Crack! The sound of football helmets colliding on the field is an audible sign that fall is just around the corner.

But that sound also comes with a darker side. Mounting scientific evidence shows that repeated hits to the head — even if they don’t result in concussions — may cause lasting damage in the brain and perhaps progressive neurodegeneration called chronic traumatic encephalopathy, or CTE.

The problem has set off alarms at all levels of the sport, with coaches, trainers, parents and leagues wrestling with tough questions about whether football can be made safer for athletes.

In just the past month, three young players have died after playing football, two of them with head injuries. The third death is still under investigation.

After his son’s death, Ryan Craddock, the father of 13-year-old Cohen Craddock, called for all student players to wear additional soft-shell coverings called Guardian Caps. They are not helmets but pliable padded coverings that slip over a player’s existing helmet.

“I believe if my son would have been wearing something like this, this would have made a totally different outcome,” Craddock told CNN. “You’re not modifying the helmets. It’s just something that attaches directly on. So why not just have that extra layer of protection?”

Intuitively, putting more padding around a football player’s head might sound like a good idea, but there’s little independent research showing that it reduces the force of blows to the head or that it prevents head injuries.

Lab studies in which researchers simulate hits to the head have shown that the caps can reduce impact forces. But the handful of published studies that have tested the caps on college football players running drills have failed to find any benefit compared with helmets alone. There are no published independent studies that have tried to measure whether Guardian Caps reduce concussions or head injuries in players and no testing to see if they might work for younger players.

Guardian Caps have gotten a big boost from the NFL, which now allows all players to wear them during regular-season games. The league also mandates them for most players during every preseason practice, as well as regular and postseason practices with contact.

The caps’ maker, Guardian Sports, says its own extensive testing — as well as years of use by many large college football teams — proves that they reduce injuries on the field.

“We will tell anyone who asks that no product can make a player concussion proof,” Erin Hanson, who started Guardian Sports with her husband in 2011, said in a statement.

“Our product, however, is well past the point of proof of concept. We now have over 12 years of on-the-field data and feedback with over 500,000 players using Guardian Caps. Schools like UGA, Ga Tech, Alabama, Tennessee wouldn’t use a product year after year that isn’t working for their players. The NFL and CFL certainly wouldn’t mandate Caps if the data wasn’t clear,” Hansen wrote.

Dr. David Camarillo, an associate professor of bioengineering at Stanford University and a former college football player who has tested the Guardian Caps, said he doesn’t think the evidence is so clear.

“I think the rationale is, football’s in big trouble. People are worried about CTE. We’re seeing kids dying,” Camarillo said. “People want to try to do something, right, as long as they don’t think it’s going to hurt.”

On its website, Guardian says its XT model reduces the impact to a player’s head by up to 33%, while the beefier model used by the NFL reduces head acceleration in a collision by up to 40%.

NFL Vice President Jeff Miller told CNN’s Coy Wire that the caps have made players safer.

“The last two seasons, we saw about a 50% decrease in concussions in the positions that we mandated to wear them. So of course, we said, ‘OK, all positions will now wear them in the preseason,’” Miller said.

The NFL has not released the data behind its claims, however, leading to skepticism among some scientists.

“It’s hard to take what the NFL says at face value,” Camarillo said. “What they really ought to do is publicly share the data, not just publish it but release it,” and follow up by funding independent studies that could confirm their conclusions.

The league says it plans to publish research backing its conclusions soon.

“We actually have papers in progress that have been submitted to journals right now. They’re under review, so you should see that in print. You know, takes a while to get things in print, but it’s in the process,” NFL Chief Medical Office Dr. Allen Sills said.

Sills said one reason the published studies may seem to conflict with the NFL’s results is that the Guardian Cap has changed over time.

“Some older models of the cap did not show a benefit in our testing, but the newer version of the cap did show this laboratory reduction,” he said.

In a 2022 position statement, the National Athletic Trainers’ Association cautioned about the use of after-market helmet add-ons like Guardian Caps, saying that “The current evidence describes no benefit.”

Several groups of independent researchers have recently tested Guardian Caps in laboratory settings and in the real world, and they say the jury is very much out on the question of whether they work.

Studies in two labs measuring reductions in the force of twisting and side-to-side impacts to the head show a measurable — if small — benefit.

In Virginia Tech’s Helmet Lab, researchers put several models of the caps through repeated collision tests that were designed to mimic the way players can hit their heads on the field.

Lab director Dr. Steve Rowson, a biomedical engineer, said that testing has found that Guardian Caps reduce the force of an impact, especially if the thicker model deployed by the NFL is used and if both players involved in a hit have them on.

