In the postwar era, many technological advances were made in the manufacture of motorcycle helmets. Energy-absorbing foams were developed which could absorb and scatter the force of impact and lessen head trauma from crashes. Improvements were made in all four of the basic elements of the motorcycle helmet: The outer shell, interior lining, flexible foam lining and chin strap.
The most important of these components as regards safety in a crash is the foam lining inside of the helmet. Polystyrene foam (to be precise, expandable polystyrene foam or EPS) is present in a thick layer in contemporary motorcycle helmets. EPS is lightweight and can withstand compression, making it a perfect material to use for the lining of motorcycle helmets. This lining absorbs and diffuses the energy of an impact, which the skull of the rider would otherwise bear the brunt of. EPS comes in different types and is used in various thicknesses in the manufacture of motorcycle helmets. Basically, the more foam that is used, the more impact resistance the helmet possesses.
The interior lining (not to be confused with the foam cushioning is usually removable; all the better to adjust for fit and of course, removal for washing. This liner comprises the top two layers of the interior of a motorcycle helmet. The outer shell is made from extremely durable materials and must go through extensive DOT testing. These tests include resistance to puncture and abrasion (for instance, from skidding along pavement). These days, the shell is likely to be made of fiberglass or plastic due to cost and weight. However, more high end helmets are made from (equally light, yet far more expensive) materials such as Kevlar and carbon fibers.
And last but not least is the chin strap. Usually made of nylon, this vital piece holds everything together and ensures that the helmet will remain on the rider in the event of a crash.
The manufacturing process of motorcycle helmets is undergoing continual improvement by the companies engaged in making them. Lighter materials and better construction mean constant improvement in motorcycle helmets. There are heavier materials out there for helmets; fiberglass for instance. While offering good protection, they can crack if dropped, ruining their utility as protective gear. Injection molded plastic is another common material. However, this is not as long lasting as fiberglass and can be broken down by paint and decals. Great care must be taken with what goes onto a plastic helmet.
Keep in mind what sort of bike you intend to ride while wearing any helmet you are considering purchasing. Discuss it with the staff at your local shop; the type of helmet that can best protect you in a crash can depend to some extent on what model and make you ride.