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The Bike Helmet Paradox

Recently a good friend told me over a pint in our local pub that he hadn't been able to sleep a wink for two nights. He'd been left traumatised by the sound of a skull cracking and the sight of brains spread over concrete. No, he hadn't just returned from fighting in Afghanistan, he'd witnessed one of the many cycling deaths that occur on the roads every year. We all have stories to tell about the wearing or not wearing of helmets and this is what makes the wearing of helmets such an emotive issue, which can blind us to the facts on both sides of the debate. I was subsequently intrigued to read a viral post on a friend's Facebook wall titled: "Why It Makes Sense To Bike Without A Helmet". The post currently has over 91,000 likes on Facebook, which I thought warranted it some investigation.


The author begins, like any good debater by outlining the very strong case for helmets:

"Let’s first get one thing out of the way: if you get into a serious accident, wearing a helmet will probably save your life. According to a 1989 study in the New England Journal of Medicine, riders with helmets had an 85% reduction in their risk of head injury and an 88% reduction in their risk of brain injury. That’s an overwhelming number that’s backed up study after study. Nearly every study of hospital admission rates, helmeted cyclists are far less likely to receive serious head and brain injuries. These studies confirm what we feel when we’re out for a spin on our bikes: We are exposed. Vulnerable. Needing of some level of protection."

After this good start, the author decides to take a "broader look at the statistics" by zoning in on one study of head injuries in San Diego in 1978. The author points out that only six percent of head injuries were amongst cyclists compared to 53% amongst drivers - seemingly oblivious to the fact that there would have been far more daily drivers than cyclists in San Diego in 1978, making the statistic more than a little misrepresentative. The author provides the pie chart on the left, which is considerably less impressive when placed alongside the pie chart on the right showing the rate at which American commuters actually cycle (the data on the right is from 2005 but the proportion of commuters cycling in the US has remained below 1% since the 70's).

Next the author points out that in the San Diego study and in another French study that was also not weighted for population: "more people were hospitalized after walking down the street than riding on a bicycle", a statistic that can likely be explained by the fact that while more or less everyone walks to some extent, relatively few people regularly cycle and old people tend to hurt themselves falling over a lot.

This is where things begin to get messy. The author cites a 1996 study which looks at injuries per hour travelled and suggests that motor vehicle occupants are actually slightly more likely to suffer head injury than cyclists. On the face of it, this is a shocking statistic, but considering the speed that cars travel, it is should perhaps not actually be surprising that cars may even be more dangerous to their occupants (and to others) than push bikes - and the suggestion that car drivers should wear helmets is certainly a compelling thought to say the least. Whether or not this is relevant information to whether cyclists should be singled out for wearing helmets is certainly an interesting topic for debate. In this respect, it is difficult to argue with the author who writes:

"In other words, if the reason we are supposed to wear helmets while biking is to prevent serious head injury on the off-chance we get into an accident, then why is it socially acceptable for pedestrians and drivers to go about bare-headed? Why has cycling been singled out as an activity in need of head protection?" 

Things become yet more interesting when we scratch further beneath the surface. The author of the blog post writes:

"There's an important caveat to the results of that 1989 New England medical study: Bike helmets may reduce the risk of head and brain injury by 85-88%—but only for those who get into accidents.

If we take a closer look at the article we see that both the experiment and the control groups studied are those who have already been hospitalized for bike injuries. If one were to examine the medical and epidemiological literature on bike helmet effectiveness, you'll find the exact same condition over and over: Studies show that helmeted cyclists who are hospitalized are far less likely to have serious head trauma than bare-headed cyclists that have been hospitalized.

But wouldn't this be true, regardless of the activity? Logically, helmeted drivers should also receive significantly fewer head injuries than bare-headed drivers. Similarly, helmeted pedestrians should be less likely to receive serious head trauma than bare-headed ones. But such studies don't exist because there aren't enough helmeted drivers or pedestrians to make a comparison. In other words, one of the reasons we think helmeted cyclists are safer than unhelmeted ones may be due to availability of information more than actual levels of head safety.

Maybe that explains why there's no comparable fear of driving or walking without a helmet."

So the evidence remains clear that cyclists who wear helmets who have accidents are less likely to suffer brain injuries than cyclists that don't wear helmets that have accidents - which in my mind is evidence enough to more than justify wearing a helmet. The obvious next question is - are cyclists who wear helmets somehow more likely to get into accidents than cyclists who do not wear helmets - this is a complicated and fascinating debate, which we'll come back to later in this post. But this is not the case the author makes. The author takes the cognitive leap to suggesting that helmets in and of themselves might actually be harmful.

The author cites a New York Times article which reports an increase in bicycle head injuries during a time when helmet use became widespread which coincided with an overall decrease in cycling. There is an obvious correlation ≠ causation issue here which we could talk about all day from a great many different angles, so we'll begin by looking at the author's arguments one by one.

