In its earliest days, the purpose of the football helmet was to prevent the sport’s scariest, most visible injuries: grisly skull fractures and broken necks. Today’s safety standards for helmets were developed in response to the high number of fatalities in 1968, when 36 players were killed across all levels of the sport. But in recent years, a growing body of evidence has shown that these helmets still aren’t nearly enough to prevent dangerous head injuries. The concussion-proof helmet has become the holy grail of helmet design. But can such a thing really exist?

“A concussion can actually happen without actually hitting your head,” said Kyle Lamson, the head of research and development at Xenith, a helmet company based in Detroit, Michigan. And “if you can get a concussion without actually hitting your head, then a helmet can't possibly prevent every concussion that can actually happen.”

Glenn Beckmann, the director of marketing communications for Schutt Sports, echoed those sentiments. “There is no such thing as a concussion-proof helmet. And there won’t be for the foreseeable future,” he wrote in an email. “We still aren’t anywhere close to knowing enough to say we can prevent concussions. There are just too many unknowns when it comes to defining a concussion and what causes it."

Still, many athletes, parents, and coaches view the helmet—perhaps the most recognizable safety measure in modern sports—as something of a savior, the thing that will keep most concussions at bay.  According to a University of Pittsburgh Medical Center poll, only one in four Americans "understand[s] that safety equipment—such as helmets or mouth guards—cannot prevent the majority of all concussions." The company that makes the safest helmet can sell its product at a premium. (For example, Riddell’s SpeedFlex, which topped Virginia Tech's STAR safety-ratings study in 2014, sold for $400, or $100 more than most competing helmets.) Consequently, most helmet companies pour a large percentage of their resources into research and development.

But what exactly are they moving towards? Consumers demand safety, but safety can be hard to define. Several athletic-equipment companies’ promises of concussion reduction haven’t held up to scrutiny. In 2013, for example, the Federal Trade Commission ruled that Riddell could not claim that its Revolution model reduced concussion risk by 31 percent; the following year, the agency sent a letter asking several sporting-goods retailers to substantiate the descriptions of mouth guards on their websites as protecting against concussions. Public studies like Virginia Tech’s, then, are a way for companies to imbue their safety claims with some authority.

Since it was developed by the Virginia Tech engineering professor Stefan Duma in 2011, the STAR (Summation of Tests for the Analysis of Risk) system has been influential in determining which helmets see higher sales than others. Some high-school coaches and even school boards have made five-star helmets mandatory for their players. In response, many companies have altered their models to better conform to the system’s criteria—“The STAR system has significantly affected how we design helmets,” Beckmann wrote—but not all companies are happy about STAR’s power in the field.

“These ratings are misleading people,” Robert Erb, the CEO of the helmet company Schutt, told Bloomberg in January. “People are now using them to determine which helmets to put their youth leagues into, which is truly insane.” (The safest helmet for a kid isn’t necessarily the safest for an adult, and vice versa.) When Schutt released its “VTD” helmet line (short for “variable thickness and durometer”) in 2014—which included two helmets that received a five-star safety rating—the company’s press release noted Schutt’s disagreement with the Virginia Tech methodology. "To date, there is scant evidence to support the conclusion that the VA Tech study is predictive," the release reads, "that is, that a higher STAR rated helmet will reduce the likelihood of concussive episodes in football players."

Some helmet-safety researchers feel the same way. The STAR system has been come under fire in recent years for failing to incorporate rotational acceleration into its criteria. The Virginia Tech methodology only looks at linear acceleration, or straight-line blows. But evidence shows that rotational acceleration—the kind of force that comes from a head-spinning blow—is the more important factor in determining concussion risk. Worse, a helmet built primarily to withstand linear forces may actually leave the athlete more exposed to rotational forces. "If you make a big, fat, soft helmet," Blaine Hoshizaki, a researcher at the University of Ottawa’s Neuro-trauma Impact Science Laboratory, told Washingtonian in 2014, "you're making a huge helmet that will increase the risk of getting hit and may create higher rotations."

A recent study from the NFL and the NFL players association addressed this gap, issuing a new safety ranking of 17 helmets that incorporated rotational acceleration.

Thomas Mayer, a physician with the NFL Players Association, emphasized that there was no statistically significant difference between the 10 top-performing helmets—but that hasn’t stopped Xenith, the manufacturer of the highest-rated helmet, from loudly touting its score as a victory. "That's obviously one of our bigger marketing tools we've ever had," said Joe Esposito, Xenith’s CEO. On Xenith's website, the first thing consumers see is a large image celebrating the Epic Varsity helmet’s top ranking in the study. "We've seen a big jump in the percentage of our sales tied to Epics."

The company has also seen a boost in sales of its youth helmets, even though the NFL-NFLPA safety ratings don’t transfer from one product to another—the study specifically examined conditions typical to National Football League games. A memo from the NFL and NFLPA study authors to the league’s players, trainers, and equipment managers noted: " It is important to emphasize that these results were based on testing intended to represent NFL impacts and thus, the conclusions on helmet performance cannot be extrapolated to collegiate, high school, or youth football."

The caveat highlights a broader problem with the helmet industry as a whole, one that’s become clearer as concussion awareness has caused the industry to swell over the past decade. Football helmets cannot possibly be a foolproof measure against concussion, which is an extremely complicated injury to measure and diagnose. This leaves helmet companies scrambling for ways to separate their products from the rest of the pack—and with so little concrete evidence around, the companies chase down any accolades they can, no matter how incomplete or irrelevant the methodology.

"Helmets do two things very well: protect the skull, and absorb direct linear force impact. But protecting the brain from injury? There’s no real way to do that," Beckmann writes. "Think of a carton of eggs. We’ve figured out how to protect the shells and keep them from cracking, but if you shake an egg, the yolk can still get scrambled. So, it’s unwise to expect a helmet to do something it’s not designed to do.”

“We’re seeing a false sense of security in the marketplace, driven by the desire to find simple answers to a very complex biological injury,” he added. “But, ultimately, there is no simple answer to the question of how to prevent concussions."