There has been a fair amount of attention paid recently to the relationship between contact sports and traumatic brain injuries, and no area is off limits, from MMA to high school-level football. Adding to the amount of information out there is a new study, published on Monday, that reveals football helmets aren’t as protective against traumatic brain injuries as professionals have believed them to be.

Resulted showed that football helmets currently do not protect against injuries to the side of the head, which may lead to traumatic brain injuries and encephalopathy. Rather than use the typical test for impact and helmet safety, researchers added sensors to the dummies being observed, measuring the linear and rotational response to a 12mph hit.

Helmets went through 330 tests, with researchers concluding that even the best only decrease TBI risks by 20 percent, when compared to not wearing one at all. Strength, as well, varied across brands; one brand known for better concussion protection scored particularly low.

The study’s co-author Frank Conidi explained in a statement: “Alarmingly, those that offered the least protection are among the most popular on the field. Biomechanics researchers have long understood that rotational forces, not linear forces, are responsible for serious brain damage including concussion, brain injury complications and brain bleeds. Yet generations of football and other sports participants have been under the assumption that their brains are protected by their investment in headwear protection.”

This isn’t to say that helmets are useless in protecting against TBI. Results revealed helmets decrease risk of fractures by 60 to 70 percent and focal brain tissue bruising by 70 to 80 percent.

Traumatic brain injuries fall into two distinct groups: closed head injuries (CHI) and open head injuries (OHI), with the latter being more apparent. Closed head injuries display no external signs but, instead, may move the brain around, causing it to hit the skull in the process.

In both instances, a TBI may surface months after the incident, which could be as seemingly-uneventful as a fall. The individual may start displaying signs like a change of personality and forgetting basic physical and social skills.