First, the author argues that "wearing a helmet changes how drivers perceive the cyclist" citing a study that suggests drivers pass closer to a cyclist wearing a helmet. The naturalistic study involving only one participant (who was also the experimenter) is interesting, but obviously potentially vulnerable to the same kind of conscious or unconscious bias that might lead a driver to drive closer to a cyclist. On its own it is not compelling evidence for the argument that one should not wear a helmet.

Next the author argues that "the design of the helmets themselves may increase the chance of some types of injuries when incidents do occur" linking to a meta-analysis but conveniently failing to mention what the meta-analysis actually found overall

"In conclusion, the evidence is clear that bicycle helmets prevent serious injury and even death"

The author also failed to mention that the meta-analysis concluded that the supposed increase in (neck) injuries was found in old data and may not be applicable to the lighter helmets now in use. So that argument is also null and void - and a textbook example of cherry-picked data amongst a sea of data showing the precise opposite.

The author ends their case for the argument that helmets may be harmful with a crucial point which is worth thinking long and hard about if you are a cyclist:

"Finally, wearing a helmet may create a false sense of security and induce risk-taking that cyclists without head protection might not make. Those wearing helmets may take risks that they wouldn't otherwise take without head protection."

Due to the ethical problems that prevent researchers asking cyclists to ride with or without a helmet, this is a difficult hypothesis to test - but it certainly seems likely that wearing a helmet might lead cyclists to overcompensate by taking greater risks. It is worth reminding yourself that a helmet only provides limited protection even though it might lead some people to behave like they are invincible. If helmets really do make cyclists take greater risks then making their use obligatory presents an interesting public health conundrum. Is it possible that helmets could make people safer if they have an accident whilst simultaneously making them behave even more dangerously? It is next to impossible for us to know for sure if this is occurring due to the immense amount of additional uncontrollable variables in the equation.

The author of the blog post makes a number of good points but seems to have overstated the case. These arguments highlight the fact that when it comes to human behaviour, epidemiological data gets incredibly messy and it is can be all too easy to intentionally or not, make whatever argument we want based on what data we look for. We've not even touched on the possibility that the type of casual cyclist who chooses not to wear a helmet may already behave very differently to the type of cyclist who does choose to wear a helmet. As Ben Goldacre explained in an editorial on bike helmets in the British Medical Journal, we are dealing with "confounding variables that are generally unmeasured and perhaps even unmeasurable."

What we are left with is a paradox. On an individual level it is clear that helmets can and do save cyclists from serious head injury and death provided that cyclists and the drivers around them don't overcompensate by taking greater risks. On a societal level, it seems that laws enforcing helmet use have done nothing to make cyclists safer and have driven a great many casual cyclists off the road - which as the author of the post rightly points out, indirectly increases the danger to cyclists in the long run, as cyclists are protected by strength in numbers. Furthermore, in places where helmet requirements have driven cyclists off the road it has been argued that the negative effects on public health outweigh any possible benefits in prevented injuries. In Australia, when a helmet law was introduced at a time when the popularity of cycling was on the rise, a 44% decrease in children cycling was observed, which was five times the size of the increase in children wearing helmets. According to a paper published in the BMJ, it would take "at least 8000 years of average cycling to produce one clinically severe head injury and 22,000 years for one death". It has also been estimated that the health benefits of cycling outweigh the life-years lost by a factor of twenty to one.

The whole argument reminded me of an anecdote regarding the introduction of helmets for soldiers in WW1 and the supposed consequent increase in recorded head injuries. As the story goes, generals nearly recalled the helmets before it was realised that the rise in head injuries could be explained by injuries that before the introduction of helmets would have been recorded as deaths. I've been unable to track down a bona fide citation for this anecdote amongst the many repetitions of it online (but I've not found any attempts to disconfirm it either). In any case, this has certainly not been true for bike helmets - where deaths still make up a tiny fraction of outcomes from bike accidents, but it is an interesting demonstration of how statistics can mislead - something that seems to be going on left, right and centre in the bike helmet debate.

After looking at the evidence, I'm happy to conclude that I'll choose to wear a helmet, but I'll not be beating the drum that cyclists should be forced to wear helmets - as the health benefits of cycling with or without a helmet are so great, that the risks of riding with or without a helmet pale in comparison. The evidence paradoxically seems to show that while wearing helmets does make cyclists safer, helmet laws don't make cyclists safer and actually harm public health in the long run.

References:

Attewell R.G., Glase K. & McFadden M. Bicycle helmet efficacy: a meta-analysis., Accident; analysis and prevention, PMID:

Goldacre B. & Spiegelhalter D. (2013). Bicycle helmets and the law., BMJ (Clinical research ed.), PMID:

Robinson D. (1996). Head injuries and bicycle helmet laws, Accident Analysis & Prevention, 28 (4) 463-475. (PDF)

Walker I. (2006). Drivers overtaking bicyclists: objective data on the effects of riding position, helmet use, vehicle type and apparent gender., Accident; analysis and prevention, PMID:

Wardlaw M.J. Three lessons for a better cycling future., BMJ (Clinical research ed.), PMID:

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A "very massive star" in the Kinman Dwarf galaxy caught the attention of astronomers in the early years of the 2000s: It seemed to be reaching a late-ish chapter in its life story and offered a rare chance to observe the death of a large star in a region low in metallicity. However, by the time scientists had the chance to turn the European Southern Observatory's (ESO) Very Large Telescope (VLT) in Paranal, Chile back around to it in 2019 — it's not a slow-turner, just an in-demand device — it was utterly gone without a trace. But how?

