Airbags and Crash Injury: Effectiveness and Risk
by Jeffrey Runge, MD, FACEP
Since the invention of the airbag in 1952, safety engineers have recognized the benefits of deploying a distensible mass to cushion the blow during a frontal car crash, increasing the distance over which the body decelerates. But only since the widespread implementation of the airbag in the mid 1980s have the inherent risks of the safety device been recognized. As more and more of the U.S. automobile fleet becomes equipped with airbags, physicians will more often encounter the product of airbag-human interaction. Emergency physicians should be on the cutting edge of this knowledge.
On November 18, 1997, the U.S. Department of Transportation announced a new regulation to allow on-off switches to be added to existing automobiles and pickup trucks meeting certain conditions. This new regulation was created recognizing that conditions exist in which the risks of a deploying airbag outweigh the benefits to the occupant. Citizens were asked to consult their physicians as to whether they may be at increased risk in the event of airbag deployment. Therefore, it behooves physicians to become very familiar with the biomechanics of airbag deployment. While the scope of this article is insufficient to provide detailed information about airbag-human interaction, there are certain things every emergency physician should know.
Airbags constitute only a part of an occupant restraint system and are not as effective without lap and shoulder belts. This restraint system, together with safety features in automobile construction technology, such as collapsing steering columns, crumple zones in the body, knee boosters, strengthened roof pillars, and other design enhancements, have produced a much safer automobile than in previous decades. For these reasons, motor vehicle crash fatalities have decreased from an annual rate of near 50,000 in the early 1970s to a little more than 40,000 today. Concurrently, there has been a proliferation of drunk-driving laws and, until recently, a reduction in speed on our highways. Highway technology has also improved with energy-absorbing barriers, better road materials, and better lighting. Further gains in fatality and injury reduction will occur through a combination of engineering enhancements, better laws and enforcement, and public education.
Because of the physical characteristics of deployment, it is absolutely mandatory that seatbelts be worn properly whenever an automobile is in motion. Emergency physicians should make seatbelt wearing a standard part of every history taken and discharge instruction given. Airbags are designed to deploy when the change in velocity (delta V) during a crash reaches 10-12 mph. This is the velocity at which most crashes occur, while most tow-away crashes happen at 15-19 mph and less than 5% of crashes occur at a delta V of more than 30 mph. At the speed of most crashes, a properly worn lap and shoulder belt is sufficient to protect people from fatality and serious injury, particularly in frontal, non-rollover crashes. Thus, the current airbag deployment threshold at relatively low delta V crashes may offer no benefit to the belted occupant but may prevent head contact with the steering wheel or dashboard in the unbelted. At high delta V crashes, the airbag plays a much more important role by increasing the distance over which the head decelerates, subjecting the brain to far less G force and preventing the head strikes that occur even in belted drivers at this velocity.
The speed of bag deployment is also an issue. In order to protect unbelted occupants, airbags must be fully deployed in less than the time it takes for an unbelted occupant to reach the steering wheel or dashboard in a crashunder 200 millisecondsabout the time it takes to blink an eye. The energy necessary to fill the airbag with gas in that amount of time is substantial, and airbag injuries are produced as that energy of inflation is transmitted to the human body in close proximity to the steering wheel or dashboard. Serious injuries to the upper extremity can occur even when the belted occupant reaches out toward the dashboard during deceleration or when the driver’s hands or forearms are in contact with the airbag assembly in the steering wheel. Future airbag technology will be aimed at producing "smart" airbags that vary the speed of deployment depending on whether the occupant is belted, the size of the occupant, and the proximity of the occupant to the airbag assembly. Therefore, it is clear why young children are at increased risk from the airbag and provide the basis for physician’s counseling about the dangers of airbags to patients with children.
The most critical issue to be addressed regarding airbags and children is the rear-facing infant seat, appropriate for children younger than 1 year old. Rear-facing infant seats placed in the front seat are in such close proximity to the airbag that, during a crash, the inflating airbag contacts the back of the infant seat with maximum force, and that force is transmitted through the infant seat into the head of the infant. This results in severe brain injuries, atlanto-occipital dislocations, and other serious internal injuries. It is absolutely critical that parents of infants be advised never to put a rear-facing infant seat in the front seat of a car equipped with a passenger-side airbag. Physicians who care for infants should bring this issue to the attention of parents as part of any patient encounter.
