The Science Behind Helmets
Protecting your head isn’t about colour and styling, it’s knowing a helmet’s limitations – and your own
by Jasper Shealy with Robert Johnson and Carl Ettlinger
from Fall 2008 issue – originally posted December 5, 2008
Jasper Shealy is Professor Emeritus, Rochester Institute of Technology, Rochester NY. This article includes material from Robert Wilson, Professor, University of Vermont College of medicine, Burlington VT and Carl Ettlinger, President, Vermont Safety Research, Underhill Centre VT
EDITOR’S NOTE: Articles on ski helmets, especially in the mainstream media, regularly use first-person anecdotes, lift-line opinions and arguments about mandatory helmet laws to discuss the subject. You’ll find none of this in the following report which was first presented at the International Symposium on Ski Trauma and Skiing Safety in Avemore, Scotland in May 2007 and published in the Journal of ASTM International. Jasper Shealy et al simply stick to the facts to explain what we can, and can’t, expect from our helmets. Although the data collected for the paper is from south of the border, the authors see little reason to believe results would vary significantly if Canadian skiers and snowboarders were included in the study.
We have now reached the point where in the U.S. close to 50 per cent of the skiing and snowboarding population is regularly using a helmet. That number has been steadily growing by about four to five per cent per year since the mid- to late-1990s.
My colleagues and I are strong believers in helmets and encourage everyone to use one. On the other hand, we are equally strong believers in the notion that helmets are not panaceas, and have an extremely limited ability to prevent serious head injuries. We encourage skiers and snowboarders to not have an exaggerated sense of protection just because they are using a helmet. We have often observed that some individuals say they only use a helmet when they do something risky, which suggests that if not for the helmet, they would not do it. In other words, the helmet is a “risk enabler” and indeed encourages risk-taking.
Some questions need to be asked. For example, what have we been promised and what are we getting for our increased helmet usage? And perhaps more importantly, what can we realistically expect from a helmet?
Let’s start with some basics. Most helmets used in North America will most likely comply with standards from various organizations such as CE, ISO and/or ASTM. While each of the helmet standards differ in detail, the outcome is not remarkably different. Experts, marketers and purists may disagree on exact specifics, but within broad boundaries they all result in helmets that do about the same thing and in the same way: the hard outer shell disperses the load over a larger area of contact, and the inner liner/shell allows for a period of deceleration as the liner material compresses and absorbs energy.
However, what often seems lost on the general public and perhaps the most avid of those who recommend helmets, is the discrepancy between the real world and the test conditions for the standards. Most standards involve some form of an impact or drop test where the helmet is mounted on a head-form and is then dropped onto a hard surface from a predetermined height or impacts at a predetermined velocity. There’s usually some maximum deceleration that’s allowed in order for the helmet to pass the standard. For example, for the ASTM F2040 Recreational Snow Sports helmet standard, the impact on a flat steel anvil is at 22.3 kph, which corresponds to a theoretical drop height of two metres, the impact on a hemispherical anvil is at 17.3 kph, and the impact on an edge anvil is at 16.2 kph. Under each of those circumstances, the peak straight-line acceleration on impact cannot exceed 300 g as registered by an accelerometer embedded in the head-form. Rotational acceleration is not measured or considered.
Keeping in mind the above figures, most snowsports fatalities due to head impact with solid fixed objects such as a tree take place at speeds of 44 kph or more. That speed is the average maximum speed seen by 650 consecutive skiers and snowboarders at three different resorts on wide, groomed blue-square trails—the sort of trails where most fatalities occur. Skilled young adult male skiers and snowboarders tend to go even faster than the rest of the population. This group is also the most commonly fatally injured. A review of most fatality reports shows that the typical fatality occurs to an experienced male between late-teens and late-30s in age, while travelling at a relatively high speed on the margins of intermediate runs.
In our study of head injuries at Sugarbush, Vermont, since 1993, we have found that only about 2.6 per cent of all medically significant injuries are what we call a potentially serious head injury (PSHI): a diagnosed skull fracture, concussion, closed head injury or traumatic brain injury (TBI). This is in contrast to the broad defi nition of a head injury as “any injury above the neck,” which includes minor injuries such as scalp lacerations and the like. Using this general defi nition of head injury, various studies around the world have found that head injuries range from about 10 to 20 per cent of all injuries. The large majority of head injuries that fi t this description tends not to be life-threatening, and in fact are minor. Helmets are most effective in the mitigation of these minor head injuries.
On the other hand, helmets are far less capable of mitigating the more serious head injuries, i.e., the PSHIs, even though fully three-quarters of the PSHIs are mild concussions. (The signifi cance of so-called “mild concussions” is an on-going discussion where many authorities believe that their signifi cance has been underestimated.) Simply based on the logic that most mild concussions are due to rather small impacts, there’s reason to believe that helmets would be effective in mitigating mild concussions. However, we have made no attempt in our studies thus far to determine the reason or motivation for individuals using a helmet, or not.
