Let’s Talk About Helmet and Helmet Safety
Here are the arguments one encounters most often for not wearing a helmet, which I have numbered arbitrarily solely for the safe of organization: (1) Although more people are wearing helmets, the number of people seriously injured each year in bike crashes keeps going up to. Far from preventing injuries, helmets are making riders more daring and less cautious. (2) Some people have sustained severe brain injuries, even died, despite wearing a bike helmet, so bike helmets have no protective value. (3) The human skull is harder than any bike helmet and provides far superior protection to the brain in a crash, as evidenced by the fact that helmets shatter but skulls do not. (4) If any helmet could possibly prevent head injuries in bicyclists, it would have to be one as big and heavy as a motorcycle helmet, a helmet no cyclist could or would wear. (5) There are no medical studies validating the theory that bike helmets prevent head injuries, just anecdotes by individual cyclists who came to the belief they survived a crash due their bike helmet without factual proof (6) Since helmets cost a lot of money and there is no medical proof that they actually prevent head injuries, why should any cyclist put up with the downside of bike helmet use such as restricted views, hot and sweaty heads and ugly helmet hair?
The first argument is a double inference that wearing a helmet over-inflates one’s confidence of surviving a crash and this over-confidence leads to reckless cycling. This is a very weak set of inferences, since the only people who wear helmets are the ones who foresee the real possibility of getting hurt while cycling and who care enough to protect themselves. Further, would you or anyone you know weave in and out of car traffic or descend a steep hill much faster than feels safe, just because you had a helmet? The likely explanation is that bike riding has taken off in popularity and many more people are riding today than in the past, which correlates with steady increases in bike sales, helmet sales and opening up of new bike lanes. As of 1993 the rate of annual production of bicycles exceeded that of automobiles by a factor of three. All the other arguments relate in some way to our medical understanding of what happens to the brain in a cycling crash; how helmets are designed, constructed and tested to assure they do protect us from serious brain injury or death during a cycling crash; and the medical evidence taken from the real world that helmets work. These are discussed below.
Bicycle Crash Dynamics As They Affect Your Skull and Brain
To understand why cyclists need helmets and how they actually protect cyclists from head injury, the starting point is to understand the dynamics of a crash. The ways in which bike crashes with head contact can occur is far too extensive for anyone to list exhaustively. Common examples include collision with another vehicle; extreme steering or braking to avoid a collision; riding through water, oil, sand or gravel on the road; striking a defect in the road surface such as a crack, pothole or tree root hump; loosing control down a hill from excess momentum or poor body/bike position in a curve; or cycling under the influence of alcohol. When a rider falls to the street the speed he was going on his bike when he fell (e.g. 20 mph) will get reduced to zero by frictional sliding over the road surface, causing scratches to the bike and road rash to the rider. What helmet designers are concerned with is the sudden downward acceleration of the rider’s head during the fall along with how and how quickly this added head velocity is stopped when the rider’s head hits the ground.
Initially the rider picks up 1G of acceleration just from coming off his seat. Then his downward acceleration speeds up markedly as he goes into free fall. When his falling head smacks into something very hard (the street, the curb, etc.) its movement stops within milliseconds and during this incredibly short interval of time all of the added velocity from the fall must be brought to zero. The instant deceleration of the naked head as it crashes against the street during free fall represents a sudden, extreme change in head velocity that can cause severe distortion of the shape of the skull and brain with brain damage. The naked human head does not negotiate this sudden change of velocity very well. If there is nothing between your head and the hard street pavement, the whole force of impact will be absorbed by your head causing inward compression of your scalp and skull towards your brain, as your brain is moving towards the street. At sufficiently high impact forces the tensile strength of the skull bones will be exceeded and the skull will fracture. Whether or not the skull fractures, when your naked head hits the street after free fall, your brain will strike the inside of your skull (an event called the coup) and flatten as it rapidly decelerates and has its shape distorted. This is accompanied by pressure changes and microscopic bubble formation on the opposite side of the skull (cavitations). It may be followed by a forceful rebound of the brain against the opposite side of the skull (the contre-coup). The collisions of the soft, gelatinous brain against the skull can produce brain injury in the complete absence of skull fracture, because the living brain is delicate and the skull is lined with sharp ridges of bone.
Skull fractures can be non-displaced or displaced. If they are displaced, the fracture fragment moves out of alignment with the rest of the skull. If they are depressed, the fracture fragment has been pushed inwards toward the brain.
