Air bags were introduced to reduce serious injuries and deaths in automobile crashes, especially in those individuals not using seat and lap belts. They are intended to provide protection only in frontal crashes and to be used in conjunction with seat belts. Compared with lap-shoulder belts, air bags are significantly less effective. Thus, airbags alone have an estimated effectiveness of 14% in reduction of fatalities in drivers in crashes of all types compared with 45% for lap-shoulder belts used alone. The estimated reduction in fatalities when used together is 50%.7
To perform their function, air bags should fill as much of the space as possible between the driver and the steering wheel or the passenger and the dashboard. In addition, the air bag must deploy as rapidly as possible. As with any life-saving device or drug, there is the potential for adverse side effects. Thus, air bags can produce injury, and, on rare occasion, death.
There is a wide variance in the design of air bags including:
• Collision speed that triggers deployment
• Speed of deployment
• Distance of extension
• Physical characteristics of the airbag, etc.
Deployment of airbags occurs when crash sensors detect an impact equivalent to hitting a solid barrier at 10-15 mph.7 There is some thought that the threshold for deployment is too low, and that there is no risk of severe injury or fatalities to drivers until impact speed is approximately 30 kph (18 mph). Some suggest that 18 mph should be used for the threshold for deployment, at least for belted individuals.13
Some vehicles have a higher threshold than the 10-15 mph while others have dual thresholds. An example of dual threshold deployment is found in Mercedes automobiles, and has been so for driver airbags since 1988 and passenger air bags since 1989.13 The threshold for deployment is 12 mph if the passenger is unbelted and 18 mph if belted.
Inflation of the air bag is usually by a pyrotechnic device with production of gas. The velocity of deployment ranges from 100 to 200-plus mph, with older air bags deploying faster than newer. Deployment time is at least 30 milliseconds. Airbags can be tethered or untethered. In the former, tethers control the excursion of the bag toward the occupant as well as the shape. The distance that the air bag can travel from wheel to driver can range from 12 to 20 inches, with the untethered bags traveling farther. Newer air bags will be less powerful and able to modify the amount of inflation depending on the size of the driver or passenger. Side-impact airbags are smaller than front-impact bags and must inflate faster.
Just like seat belts, air bags can cause injuries, but, unlike seat belts, the injuries can be immediately lethal. Deaths are usually associated with women of small stature and children below the age of 13 years, especially when the children are unrestrained or out of position. Rear-facing infant or child restraints should never be used in front seats, as they place the child's head and body very close to the air bag housing. Infants in the front seat, in rear-facing infant seats, have predominantly craniocerebral injuries. Drivers seated too close to the steering wheel (less than 10 inches) can be seriously injured or killed by deploying air bags. Short drivers are injured more frequently because they must sit closer to the steering wheel to reach the gas and brake pedals.
In some vehicles, e.g., Mercedes, there are "pre-tensioners" that are activated by the same sensor system that activates the air bag. Pre-tensioners pull the slack out of the shoulder strap before the airbag deploys, pulling wearers back in the seat before they begin to move forward, reducing the subsequent force of impact between the air bag and the person. This reduces the likelihood of air bag injury.
Fatal injuries ascribed to airbag deployment include cervical spine dislocations or fractures, basal skull fractures, and injuries to thoracic and abdominal viscera. The injured person may show characteristic abrasions of the anterior neck and under surface of the jaw (Figure 9.12). Abrasions of the chest might also be present. In one relatively minor accident reviewed by the authors, there was blunt trauma to the right internal carotid artery, just
distal to the bifurcation of the common carotid artery. At the scene, the driver appeared somewhat disoriented. She was seen at a hospital and sent home, only to be re-admitted 12 hours later, with a history of convulsions. She was alert, oriented, and could speak but had left-sided hemiparesis. An angio-graph showed complete occlusion of the right internal carotid artery caused by thrombosis. She gradually deteriorated over the next few days, dying of widespread infarction of her right cerebral hemisphere. The traumatic injury of the internal carotid artery was demonstrated at autopsy.
By November 2000, the National Highway Traffic Safety Association (NHTSA) had documented 169 deaths ascribed to airbags since 1990.14 One hundred involved children 12 years of age or younger. Ninety-eight were killed by passenger bags; two by driver's air bags. Of the 98 children fatally injured by deployment of passenger air bags:
• 69 were unrestrained or improperly restrained
• 18 were in rear-facing child safety seats
• 5 were in forward-facing child safety seats
• 6 were wearing lap and shoulder belts
Of the 69 adults, 63 were drivers and six were passengers. Only 18 of the drivers were belted, and 23 of the drivers and three of the passengers were 62 inches or less in height.
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