You can leave your hat on!
Helmets help to protect your brain by reducing the amount of energy transferred to your brain during a crash. The energy is absorbed by crushing the foam cells of the lining. Once the cells are crushed, their ability to absorb shocks is gone; so you need a new helmet. The force is measured in gravitational force (g’s). A force greater than 300 g’s will lead to concussion, head trauma or death. So your helmet should not transmit over 300 g’s to your brain on impact. There are lots of helmet standards in sports; different types of helmets help to protect you from different types of impacts. A bike crash is different from rock fall protection. Different organizations and countries also have different safety standards for the same helmets.
Even before the First World War, people tried to protect the head of the rider from falls. There were many accidents during races and six-day events. The speeds increased considerably during crashes of races with pacemakers (motorcycles). The stayer track helmets from the thirties protected the head, but hardly absorbed the energy of a crash. Until the nineties roadracers used the "hairnet"-style headcover we see in FIG.3 : number 2 and 7. Of course these foldable leather strips, filled with horsehair, offered little protection. Number 3 and 5 in FIG.3 were hard shells, but they had little shock absorbing properties. Number 4 and 6 have a real Poly-Styrene (PS) inner helmet. The firm Bell, well known for its motor racing helmets, produced the first modern helmet especially for cyclists (FIG.2).
Until then, there had been riders using ice hockey and mountaineering head protection. Now a whole new market started to develop. Experiments of helmets without hard shell, "soft shell helmets", showed them to be too vulnerable; they soon disappeared again. The aero-helmet was the next new phenomenen.
FIG. 1 a/b Stayer track helmets from the thirties
FIG.2 Bell was the first modern cycling helmet (mid 70's)
FIG.3 A series of available cycling helmets from 1987
Bicycle helmets in the European Union must comply with the European standard EN-1078 for adults and with EN 1080 for children's helmets. These helmets have been tested for their ability to absorb impact. An approved helmet . The EN-1078 standard was implemented in 2012 for all cycling helmets sold within the EU; there should be a CE marking on the inside.
Since 1999, all bicycle helmets sold in the United States have been required to pass the helmet test of the Consumer Product Safety Commission: the CPSC 1203; this test subjects helmets to slightly greater impacts than the EN-1078 standard. EN-1078 permits lighter, thinner helmets than some of the other standards, because it subjects helmets to impacts from lower heights than Snell or CPSC. But the EN-1078 requires a lower test line, so the helmet should provide more coverage. Helmets that transmit more than 250 g’s to the chrash-test dummy during impact, fail EN-1078. Other standards allow up to 300 g’s. If you buy a helmet on the internet, it could be missing the proper certification. If you race in Europe, you might need a different certification than in the U.S. Be carefull: Wim van Est ended up in the ravine in 1953
The downhill mountain biking helmet must be able to absorb greater impact energies (standard ASTM F1952). This also requires a lower test line on the sides and back of the helmet than most other bike helmet standards. The same applies to the SNELL B-95 tests; this is a higher voluntary standard. These helmets offer more protection, but are heavier and more expensive.There are developments from Sweden to absorb any rotational energy that may occur by sliding layers in the helmet. To what extent this actually occurs and how much safer the helmet becomes, has yet to be proven. The system is called Mips; it seems like a solution in search of a problem. More info on testing: https://helmets.org/ and helmetfacts.com