A significantly depressed skull fracture can lacerate the surface of the brain where our brain cells are densely packed and folded into a thin layer of cortex called the gray matter. The cortex is not a good thing to damage as it houses our higher brain functions such as foresight, planning, reasoning, judgment, complex decision-making and self-awareness. Any depressed skull fracture, even a mild one, has the capacity to tear the membrane between the brain and skull called the dura. This can create an epidural hematoma by tearing a dural artery and allowing blood to pump from that torn artery into the epidural space created by dural tear. An epidural hematoma can kill you if not detected and removed quickly, because the clot expands rapidly against the brain, forcing it down through the hole in the base of the skull where the spinal cord emerges from the brain stem. Depressed skull fractures allow air to move into the intra-cranial space and allow cerebro-spinal fluid to leak out, creating a risk of intra-cranial infection. Skull fractures with frontal lobe injury are associated with lost sense of smell and taste; changes in mood, personality and behavior; and difficulties with concentration, memory, multi-tasking and organization. Temporal skull fractures with temporal lobe damage are associated with chronic hearing loss, reduced auditory processing, balance problems, ringing in the ear and sometimes seizures. Occipital skull fracture is associated with vision loss.
Closed head injury occurs when a hard blow to the head does not fracture the skull, but causes damage to the internal white matter of the brain by application of violent shearing force or causes damage to its outer cortical surface from forceful collisions of the brain against the inside of the skull. Shearing force involves sudden rotational acceleration of the brain associated with back and forth or side to side motion of the head in a crash. The brain is composed of layers of different densities that accelerate and decelerate at differential speed during a crash with a component of head rotation. This lag in the movement of different parts of the brain results in injury to the long, incredibly thin axons that “wire” the brain’s nerve cell bodies together in the white matter tracts. The injury involves sudden stretching and straining of the axons, which are partially or fully torn and rendered dysfunctional. Because it occurs in a widespread or diffuse pattern throughout the brain it is called diffuse axonal injury or DAI. DAI is associated with slowing of one’s cognitive processes that has a real world impact on one’s cognitive efficiency and requires people to work much more slowly to stay accurate. Hard collisions of the brain against the intact skull can cause localized bruises to the cortex with or without bleeding called cortical contusions. They can also rupture bridging veins in the arachnoid membrane covering the brain and cause subdural hematomas.
Head injuries at the higher range of forces acting on the brain that are of sufficient magnitude to cause considerable DAI or subdural hematomas involve extended loss of consciousness and are classified as either severe or moderate. Such brain injuries show up on CT or MRI post-trauma. They typically require extensive and expensive medical care, therapy and rehabilitation and cause prolonged or permanent disability from work. Mild head injuries from lesser forces are associated either with brief periods of loss of consciousness or a brief episode of dazing and confusion. These injuries do not show up on CT or MRI post-trauma, because the damage is microscopic in nature. While “mild” in relation to brain injuries that cause death, seizures, paralysis or permanent total disability, the so called “mild” head injury is nothing to scoff at. Even in healthy, young people they can cause days, weeks or months of headache, nausea, blurred vision, dizziness, interrupted sleep, depression, irritability, slowed thinking, difficulty concentrating, distractibility or poor memory. In certain high risk groups, mild head injuries can cause permanent problems with cognition, mood or both. These include people with one or more previous concussions, persons over age 50, persons with a history of depression and persons with the APOE-e4 gene mutation associated with Alzheimer’s Disease. Mild head injury is equivalent to a concussion. Concussions are graded at Levels I, II, III and IV depending on severity and duration of disruption of consciousness, amnesia and other symptoms. While the very mildest of concussions will generally not produce permanent problems, the same is not true for more significant concussions.
Bike Helmet Design and Construction
A bike helmet is composed of a plastic micro shell (for aesthetics), Styrofoam to absorb impact force and a retention system using a chinstrap (and sometimes a head cage) to keep the helmet in place during a crash. The Styrofoam in a bike helmet places shock-absorbing material between your head and the hard surface it strikes during a fall. The purpose of the material is to manage the force of impact by crushing. The Styrofoam is composed of tiny beads that burst when they are compressed and, as they burst, provide a little extra room for the other beads. Because they burst in sequence, they provide a controlled braking force to the moving head while they give way. This avoids an instantaneous stop and allows the head to remain in motion while coming to a comparatively more gradual stop. The Styrofoam changes shape during the crush process, reducing the amount of distortion necessary in the head. This slows the deceleration of the brain by providing a more controlled, gentled crush – sort of like the crumple zone built into a Mercedes car.
