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Head Injuries

Head injuries are fairly common in industry and account for 3 to 6% of all industrial injuries in industrialized countries. They are often severe and result in an average lost time of about three weeks. The injuries sustained are generally the result of blows caused by the impact of angular objects such as tools or bolts falling from a height of several metres; in other cases, workers may strike their heads in a fall to a floor or suffer a collision between some fixed object and their heads.

A number of different types of injury have been recorded:

* perforation of the skull resulting from the application of an excessive force to a very localized area, as for example in the case of direct contact with a pointed or sharp-edged object.

* fracture of the skull or of the cervical vertebrae occurring when an excessive force is applied on a larger area, stressing the skull beyond the limits of its elasticity or compressing the cervical portion of the spine.

* brain lesions without fracture of the skull resulting from the brain being displaced suddenly within the skull, which may lead to contusion, concussion, haemorrhage of the brain or circulatory problems.

Understanding the physical parameters that account for these various types of injury is difficult, although of fundamental importance, and there is considerable disagreement in the extensive literature published on this subject. Some specialists consider that the force involved is the principal factor to be considered, while others claim that it is a matter of energy, or of the quantity of movement; further opinions relate the brain injury to acceleration, to acceleration rate, or to a specific shock index such as HIC, GSI, WSTC. In most cases, each one of these factors is likely to be involved to a greater or lesser extent. It may be concluded that our knowledge of the mechanisms of shocks to the head is still only partial and controversial. The shock tolerance of the head is determined by means of experimentation on cadavers or on animals, and it is not easy to extrapolate these values to a living human subject.

On the basis of the results of analyzes of accidents sustained by building workers wearing safety helmets, however, it seems that head injuries due to shocks occur when the quantity of energy involved in the shock is in excess of about 100 J.

Other types of injuries are less frequent but should not be overlooked. They include burns resulting from splashes of hot or corrosive liquids or molten material, or electrical shocks resulting from accidental contact of the head with exposed conductive parts.

Safety Helmets

The chief purpose of a safety helmet is to protect the head of the wearer against hazards, mechanical shocks. It may in addition provide protection against other for example, mechanical, thermal and electrical.

A safety helmet should fulfill the following requirements in order to reduce the harmful effects of shocks to the head:

1. It should limit the pressure applied to the skull by spreading the load over the largest possible surface. This is achieved by providing a sufficiently large harness that closely match various skull shapes, together with a hard shell strong enough to prevent the head from coming into direct contact with accidentally falling objects and to provide protection if the wearer’s head should hit a hard surface. The shell must therefore resist deformation and perforation.

2. It should deflect falling objects by having a suitably smooth and rounded shape. A helmet with protruding ridges tends to arrest falling objects rather than to deflect them and thus retain slightly more kinetic energy than helmets which are perfectly smooth.

3. It should dissipate and disperse the energy that may be transmitted to it in such a way that the energy is not passed totally to the head and neck. This is achieved by means of the harness, which must be securely fixed to the hard shell so that it can absorb a shock without being detached from the shell. The harness must also be flexible enough to undergo deformation under impact without touching the inside surface of the shell. This deformation, which absorbs most of the energy of a shock, is limited by the minimum amount of clearance between the hard shell and the skull and by the maximum elongation of the harness before it breaks. Thus the rigidity or stiffness of the harness should be the result of a compromise between the maximum amount of energy that it is designed to absorb and the progressive rate at which the shock is to be allowed to be transmitted to the head.