The second concern is the young child who, by virtue of size, cannot be properly restrained on the passenger side using a lap and shoulder belt. Children tend to sit forward on the seat, even when belted, because they are more comfortable when their legs bend over the front of the seat, and they usually prefer to look out the windows. Although the seat belt may protect them from dashboard or windshield contacts during a crash, the airbag is in a position that may be fatal to the child sitting close to the dash, restrained by lap belt only, or unrestrained. This situation may result in serious injuries to the brain, spinal column, and cord, including atlanto-occipital dislocation. Even children riding in age-appropriate booster seats currently on the market are not necessarily out of danger. Therefore, all children younger than 12 years of age should be seated in the back seat and properly belted. Age may not be the most appropriate measuring stick; any child who is not tall enough to sit comfortably in the passenger seat (including all children under 55 inches in height) with the lap-shoulder belt fitting properly, should be seated only in the back.
Airbags have also injured adults while deploying, but serious injuries to the head, neck, and torso occur almost exclusively in people who are not properly belted. Upper extremity injuries may occur when people are properly belted due to the position of the extremity pre-crash during the braking phase. Other injuries that are more frequently seen have to do with airbag contact after full deployment. In general, these injuries are not serious and are limited to swollen lips, skin abrasions, and corneal abrasions. These injuries are generally of less severity than would be seen from the head striking the steering wheel, and they are managed conservatively.
There is another issue of which EMS medical directors should be aware. There is often an unwarranted fear of fire after the airbag deploys. Frequently, patients are removed hastily from crashed cars because of "smoke" in the passenger compartment. Although there may be a small amount of smoke released by the detonator and inflation medium, the "smoke" is usually the talc-cornstarch lubricant in which the bag is packed for deployment. There have been case reports of bronchial spasm induced by this lubricant, but it does not cause pneumonitis or burns.
Although the seriousness of the more than 80 deaths attributed to airbags should not be minimized, one must place this number in perspective with regard to those who were spared death or serious injury by virtue of the presence of an airbag. The Insurance Institute for Highway Safety estimates approximately 2500 lives saved by the airbag since 1985, and the reduction of serious injury to the brain or torso is much higher. The National Highway Traffic Safety Administration estimates that the airbag plus lap-shoulder belt provide about a 60% reduction in overall injury risk, compared with a 49% reduction in risk for the lap-shoulder belt alone. Thus, the value of the system is realized when the whole system of seat belt and supplemental restraint system is used. Turning off the airbag, except in particular circumstances, will increase injury and fatality risk.
If your patients have questions about whether they should request an airbag turnoff switch, they should be informed that they are much safer with an airbag than without, as long as they buckle their lap and shoulder belts. This includes patients with pregnancy, recent sternotomy, breast implants, cataract surgery with intraocular lenses, and virtually any other medical condition. Short stature individuals should position themselves so that they are 10 inches or more from the steering wheel, the recommended safe distance from the deploying airbag. The conditions under which the on-off switch may be beneficial are: drivers with achondroplastic dwarfism; children who must be transported in the front seat because the vehicle has no back seat; a rear-facing infant seat that must be placed in the front because of a medical condition mandating constant monitoring; and when there are insufficient numbers of rear seats for all children who must be transported in that vehicle. There will be no verification process by the government; only a signed affidavit is necessary to have the on/off switch installed. Hopefully, patients will ask their physicians for an opinion about whether the on-off switch would be beneficial. Available data have made it clear that individuals who can position themselves 10 inches from the bag assembly and can seat children in the back seat would be ill- advised to turn off their airbags.
Airbags:
a. obviate the need for seat belts.
b. obviate the need for shoulder, but not lap, restraints.
c. are hazardous to infants in rear-facing infant seats placed in the front passenger seat.
d. contain a talc-cornstarch material that often causes pneumonitis or burns.
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