module or placed loose on or near an air bag module, since they can be propelled with great force by a deploying air bag, potentially causing serious injuries.

The increasing use of air bags may actually make rescue work for firefighters, EMS and police officers more dangerous. Every first responder should be properly trained on how to safely deactivate air bags or be aware of the potential hazards. Removing the car battery does not deactivate the air bags.

Improvements in sensing and gas generator technology have allowed the development of second generation air bags systems that can adjust their deployment parameters to size, weight, position and restraint status of the occupant. These improvements have demonstrated a reduced injury risk factor for small adults and children who had an increased risk of injury with first generation air bag systems.

Safe use of air bags
An unrestrained or improperly restrained occupant can be seriously injured or killed by a deploying air bag. The National Highway Traffic Safety Administration (NHTSA) recommends drivers sit with at least 25 cm (10") between the center of their breastbone and the center of the steering wheel. Children under the age of 13 years should always be properly restrained in a rear seat. A rear-facing infant restraint must never be put in the front seat of a vehicle with a front passenger air bag. A rear-facing infant restraint places an infant's head close to the air bag module, which can cause severe head injuries or death if the air bag deploys. Modern cars include a switch to turn off the air bag system of the passenger seat, in case a child-supporting seat is used there.

Smoking a pipe should be avoided while driving. If the air bag inflates and hits the pipe, it is likely to be fatal, even if the crash is moderate.

NHTSA Rule Changes
On July 11, 1984, the U.S. government required cars being produced after April 1, 1989 to have driver's side air bags or automatic seat belts (the automatic seat belt was a technology, now discarded, that "forced" motorists to wear seatbelts). Airbag introduction was stimulated by the U.S. DOT. However, airbags were not mandatory on light trucks until 1995.

In 1998, dual front airbags were mandated by the National Highway Traffic Safety Administration (NHTSA), and de-powered, or second-generation air bags were also mandated. This was due to the injuries caused by first-generation air bags that were designed to be powerful enough to restrain people who were not wearing seatbelts.

Elsewhere
In the United Kingdom and most other developed countries there is no direct legal requirement for new cars to feature airbags. Instead, the compulsory Euro NCAP vehicle safety rating encourages manufacturers to take a comprehensive approach to occupant safety; a good rating can only be achieved by combining air bags with other safety features. Thus almost all new cars now come with at least two airbags as standard.