Some of you are going to hate me for flogging a dead horse. Some of you are going to love the opportunity to rear your stallion and try to kick us into agreement with the helmet crusade. Either way I’ll give you the last word in the comments but I’m taking the bully pulpit for a moment. (Disclaimer: I’m not a physicist or a medical researcher, following is simply gleaned from lots of reading along with attempting to be realistic.)
First, we need to get straight on types of head injuries. To keep it simple (apologies to medical pros), we’ll divide the nuances into two types: Direct injury is a surface bruise, laceration, abrasion or skull fracture. These can include brain injury but not necessarily. The other type of injury is brain damage caused by your grey matter banging into the inside of your skull when your fast moving head quickly stops moving. In the latter (and sometimes former) case, the result is a “concussion,” simply meaning your brain gets bruised and damaged. Also, we should clarify that in this discussion we’re talking about a moving athlete hitting his or her head on something. Helmets also protect against things like rocks falling on your head, but that’s another subject altogether as it involves properties such as penetration resistance.
Scalp lacerations and surface bruises can be spectacular and painful. Blood. Stitches. But without associated concussion or other types of brain damage they heal with no lasting effects. Ski helmets do a great job of protecting you from such injuries.
Concussions are different. Each time you undergo a concussion, you get a poorly understood form of brain damage that is known to be cumulative. Eventually, your brain becomes more prone to concussion at lower impacts, and you begin to exhibit 24/7 brain damage symptoms. These effects are said to sometimes happen after as few as three concussions — even over fairly long periods of time. What is more, it doesn’t take much of a hit to cause a concussion.
When your head hits an object, the likelihood of concussion can be measured in G force of the deceleration. You get a possible concussion at 95 g’s, certain concussion at 150 g’s, and serious injury or death at 275 g’s.
Ski helmets are certified to the ASTM F2040 standard for snowsports helmets. This article refers to a study where they tried to emulate a real-life skiing accident and measure G forces on a helmet “protected” head. The testing was done as if the skier was moving at 30 kph, 18.6 mph. Such moderate speed is frequently attained by nearly any skier.
During the study, measured force at 18.6 mph was 333 g’s when the helmet/head hit a wooden post. That’s significantly above the threshold for serious injury or death. What is more, G force when the head hit hard icy snow was still up at 162 g’s.
That, my friends, is the problem with ski helmets.
So, why? As I mentioned in a comment, simple physics is the main reason why it’s tough to engineer better helmets. Put as simply as I can, the G force your brain is going to undergo in collision increases as a square of your speed. In other words, a helmet that definitely protects you at 10 mph needs four times the protecting at 20 mph and fully SIXTEEN times the protection if you’re going 40 mph (not an uncommon speed for good skiers). You can even reverse the math for the study I used, and realize that if you’re moving at half of the 18.6 mph, around 9 mph, you’d have 1/4 the G force (is my math correct?), thus, yeah, that helmet would possibly work when you hit that fence post at 9 mph and got 83 g’s on your brain. Though that still sounds iffy, since the lower threshold for concussion is 95 g’s.
What is more, it is common but misleading to assume that a helmet protects you by spreading out force like hardshell knee pads do. Yes, if a rock falls on your head the helmet needs to resist penetration and subsequently spread out the force. But in the case of hitting something, spreading out the force does zilch to change the deceleration that causes concussion. Thus, though it is somewhat of a paradigm shift, try to put out of your mind that the shell of your helmet spreading force over your head is in any way protecting you from concussion. The only thing that protects you from concussion is to increase the time your head takes to decelerate. This is usually accomplished by the crushing of foam inside the helmet, or sometimes by a suspension system releasing or stretching.
Conclusion? Wear a helmet if you choose; good ones do offer a small amount of protection. But even with ASTM certified snowsport helmet protection the possibility of receiving a brain concussion is very real in even low speed accidents. As for helmet evangelism, exhorting others to take up the helmet crusade looks a bit ridiculous in light of all this. Yes, once they’ve got better helmets on the market by all means shout it out. Till then, perhaps all the shouting should be at the helmet industry to make better product, rather than at other folks to buy one and thus perpetuate the status quo.
Following is from the study as quoted in the article I refer to above:
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.