The two leading theories about what happened are that either it's still there, still erupting its way through its death throes, with less luminosity and perhaps obscured by dust, or it just up and collapsed into a black hole without going through a supernova stage. "If true, this would be the first direct detection of such a monster star ending its life in this manner," says Andrew Allan of Trinity College Dublin, Ireland, leader of the observation team whose study is published in Monthly Notices of the Royal Astronomical Society.

So, em...

Between astronomers' last look in 2011 and 2019 is a large enough interval of time for something to happen. Not that 2001 (when it was first observed) or 2019 have much meaning, since we're always watching the past out there and the Kinman Dwarf Galaxy is 75 million light years away. We often think of cosmic events as slow-moving phenomena because so often their follow-on effects are massive and unfold to us over time. But things happen just as fast big as small. The number of things that happened in the first 10 millionth of a trillionth of a trillionth of a trillionth of a second after the Big Bang, for example, is insane.

In any event, the Kinsman Dwarf Galaxy, or PHL 293B, is far way, too far for astronomers to directly observe its stars. Their presence can be inferred from spectroscopic signatures — specifically, PHL 293B between 2001 and 2011 consistently featured strong signatures of hydrogen that indicated the presence of a massive "luminous blue variable" (LBV) star about 2.5 times more brilliant than our Sun. Astronomers suspect that some very large stars may spend their final years as LBVs.

Though LBVs are known to experience radical shifts in spectra and brightness, they reliably leave specific traces that help confirm their ongoing presence. In 2019 the hydrogen signatures, and such traces, were gone. Allan says, "It would be highly unusual for such a massive star to disappear without producing a bright supernova explosion."

The Kinsman Dwarf Galaxy, or PHL 293B, is one of the most metal-poor galaxies known. Explosive, massive, Wolf-Rayet stars are seldom seen in such environments — NASA refers to such stars as those that "live fast, die hard." Red supergiants are also rare to low Z environments. The now-missing star was looked to as a rare opportunity to observe a massive star's late stages in such an environment.

Celestial sleuthing

In August 2019, the team pointed the four eight-meter telescopes of ESO's ESPRESSO array simultaneously toward the LBV's former location: nothing. They also gave the VLT's X-shooter instrument a shot a few months later: also nothing.

Still pursuing the missing star, the scientists acquired access to older data for comparison to what they already felt they knew. "The ESO Science Archive Facility enabled us to find and use data of the same object obtained in 2002 and 2009," says Andrea Mehner, an ESO staff member who worked on the study. "The comparison of the 2002 high-resolution UVES spectra with our observations obtained in 2019 with ESO's newest high-resolution spectrograph ESPRESSO was especially revealing, from both an astronomical and an instrumentation point of view."

Examination of this data suggested that the LBV may have indeed been winding up to a grand final sometime after 2011.

Team member Jose Groh, also of Trinity College, says "We may have detected one of the most massive stars of the local Universe going gently into the night. Our discovery would not have been made without using the powerful ESO 8-meter telescopes, their unique instrumentation, and the prompt access to those capabilities following the recent agreement of Ireland to join ESO."

Combining the 2019 data with contemporaneous Hubble Space Telescope (HST) imagery leaves the authors of the reports with the sense that "the LBV was in an eruptive state at least between 2001 and 2011, which then ended, and may have been followed by a collapse into a massive BH without the production of an SN. This scenario is consistent with the available HST and ground-based photometry."

Or...

A star collapsing into a black hole without a supernova would be a rare event, and that argues against the idea. The paper also notes that we may simply have missed the star's supernova during the eight-year observation gap.

LBVs are known to be highly unstable, so the star dropping to a state of less luminosity or producing a dust cover would be much more in the realm of expected behavior.

Says the paper: "A combination of a slightly reduced luminosity and a thick dusty shell could result in the star being obscured. While the lack of variability between the 2009 and 2019 near-infrared continuum from our X-shooter spectra eliminates the possibility of formation of hot dust (⪆1500 K), mid-infrared observations are necessary to rule out a slowly expanding cooler dust shell."

The authors of the report are pretty confident the star experienced a dramatic eruption after 2011. Beyond that, though:

"Based on our observations and models, we suggest that PHL 293B hosted an LBV with an eruption that ended sometime after 2011. This could have been followed by
(1) a surviving star or
(2) a collapse of the LBV to a BH [black hole] without the production of a bright SN, but possibly with a weak transient."

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