Rowson said their experiments showed that the standard model of the Guardian Cap reduced the force of acceleration by up to 5% while the model used by the NFL reduced the force of linear acceleration by as much as 14%.

When the testing simulated the impact of two players who were each wearing a Guardian Cap, the reduction was even greater.

“We saw that it pretty much doubled the effect,” Rowson said. The lab tests suggests it could reduce concussion risk by 15 to 35%, he said.

The Virginia Tech study hasn’t been published in a peer-reviewed journal, however. The team says it has a paper under review, and it has published a summary of the findings online.

Several more published studies by independent researchers have looked for the reduction in impact forces in players on the field but haven’t seen it.

Two studies by researchers at the University of North Carolina and the University of Nevada, Reno, used special sensors to measure the force of head impacts in college football players and found no difference between players who wore the caps and those who didn’t.

The University of North Carolina study followed 10 Division I college football players across 14 practices in the fall 2022 season. Half wore Guardian Caps over their regular helmets; the other five did not.

All the helmets were fitted with special sensors to measure head impacts. Cameras were used to make sure the measurements corresponded to actual hits and weren’t just false positives.

After measuring and observing nearly 1,000 hits, the researchers found that there was no difference between the forces experienced by players wearing Guardian Caps and those who did not. The research was published in October 2023 in the International Journal of Research and Public Health.

Similarly, the study by researchers at the University of Nevada, Reno, in football players didn’t find a benefit to using Guardian Caps.

The study followed seven players across six practices. For half of the practices, they wore the extra padding on their helmets; for the other three, they did not. All the players wore special mouthguards with sensors to detect head impacts.

The researchers recorded more than 800 video-confirmed hits.

“There was no difference after implementation of the Guardian Caps to reduce the forces that are at the brain and head,” said Dr. Nicholas Murray, an associate professor at the University of Nevada, Reno, who worked with his graduate student Kristen Quigley. Their study was published in June in the Journal of Athletic Training.

Considering all the known science on Guardian Caps, “I don’t think we can answer that question, honestly, if these are protective or not,” Murray said.

A third recent study, by researchers at Stanford, split the difference. When Camarillo and his team tested Guardian Caps in their lab, they found that the extra padding measurably reduced the force of impacts. When they put them on players in the field, however, there was no significant differences between those who wore Guardian Caps and those who weren’t.

Rowson, at Virginia Tech, thinks the studies that have tested the caps on players in the field perhaps haven’t found a benefit because they have been small and may not have enough data to pick up the level of benefit the caps are providing.

Murray, at the University of Nevada, Reno, says there’s no doubt that laboratory testing is important. Conditions in a lab can be carefully controlled and the tests carried out again and again.

But real-life studies are important too, he said. The results of lab studies don’t always translate directly to real life.

“On the field is where real people are engaging and real behavior is being acted out,” Murray said.

Murray said forces measured in a lab don’t directly translate into concussions or head injuries on the field, and head injuries are what they’re really trying to prevent.

In a lab, scientists measure acceleration in terms of g-force, or gravitational forces. There’s no threshold of g-forces that determines when a hit would definitely cause an injury to the brain.

Increasingly, he said, science suggests that it’s repeated blows to the head that damage the brain, perhaps even more than the force of those blows.

“I’ve seen unremarkable concussions, meaning that it’s just the lineman getting off his block, hitting his head, and comes over to the sideline and says, ‘Oh my goodness, I feel terrible.’ And I’ve seen wide receivers who have just gotten creamed, completely creamed, and are totally fine and haven’t had a concussion. And so not every blow to the head is a concussion,” he said.

For parents who are trying to figure out how best to protect their kids, Rowson’s advice is to choose a good helmet before considering adding a Guardian Cap.

“The most important factor is which helmet the player is in, not necessarily whether or not they’re wearing a cap, because some helmets test really good, and that helmet without a cap could be better than putting a cap on a poor performing helmet,” he said.

Rowson says there’s no evidence that adding padding to the outside of a helmet is going to cause a player harm.

Guardian Caps cost about $70 when ordered from the company. The models used by the NFL run $125 each.

Murray said that if teams are considering how to use limited funding to enhance player safety, he would advise hiring an athletic trainer.

“They save lives. They’re instrumental in the health and safety of the athletes,” he said. “I would argue that your money would be better spent elsewhere.”

CNN’s David Close, Coy Wire, Holly Yan, Amanda Sealy and Nadia Kounang contributed to this report.

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