With head impacts of sufficient speed or hardness, the crush will be accompanied by an actual cracking of the Styrofoam. This does not mean the helmet failed. What about the argument that helmets are inferior because they crack before the human skull does? Its wrong. Helmets have to be softer than the human head, or they would act as a battering ram against the head when crunched between the head and the pavement. If we made bike helmets out of unbreakable steel, head injuries would be worse because the head would slam against a layer of street and a layer of steel without progressive slowing and softening of the deceleration. Helmets are like Kleenex. After one use they should be thrown away. If you have struck your helmeted head on the ground and walked away, the helmet has done its job and should be discarded even if you can’t see cracks with your naked eye. Once crushed it loses its ability to protect you. Some people get angry when their helmets crack in a cycling crash, not realizing that this is exactly what the helmet is supposed to do. It’s much better to crack a helmet than your skull.
The Styrofoam crush system works most of the time for most crashes. Yes, there are some bike crashes that are so horrendous with regards to the speed and hardness of impact, that no helmet could prevent serious brain injury or death in those circumstances. However, these crashes are so rare, that they provide no rational justification for not wearing a helmet. The same is true of seatbelts and air bags. They cannot prevent all serious injuries and deaths, but they prevent so many that not to use them is folly. Should someone forego a condom because it was only 99% effective at preventing unwanted pregnancy or sexually transmitted diseases?
Epidemiology of Head Injury in Bicycle Crashes
What is the medical evidence that helmets provide substantial, while less than perfect, protection against head injuries in the real world? In the early 1990s the Snell Memorial Foundation provided grant money to physicians at the Harborview Injury Research and Prevention Center in the State of Washington to do an epidemiological study on the protectiveness of bike helmets against head injury in real world cycling crashes. The study involved 7 participating hospitals (including Harborview Medical Center) in Western Washington State, who followed up 3,390 victims of bike crashes treated at their hospitals from March 1992 to August 1994. Of these 50. 6% wore bike helmets and the rest did not. The physicians gathered pertinent details by reviewing medical records, using questionnaires and in some cases measuring the cyclist’s head and examining the cyclist’s helmet. The doctors wanted to find out which of the cyclists had head injuries, the severity of those injuries, whether the cyclist was wearing or not wearing a helmet and whether the helmet was being worn correctly so that it did not become displaced during the crash. They found that helmets decreased the risk of all serious mid to upper facial injuries by 50%, all types of head injury by 69%, mild to moderate brain injury by 65% and severe brain injury by 74%. Although these figures came out lower than the 85% figure one sees in other research papers, the researchers think it had something to do with sample size, and that with a larger number of subjects the final figure would have matched 85% protection. Even still the difference between wearing and not wearing a helmet is huge.
The Harborview paper says that properly worn helmets worked equally well for all age groups and equally well for cyclists involved with a motor vehicle vs. those who were not. The type of shell (hard shell, thin shell or no-shell) had no effect, since the protective effect of the helmet came from the Styrofoam. Cyclists who fit their helmets improperly (e.g. with the helmet pushed back on top of the head) were twice as likely to have a head injury than cyclists whose helmets fit snugly over the tops of their eyes. As expected no helmets prevented injuries to the lower face. The researchers also concluded that some crashes involve so much force, that no bicycle helmet could have prevented brain injury. If you are struck by a truck or SUV and sent into the pavement like a cannonball, there is no helmet that will save you, but this is not a valid reason to go helmet less. Protection against most accidents is far better than protection againstnone. The naked human head is fragile in a fall. Many of us know, or have heard of someone, who died after falling off a short stepladder changing a light bulb.
Epidemiologic studies done in Australia and other countries have shown that wearing of bicycle helmets causes a significant reduction of the number of head injuries in a given population. Persons interested in reviewing other epidemiological studies may wish to consult “Injuries to Bicyclists: A National Perspective” published by The John Hopkins Injury Prevention Center (1993). The research was funded by the Snell Memorial Foundation. The booklet is distributed by CDC’s National Center for Injury Prevention and Control. It can be ordered from CDC by calling 404-488-4400 or faxing them at 404-488-4389.
Bike Helmet Protection and Test Line
The shock absorbing capacity of a helmet is not equal throughout the structure. Helmet manufacturers design and build them with a “test line,” above which they are intended to meet or exceed the applicable standard of crash force resistance, and below which all bets are off. The test line cannot be seen with the naked eye inside or outside the helmet. The test line is described in the written specifications for the construction of the helmet. It can be located by putting the helmet on a head form and setting a computer-guided laser to shine on the helmet according to the product specs for the test line.
