Summary: Here are some basic ideas on helmets including design principles and what they can or can't do for you. We also explain what is included in standards for helmet testing.
Why use a helmet?
Helmets are useful as safety gear to prevent injuries in an uncontrolled environment. If you can't prevent a crash or impact, but you know it will occur, a helmet can prevent or minimize injury to the head and brain.
We can not emphasize enough that the first step in preventing injury is to minimize the probability of crashing or being impacted. Not hitting something hard is infinitely better than hitting with a helmet on!
There are many reasons for that statement. No helmet can protect against all possible impacts, and the impact may exceed the helmet's protection. No helmet protects any part of the body that it does not cover, so even if the head injury is avoided you may have a smushed face, broken bones or worse.
Safety always involves compromises. A helmet that could protect completely against every impact might be huge. A strong strap that keeps the helmet in place in a crash may strangle a child catching that helmet in the monkeybars on a playground. Helmets are designed to keep expected impacts within the range of human brain tolerance. But what if the brain is in a senior citizen and has become more brittle and less flexible, or what if it has already been injured in the past and is easier to re-injure? Or what if it is just more fragile than the average due to hereditary factors? Current helmet standards do not even attempt to address this problem. Concussions are reduced, but still acceptable as long as catastrophic injuries are reduced. Eliminating the crash or eliminating hard objects in the crash environment may be a more effective means of addressing a head injury problem than wearing a helmet. The benefits of that approach extend to those other parts of the body that a helmet does not even attempt to protect.
How does a helmet work?
Human brains can be injured by impact, of course, or by exceptionally violent rotation of the head, when the brain remains stationary, giving blood vessels and nerves a yank. Internal blood vessels and nerves yank parts of the brain around too in different ways, straining the vessels and nerves in the process.
Helmets designed to handle major crash energy generally contain a layer of crushable foam. When you crash and hit a hard surface, the foam part of a helmet crushes, controlling the crash energy and extending your head's stopping time by about six thousandths of a second (6 ms) to reduce the peak impact to the brain. Rotational forces and internal strains are likely to be reduced by the crushing.
In a lab test, graphs of the impact energy the brain sees look like this, with a smooth curve extending over 6ms for the good helmet
and a huge spike like the one below for a bare head.
About half way up that spike is where permanent brain damage begins.
Thicker foam is better, giving your head more room and milliseconds to stop. If the foam is 15mm thick it obviously has to stop you in half the distance of a 30mm thick foam. Basic laws of physics result in more force to the brain if the stopping distance is shorter, whatever the "miracle" foam may be. Less dense foam can be better as well, since it can crush in a lesser impact, but it has to be thicker in order to avoid crushing down and "bottoming out" in a harder impact. The ideal "rate sensitive" foam would tune itself for the impact, stiffening up for a hard one and yielding more in a more moderate hit.
If the helmet is very thick, the outer circumference of the head is in effect extended. If the helmet then does not skid on the crash surface, that will wrench the head more, contributing to strain on the neck and possibly to rotational forces on the brain. In short, there are always tradeoffs, and a super-thick helmet will probably not be optimal. It will also fail on consumer acceptance.
If there are squishy fitting pads inside the helmet they are there for comfort, not impact. The impact is so hard and sharp that squishy foam just bottoms out immediately. In most helmets a smooth plastic skin holds the helmet's foam together as it crushes and helps it skid easily on the crash surface, rather than jerking your head to a stop. In activities that involve forward speed on rough pavement, rounder helmets are safer, since they skid more easily. The straps keep the helmet on your head during the crash sequence. A helmet must fit well and be level on your head for the whole head to remain covered after that first impact.
Helmets designed for lesser impacts do not necessarily have foam inside. Some are just hard shells with a suspension headband that provides the fit and keeps some space inside for air to circulate. Construction helmets are of this type, and do a fine job when somebody drops a brick on your head or you bump hard against an overhanging steel beam. Just don't fall off a bicycle with one, since they will not handle the impact of falling on pavement.
The foams in some helmets are crushable but do not ever recover. If you crash a bike helmet made with the usual expanded polystyrene foam, the foam is trashed and you can't use it again. If the helmet is made for hockey or skateboarding it has a slow-rebound squishy foam called butyl nitrate foam, or perhaps expanded polypropylene foam. Either will recover slowly after a blow and can be reused. Construction helmets are ok as long as the shell is not cracked and the suspension is not damaged.
Different types of helmets seem indistinguishable to most consumers, and you can't test the impact protection unless you have a lab and are willing to destroy the helmet. So the industry uses standards to designate performance levels.
What are helmet standards?
