Can Safety Helmets Protect Against Dangerous Rotational Forces?

Reducing effects of overlooked angled impacts through innovative safety helmet design.

• By Tim Gallant
• Apr 01, 2022

Since 1919, the hard hat has been the most iconic symbol of worker safety. But despite more than six million hard hats and safety helmets sold each year, more than 18,000 still suffered traumatic brain injuries (TBIs) in 2019 (BLS data).  

While hard hat technology has evolved tremendously over the last century, there is one thing that really hasn’t—a one-track focus on protection from direct, linear impacts. Though incredibly important, this focus has neglected the TBI-causing rotational forces that often result from more common, angled impacts.  

Direct Impacts VS. Angled Impacts 

To understand what an angled impact is, we first need to understand direct impact. A direct impact is, well, exactly that—a square, dead center hit to the top or sides of the head. When a perfect direct impact occurs, there is very little rotational force on the brain (more on that in a bit). Which is all fine and dandy except for that when an object, floor or beam comes toward your brain, it’s not going to give you the common courtesy of adjusting its path to hit you square on.  

More frequently, the object is going to strike your head at an angled impact—introducing rapid rotational forces against the brain’s center of gravity. Unlike a concentrated compression caused by a single direct impact, rotational forces cause the brain to twist or pull in multiple directions. These shifts and rotations can cause a tearing of nerve fibers, a side effect known scientifically as brain shear.  

Why Are Angled Impacts so Dangerous? 

Due to the pulling and stretching of brain shear, angled impacts are actually more likely to result in traumatic brain injuries (TBIs) than direct impacts. What makes things even more concerning is the frequency at which brain shear-inducing incidents occur on the worksite. Falls, which are more likely to result in angled impact, are the most common worksite accident—and also the most common incident to result in a TBI.  

As anyone who has followed the evolving saga of head trauma in professional sports knows, the effects of TBIs can be delayed, long-lasting and life-changing. In 2019 alone, there were 61,000 TBI-related deaths in the United States.  

TBIs resulting from brain shear are known as closed brain injuries. Unlike an open brain injury, these occur without breaking the skull. The type of closed brain injury that most frequently occurs as a result of an angled impact is Diffuse Axonal Injury (DAI). DAI is the shearing of the brain’s long connecting nerve fibers (axons) that happen to the brain as it shifts and rotates. “Diffuse” refers to how the resulting dysfunction spreads to a widespread area throughout the brain. Often, these tears are microscopic and cannot be seen on CT or MRI scans.  

A concussion is considered to be a mild form of DAI, resulting in symptoms such as headache, dizziness or memory loss. While symptoms of concussions and more severe DAIs can certainly show immediately after the impact, they often evolve in the following hours, days or weeks.  

What is Being Done to Protect Workers from Angled Impacts? 

Currently, both North American Hard Hat Standards (ANSI Z89.1 and CSA Z94.1) only test for direct, linear impacts. Type 1 hard hats are approved for top of head, with Type II approved for top, front, back and sides.  

To meet ANSI Z89.1 as well as European standards, a hard hat or safety helmet must pass a force transmission test. In this test, the hat or helmet is placed on a dummy and hit squarely with an 8lb (3.6kg) impactor from the top. If testing for Type II, it will also be hit from the front, back and sides.  

This method determines individual maximum force readings based on various impact velocities. To be approved for the worksite, the hat or helmet cannot transmit over 4,450 newtons (1,000 pound-force).  

Certainly, direct impact testing is incredibly important to worker safety. Yet, the limitations of this widely used method do not adequately measure rotational forces caused by angled impacts. And considering the TBI-causing risk of rotational forces, it’s clear that something’s missing.  

In 1996, Swedish neurosurgeon Hans von Holst began exploring the relationship between brain injuries and helmet construction. Upon recognizing helmets were inadequately protecting against rotational impacts, Von Holst teamed up with researcher Peter Halldin to engineer a solution.  

Their efforts resulted in the creation of a multi-directional impact protection system now known more simply as Mips. This technology is built around the discovery that relative motion between low friction layers reduces the effect of rotational force. By integrating a sliding layer into helmets, Von Holst and Halldin found a way to redirect the force of rotational impacts that would otherwise be transferred to the head.  

The technology mimics the protective properties in the human brain by adding a second layer of rotational protection. Cerebrospinal fluid is our natural protection system that allows the brain to move relative to the skull. And just as the cerebrospinal fluid allows that movement, Mips is designed to reduce the strain in the brain by allowing the helmet to briefly rotate a short distance around the head of the wearer, so the most severe rotational aspect of the crash can essentially be taken on by the motion of the helmet, not the brain. 

To test effectiveness, Mips helmet test lab includes four testing machines designed to mimic real-world impact scenarios. Of the many comprehensive and helmet-specific tests conducted in the lab, Mips has found a vertical drop onto an angled surface to be the most simple and robust method for determining reduction of rotational force.  

Though no testing standard currently exists for hard hats and climbing-style safety helmets regarding rotational motion, the technology has been proven via third-party validations from external institutes and organizations, such as Virginia Tech, Folksam and Länsförsäkringar. In addition to that, there are multiple doctoral theses presented around the Mips system.  

Initially focused on the recreational space with bike and ski helmets, Mips is now expanding into the industrial space, collaborating first with leading safety helmet manufacturers in Europe before breaking into the North American PPE market in 2021.   

It’s important to note that the added safety measure is found only in climbing style safety helmets and not traditional hard hats. This is because of the added security provided by the chin strap, which is needed for the system to function properly.   

This article originally appeared in the April 2022 issue of Occupational Health & Safety.