Significance of Head Protection

Head Protection in PPE terms is considered as protection against impact injury and some burn injuries. It generally protects the scalp area , and sometimes the jaw. The face is generally not included , except for some sports helmets. 

How does head protection reduce the level of the force/acceleration on the head?

There are two key mechanisms:-

Some of the energy of an impact is absorbed by the head protection which reduces the force/acceleration sustained by the head. The duration of the impact is increased so that , for the same energy , the maximum force/acceleration on the head is lower. In simple terms , head protection  will act like the crumple zone on your car. It absorbs some of the energy and spreads out the time of the impact.High speed video of a hard hat during impact shows the helmet looking very like a jelly - it wobbles and deforms which absorbs energy.