Standards define laboratory tests for helmets that are matched to the use intended. If a helmet can pass the tests for a sport or activity, it provides adequate impact protection. A construction helmet will not pass the more severe bicycle helmet tests. A bicycle helmet will not pass the more severe motorcycle helmet tests. None of them provides the protection against shrapnel that is required of a military helmet.
Standards also define other tests for such parameters as strap strength, shell configuration, visor attachments, and the head coverage that must be provided, depending on the activity.
Standards are developed and published by various standards-setting organizations. We work with ASTM, the American Society for Testing and Materials, an organization that publishes a wide range of sports helmet standards. Helmets for US football must meet the standard of the National Operating Committee for Sports and Athletic Equipment (NOCSAE). The American National Standards Institute (ANSI) was formerly active in publishing helmet standards, but is less so today. There are military specifications for helmets for infantry, pilots and lots of others. There is a NASA standard for astronaut helmets. There are standards from a number of sports organizations for helmets related to their sport. And there are standards for other countries, including European CEN standards and those from Australia/New Zealand, Canada, Japan and others.
A typical standard specifies impact tests, strap tests, characteristics of materials to be used, required coverage, labeling and other requirements. Some have tests to simulate low temperature performance, hot performance, wet performance and sunlight ageing. Test equipment is described as well as the severity of the testing. For a look at a complete helmet standard, check out the Snell Memorial Foundation site, where their standards are all available. Or you can read the US CPSC bicycle helmet standard, probably the most-used standard in the world. For a look at a point-by-point comparison of bicycle helmet standards, check out our short comparison or our more detailed long comparison.
How is testing done?
There are various types of tests included in most standards. For impact testing, the typical test apparatus consists of a rig that drops a helmeted headform in a guided freefall to an anvil on the floor. You strap the helmet on the headform, turn it upside down so the helmet hits the anvil first and drop it onto the anvil. The helmet is oriented before each drop to test it's most vulnerable areas. The variables in the test include the drop height and the shape of the anvil: flat, round, ridge-shaped, pointy or in one case a shape that simulates a horseshoe. Instruments inside the headform register how much shock the headform experienced. The unit of measurement is normally the g, for gravity. (We have put up a page explaining g's.) Guided freefall is used because gravity is almost completely uniform everywhere, and the velocity of the test helmet just before impact is therefore very uniform.
There are other types of impact tests in some standards, including some where a weight or striker is dropped on a stationary helmet rather than dropping the helmet and headform on an anvil. There are also penetration tests where a sharp object is dropped in guided free fall to strike the helmet shell.
Testing is usually required with hot, cold, and wet helmets as well as those at ambient room temperature. Some foams are adversely affected by heat, others stiffen in cold, or may take up water and lose their effectiveness because the water does not compress. Temperatures chosen vary with the activity the helmet is designed for.
Strap testing is either dynamic or static. A dynamic yank is usually delivered by hooking a rod on the strap with a weight on it, lifting the weight and allowing it to fall to a stop at the end of the rod, delivering a calibrated yank. Some labs have production machines to reproduce this effect. A static test is done by simply hanging weight on the strap. In all of those tests, the strap must not release, and must not stretch or give more than a set amount.
What helmet is ideal for my activity?
We have a Web page up describing the ideal bicycle helmet in detail. We also have a page on other helmets. This section takes a more generalized approach.
Why can't the crash be prevented?
In all cases it is obviously better to prevent a crash than to have one occur and be wearing a helmet. The helmet may or may not be capable of handling the energy of the crash, and in any event the helmet protects only the head, leaving the rest of the body to be smushed. In a sport like pole vaulting, for example, where the organizers have control over every feature of the runway and pits, there is no excuse for having an "accident" where jumpers who go astray find something hard like pavement or a cinderblock to hit their heads on. Pole vault organizers have recognized this, and recently improved their pit standards before working on a helmet standard for their sport.
The road environment is different in the sense that it is everywhere, it is fairly uniform, and in the case of the US there is a blind spot among the general population to road injuries. We kill about 34,000 people in the US every year on our roads, more than 650 per week. Yet a sniper who shoots ten people gets front page headlines for days while the thousands of road kill are seldom worthy of any media coverage at all. We have in fact reduced US casualties considerably with better roads, safer cars and a modest amount of driver education, but bicycle safety on the road has lagged. That is why we support the National Strategies for Advancing Bicycle Safety. Road crashes could be reduced, if not prevented, and cyclist injuries could be reduced by substantial amounts if the Strategies were applied.
More: The Ideal Helmet
See our page on the ideal helmet for a discussion of the principles for the best helmet, and why you will not find one among today's bicycle helmets.