Kinetic energy goes up as the square of the velocity, so since the average maximum of 44 kph is roughly twice the 22.3 kph test impact speed, a body travelling at 44 kph has roughly four times as much kinetic energy as that same body would have at 22 kph. That 300 g maximum peak acceleration limit needs to be seen in the context that brain injury can start at as little as 150 g, and by 275 g fairly serious brain injury is almost a certainty.
If those hard, cold facts are kept in mind, it’s easier to see and understand why helmets are fairly effective at preventing minor head injuries such as scalp lacerations, but not so good at preventing the more serious forms of head injury, especially fatalities due to direct impact with fixed objects.
The public expects far more than a helmet could ever be expected to deliver. Most famously, the U.S. Consumer Product Safety Commission (USCPSC) as much as promised in 1999 that if everyone wore a helmet while skiing and snowboarding, there would be no more head-injury deaths on ski slopes.
Our research and the research of others has consistently shown a 35- to 50-per-cent reduction in head injury if a head injury is defined as “any injury above the neck.” Helmets prevent close to 100 per cent of relatively minor head injuries (lacerations), but are far less effective at preventing serious head injury (concussions, closed head injury, subdural haematoma and so on). In terms of overall fatality rates nationwide in the U.S., there has been no decline (statistically significant or otherwise) even though nearly half the onslope population now wears a helmet.
Interestingly enough, what has changed is the modality of death—but not the rate. For the ski seasons starting in 2000-01 through 2004-05, there were 76 instances of fatality in the U.S. (We are not able to find reliable data from other countries, but are confident that the overall numbers wouldn’t vary significantly.) Of those 76, 28 (37 per cent) were wearing a helmet at the moment of death. This figure of 37 per cent is considerably higher than the helmet utilization rate in the general population at that time. For those who died while wearing a helmet, some form of head injury was listed as the leading cause of death 46 per cent of the time. For those who died and were not wearing a helmet, some form of head injury was listed as the leading cause of death 72 per cent of the time.
An insight as to why this study found a difference in patterns of death as a function of helmet utilization can be found in the following study. A simulation using a 50th percentile male anthropometric device (Scher, Richards and Carhart, 2005) was done of a snowboarder going 30 kph, catching an edge and falling headfirst onto soft snow, icy snow and a fixed object (a 28-cm upright wooden post). This simulation was done to assess the effect of wearing a helmet or not under the three different impact conditions. The helmet in question met the requirements of ASTM F2040. The g-loads to the head-form were measured and the associated Head Injury Criterion (HIC) values were computed. HIC is a time-weighted acceleration measure used widely in the automotive industry to measure impact severity as it relates to head injury. This study found that if the impact is onto a soft-snow surface, both the measured g-loads (under 100 g) and the computed HIC values (less than 220) are well within acceptable limits regardless of whether or not a helmet is used. When the impact was onto simulated hard, icy snow, the helmet reduced the average measured g-load from 329 to 162, and the HIC value from 2,235 to 965. When the impact was against the fixed object, the helmet reduced the values from 696 to 333, and the HIC from 12,185 to 3,299.
The study concluded that under the circumstances of impact with soft snow, the use or non-use of the helmet had no significant effect. In the matter of the impact with a solid fixed object resembling a tree, while the use of a helmet was associated with a significant reduction in both the g-load and the HIC, the likely outcome remained that of a fatal injury— with or without the use of a helmet. With an impact on icy snow, the use of a helmet could be the difference between a significant head injury (possibly life-threatening) and a minor head injury.
We believe that the kinetic energy in many death scenarios may be so massive as to overwhelm the degree of protection that any helmet could offer. Many fatalities appear to occur under circumstances that are likely to exceed the protective capacity of current helmets designed for recreational snowsports. While helmets can reduce the impact to the head, it’s inherently possible to overwhelm that degree of protection. It seems that in some snowsports fatalities, helmets are capable of preventing fatal head injuries, but still not lower the overall likelihood of death because most of the fatalities involve significant multitrauma events and thus other fatal injuries are also likely to occur.
Data indicate no decline in fatality incidence (or serious head injury incidence, for that matter, but this is less clear and we are still working on that point) even though helmet utilization within the high-risk group of skilled/ experienced young adult participants is more than 40 per cent and growing. What is clear is that the pattern of death is different as a function of helmet utilization.
Helmets will probably never have a serious impact on mitigating death due to head injury since the typical fatal scenario has so much kinetic energy that it will overwhelm the protective elements of the helmet. But the good news is that fatal injuries in snowsports are quite rare—less than one in 1.5 million days of activity.