The test line is a necessary limitation. Bicyclists can only wear so much helmet and they cannot safely tolerate much restriction of vision or head/neck articulation. Whole head helmets would not work. Motorcycle helmets are admittedly more protective against head injury than cycling helmets, but motorcycle helmets weigh about 3 pounds and cycling helmets typically weigh 8-9 ounces. The reason motorcyclists can wear more helmet is that they get excellent ventilation and cooling from riding at high speeds of 55-75 mph, and because they are seated upright with a straight back so their whole spinal column is taking the weight of their head and helmet. Cyclists riding at 25-25 mph cannot get the same level of cooling and ventilation. Because they ride with bent backs, they cannot take the weight of a heavy helmet on their necks for a sustained period of time. While bike helmets are necessarily smaller, lighter and less protective than motorcycle helmets, they are still vastly safer than riding with a naked head and having no Styrofoam crush protection. Further, great advances have been made with regards to bike helmet ventilation by increasing the number of vents and improving air flow. Admittedly the vents are not perfect. There is still some sweat and bad hair, but aren’t they a small price to pay for protecting one’s skull and brain from catastrophic injury. The “hair police” can be mollified by wearing a do-rag under one’s helmet, keeping one’s hair short or diving into a restroom to repair one’s coif before rejoining your friends sans helmet. You can keep your helmet on until safely back inside your house or garage or you can associate with riders who still like you even after you sprout corn rows.
Bike Helmet Safety Standards and The Snell Memorial Foundation
How do you know if your helmet is safe? It’s a question of what standard it was built to meet and whether it met that standard. There is more than one standard. There is the CPSC standard that helmet makers must meet in America to lawfully sell their helmets, and then there is the higher standard set by the Snell Memorial Foundation, which is a voluntary standard that helmet makers may try to attain. Snell is located at 2628 Madison Ave #11, North Highlands, CA, just outside of Sacramento. Snell was founded in 1957 by private individuals to do empirical, scientific research on how helmets protect people from head injury while engaging in auto racing, motorcycling and cycling, and to set measurable, testable performance standards for helmet safety. The original research for Snell was conducted by George Snively, a physician and chemical engineer. Using facilities at UC Davis, and working with human cadavers and live animals, he developed parameters for the minimum force necessary to cause serious brain damage or death from a blow to the head during a crash.
Snell is a private, not for profit organization that develops helmet safety standards and tests helmets for the public benefit. It is not affiliated with the government. It does not take government grants or private donations. Its money comes from fees paid by manufacturers of racing, motorcycle, cycling, ski, equestrian and other helmets to test their helmets. Snell tells the manufacturers up front that they will do everything they can to break their helmets and fail them. If Snell certifies a particular model and size helmet, it continues thereafter on its own to buy examples of the helmet off the shelf at stores and do spot testing to ensure compliance. Because Snell is a private foundation, no manufacturer is required by law to meet its standards.
Snell’s standard writers use their own data, but also acquire, review and consider data from all over the world that is submitted by scientists, manufacturers, government and others. They periodically revise their standards to make them more stringent as helmet safety technologies improve. This occurs about every 5 years. Snell tests simulate real world impacts to bike helmets strapped to an artificial human head (head form) dropped from a prescribed height at a prescribed speed onto a hard surface. The current Snell Standard is that in any place on the helmet above the test line, the helmet will not allow more than 300 Gs to be transmitted to the head/brain in any Snell test. The 300 Gs standard is based on data showing that serious brain injury or death from brain injury occurs at G forces significantly higher than 300s and that 0 Gs to 300 Gs. Since mild concussion can occur below 300Gs, the Snell standard allows for cyclists to sustain a mild concussion. It is not possible at this time to make a practical, wearable bike helmet that will prevent mild concussions. The choice is either to wear bike helmets that prevent catastrophic brain injury most of them time, and allow for mild concussions, or not to wear a helmet.
The scientists at Snell believe it is so important to prevent catastrophic brain injury that helmets must always be worn even if they fail to prevent mild concussions. They caution riders to who sustain a mild concussion to get thoroughly checked out by a neurologist and get medical clearance before returning to cycling. They also raise the point that cyclists who experience serial concussions ought to consider not riding any more. This is because the cumulative effect of concussions is not additive (one plus one equals two) but synergistic. There is a great deal of medical research showing that multiple concussions reduce vigilance, reaction time and coordination, making it harder to protect oneself. Further, multiple concussions take a bigger and bigger chunk out of one’s entire array of cognitive functioning, and at a certain point the owner of the brain will need to implement lifestyle changes to avoid any more concussions.
