HEAD PROTECTION - HELMETS AND AIR-BAGS
According to the U.S. Department of Health and Human Services (1989), over two million U.S. citizens (approximately 1% of the population) experience some form of head injury each year. One in four of these injuries require hospitalization, and approximately 1 in 25 result in death. These statistics alone indicate the importance of head impact safety.
Biomechanics of Head Injury
Head injuries can be produced either by forces applied directly to the head or rapid accelerations experienced by the head.
In the first case, external concentrated forces applied directly to the head may produce local injuries to any or all of the scalp, skull, or brain. Brain injury due to local impact can occur in one of three ways: penetration of the skull, fracture of the skull, or excessive deformation of the skull (without fracture).
In the second case, rapid accelerations to the head are commonly a combination of linear and rotational motions. This rapid motion can cause the brain to move relative to the interior skull surface. This could result in damaged connecting blood vessels as well as deformation of the brain as it contacts or tears away from irregular interior skull surfaces and membranes.
Even with today's advanced medical knowledge and technology, injuries to brain structures are still currently non-reversible. Therefore, in the transportation industry, the goal of head protection systems is to minimize head impact forces and accelerations to levels below injury thresholds. As a result, the science of head impact protection can be reduced to two fundamental components: load distribution, and cushioning.
A motorcycle helmet typically is constructed of a hard plastic outer layer (or shell) and an inner, energy-absorbing material (e.g., polystyrene). The hard outer shell spreads an impact force over a greater area, and the inner layer deforms to absorb the bulk of the impact energy. Thus, the remaining impact force and energy actually experienced by the head has been reduced to a small fraction of the original blow. Many bicycle helmets are constructed with a very thin outer shell (or none at all). However, this construction still complies with strict industry test standards.
Although helmets are designed fundamentally to protect the head against direct impacts and resulting linear accelerations, the protection that helmets provide against rotational head accelerations is relatively unknown. However, the effectiveness of helmets in protecting against bran injury has been proven unquestionably.
It must be noted that event he best helmet cannot protect its wearer if it is not used correctly. The two essential factors for effective protection be a helmet are: proper fit, and proper retention around the wearer's chin. A helmet that is too large or improperly strapped on will possibly fall off. In addition, the helmet must fit snugly in order to maximize the distribution of lead around the head, thus reducing the concentration of the impact force at any single location on the head.
Automobile air-bags are designed for one single purpose: to protect the front occupants of the vehicle during a frontal collision. Air-bags perform the same two protective actions as those described above for helmets: i.e., load distribution and cushioning. The flexible surface of the air-bag can mold to the shape of the occupant's torso and face, thus distributing the impact load over that large surface. Then, the air in the air-bag acts to cushion the forward motion of the occupant.
The protective effects of an air-bag are especially notable for the driver. Without a driver's air-bag, the driver's head would snap forward and down toward the steering wheel, ad in many cases the face would actually impact the steering wheel (even if the person was properly belted). In addition to cushioning the head from impacting the steering wheel, an air-bag minimizes the snap motion of the neck, thus reducing head acceleration and eliminating neck injuries.
As similarly noted for a helmet, an air-bag must be used correctly. In a frontal collision, an air-bag can protect the occupant from injury only if the person is properly belted and seated upright. An unrestrained occupant may actually ride up or around the air-bag, thus impacting the windshield or dashboard with great force. Therefore, occupants must always be properly belted, whether or not an air-bag is available.
When used properly and in accordance with industry regulations, helmets and air-bags save lives and reduce injuries. Many of the cases in which these protective devices actually have caused injury are directly related to the improper use of the device, or the use of the device in a situation contrary to its design objective.
A tragic example of this involves air-bags and rear-facing infant car seats situated in the front right passenger seat. Properly fastened rear-facing car seats by themselves act to restrain and protect the infant during a frontal collision. However, if situated in the front right passenger location, a rear-facing car seat protrudes forward to be directly in the explosive envelope of the air-bag. Rear-facing infant car seats should always be situated in the rear seat of the vehicle.
The industry trend to reduce the explosive force of air-bags is a positive step. The frontal impact protection will still be available, while reducing the risk of injury due to the explosive deployment of the air-bag.
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