Injuries,death and vehicle airbag deployment

Introduction

Airbags work by cushioning impact and controlling the force of impact of occupants against internal vehicle structures. They deploy within 10 ms of impact, utilising an exothermic chemical reaction that produces hot gas that rapidly inflates the bag. ¹ This creates a cushioned impact point, as well as absorbing energy and decelerating the occupant. The area protected from injury by airbags ranges from just the torso to both the head and torso. Head-and-torso side airbags have reduced the risk of death by almost 50%, ² and so airbags are now mandatory in all new vehicles in Europe, the USA and Australia. ³

Critically, the time between crash and occupant impact with internal structures is brief, and prior to airbag implementation, occupants would collide with extreme force into the dashboard, windshield and steering wheel, often resulting in severe head and chest injuries. ⁴ Airbags have decreased the severity of head injury during frontal collisions. ⁵ Side airbags are more commonly used in combination with front airbags, and they deploy from either the side pillar or the outer occupant seat. ² Side airbags are, however, constrained during deployment, with limited expansion space given the narrowness of the space between the seat, occupant and side car door.

Mechanisms of injury

Although the trauma reduction associated with airbags has been well established, airbags can cause hyper-extension of the neck, resulting in atlanto-occipital fractures, thoracic injuries or aortic dissection, all of which have proven fatal.^3,6,7 Injuries such as these occur in the second phase of airbag deployment when the inflating airbag has pushed body parts away from the sternum. ⁸ These injuries have been associated with spontaneous, unwarranted, mistimed or excessive force during deployment where the airbag action has caused greater injury than the impact otherwise would have.^5,7

A higher risk of injury is also associated with particular positions and manners of occupant seating, ⁸ for example in unrestrained or improperly restrained occupants, especially during low-speed collisions where the airbag punch-out force is disproportionate to the collision force.^3,5 Additional compounding factors include driving smaller vehicles and sustaining greater impacts. ³ A difficulty in attempting to determine the overall success of airbags previously was in determining how many vehicles were in fact fitted with them.^2,5

Children and occupants of shorter stature are more often seriously injured by airbags due to their relative position to the airbag trajectory. ⁷ Airbags are designed to deploy outward from the steering wheel with the direct force focused on the sternum. The improper application of force in deployment due to the difference in position has led to more atlanto-occipital, cervical and thoracic spinal fractures which is further complicated by increasing age with its associated physical vulnerabilities.^4,7 Hoye also suggested that women are at higher risk given that they are generally of shorter stature and thus sit closer to the steering wheel and dashboard. ³

Defective airbags

Cases of defective airbags have also been reported. These have exploded on deployment, launching metal projectiles towards the occupant’s head and neck. In one such case, fragments penetrated an occupant’s neck, causing lacerations and transection of major neck structures, including the trachea, right carotid artery and part of the cervical vertebrae. The crash itself was not considered to have been capable of causing serious injury, and so the fatality was exclusively the result of airbag malfunction. ⁴ This case has been inquested and is part of a global recall of Takata airbags.^9,10

Fatalities

The highest number of fatalities caused by airbags have been in the USA, mainly due to first- and second-generation airbags being substantially larger (70 L) and more powerful compared to western European 30 L models.^3,7 By January 2009, 296 deaths in the USA were contributed to by front airbags, which included 191 children, 92 drivers and 13 adult passengers. ¹¹ Initial testing methods for airbags in the USA that utilised an unbelted rigid-barrier test with high-impact velocity led to first-generation airbags being extremely large and powerful. ³ In 1997, crash test conditions were changed to use a 25 mph sled test, ³ which resulted in successive decreases in fatality numbers, although this was also influenced by an increase in restraint wearing and an overall improvement in road-safety awareness. The original significantly more powerful US airbags that had been designed for a higher collision performance meant that belted occupants involved in low-speed collisions were being exposed to high-energy inflations, resulting in numerous unnecessary injuries and fatalities. Whilst the European design was modelled around belted low-speed collisions, these countries are not without airbag fatalities, with the first recorded case occurring in the UK in 2000. ⁷

The balance between collision force and airbag-deployment force remains contentious. For example, while smaller lower-powered airbags reduce airbag-related injury in low-speed collisions, they may offer little protection in high-impact collisions. Conversely, high-powered airbags may cause injuries that otherwise would not have occurred during collisions at lower velocity. ³ The newest generation of airbags include collision force and occupant weight sensors to mediate the deployment power.

Non-lethal injuries

Non-lethal injuries may also be associated with airbag deployment and inflation. These injuries often occur in the head and neck region as well as to the upper limbs, and include facial fractures, lacerations, contusions, otologic injury such as tympanic membrane rupture and ocular injuries such as a ruptured globes and retinal haemorrhage.^4,7 The pattern of injuries has changed with each generation of airbag, with occupants being less likely to sustain facial or cranio-cerebral trauma from third-generation devices.^3,12 Injuries can also be caused by chemical burns from corrosive substances and by thermal burns from contact with airbag exhaust vents. ¹ It is also recognised that seat belts may contribute to other injuries, including vascular transection, airway compromise and fractures.^13–15

Conclusion

Airbags are an integral part of the safety system within modern vehicles and, when operating correctly, can not only save lives but also reduce injury. However, as with any safety device, misuse or incorrect performance may instead lead to injury. Furthermore, when significant forces are involved in a crash, the forces may surpass the operating ability of the safety device. ³ Thus, the autopsy assessment of vehicle crash victims should be conducted with an appreciation and understanding of the mechanism of action of vehicle airbags and an awareness of the range of associated lethal and non-lethal injuries that may occur.