Snell has a machine with lasers that determines precisely where the manufacturer’s test line is for each model and size of bike helmet. It tests each model and size of bike helmet from the manufacturers participating in their program using a weight drop machine hooked up to a computer. Their testers take an artificial head form with the shape and weight of a normal adult human head and attach it the drop carriage with a metal ball arm and ring. They strap on a bike helmet over the head form and drop it from a prescribed height onto a solid metal anvil to see how much force is required to break the helmet in various locations over the test line. They use 4 impact locations per sample spaced at least 12 cm apart. To make sure they are generating the right amount of force, they measure the duration of the fall in meters per second by having the helmeted head form pass by a velocity gate. The computer records and displays the duration, peak and average of the G forces exerted on the helmet.
If you go to a bicycle shop you are likely to see few if any helmets with the Snell sticker. This reflects certain historical events. Prior to the mid-1990s bicycle helmets were fairly expensive and sold in small quantities to serious cyclists. In the mid-1990s there was an explosion of popular participation in cycling that went hand in hand with huge retailers like Wal-Mart putting out vast quantities of easily affordable bike helmets. The buyers of these helmets were motivated by price. They had never heard of Snell and the presence or absence of a Snell sticker meant nothing to them. This diluted Snell’s presence in the marketplace for bicycle helmets, because the vast majority of helmets were now made for sellers who had no interest in gaining the imprimatur of Snell on their helmets as a selling point. Today, all sellers of bike helmets in the United States must meet the standard set by the CPSC.
On 3/10/99 the CPSC adopted a standard that can be met more easily and cheaply than by meeting Snell’s standard (16 CFR 1203). This standard is satisfied solely by data submitted by the manufacturer. In other words, the helmet makers certify to the CPSC that they have tested their own helmets and they are in compliance. CPSC retains the right to do spot testing, but apparently lacks the resources to do it. The testers at Snell say that the current CPSC standard and current ASTM standard is equivalent to the standard Snell had in place in 1984. In lay terms, Snell drops its helmets farther and smacks them harder than is required by CPSC or ASTM. For the physicists among you, the difference is between a drop of 72 joules versus a drop of just 58 joules. Further, Snell tests more areas of the helmet than is required by CPSC or ASTM. Are you safe with a helmet that has approval from CPSC and ASTM but not Snell? The testers at Snell say the CPSC standard is far superior to that found in other countries and that you are much safer wearing a CPSC approved helmet than not wearing one, but that the highest level of confidence can only be achieved by a helmet that meets their standard. They also say some helmets that do not have their stickers may meet their standards, but for cost reasons the makers of such helmets have decided not to pay the fees for testing at Snell.
Selecting the Best Bike Helmet for You
Do not play Russian Roulette with your brain by riding bare. Anyone who truly cares about their own safety and about maintaining their current level of functioning (for themselves and their dependents) should never ride without the best helmet they can afford worn in a proper manner. The best bike helmet is the best-made, safest helmet the consumer likes enough to always wear. There is sufficient variation of shape, size, color and style to please anyone on today’s market. Can today’s helmets be improved? Yes, improvement of the safety features of helmets is a constant goal and an ongoing process. During the Harborview study, all helmets damaged in a crash in which the cyclist had struck his head were purchased on the spot and mailed to the Snell Foundation for examination (some 527 in number). Most of the helmets had damage to the rim in the front or sides. One recommendation of the report is that more attention be paid by helmet makers to building extra energy absorbing capacity in the front and sides. The thinnest bone in human skull is the temporal bone between the lateral corner of the eye and the ear. This is a consequence of our evolution. When our caveman ancestors tripped and fell, they struck their shoulders, which absorbed the impact before the side of our heads hit. None of them rode bikes and suffered the kinds of falls in which the side of the head hits the ground before the shoulder. For this reason, when you buy your next helmet, you ought to closely examine the thickness of the Styrofoam close the ear, and opt for the helmet with relatively thicker Styrofoam there.
If you can find a Snell certified helmet, then buy it, as long as it you like it enough to wear. Obviously you should never buy a used helmet, a helmet from a country that does not certify its safety at a level equivalent our CPSC or a helmet made in America before the most current CPSC standard was in place. The testers at Snell caution against having a plastic sun visor or peak on the front of your helmet unless it snaps off easily, a quality called frangibility. If the peak does not snap right off in a crash it can push your helmet back up and over your forehead (which diminishes head protection) and can act as a lever to worsen whiplash and increase the potential for neck fracture.
Go out and ride, with a snugly fitting helmet, and have a great time. Stay safe riding and enjoy yourself. Spending the money to purchase a good, safe helmet, and making the effort to wear it snugly on every bike trip, is an investment in yourself, your future and your family’s future. Putting up with the temporary discomfort of hot, sweaty hair is a small price to pay.