Many people will experience a concussion at some point during their life – whether that's from contact sport, or just an unfortunate fall. But even a 'mild' version of this injury can cause long term health impacts for a portion of those who experience it.

A new study from a team of researchers across New Zealand has found that up to eight years later, adults who had experienced a mild concussion reported more ongoing symptoms, depression, and problems with work than those who never had one.

This is not to say that all people who experience a concussion – also called a mild traumatic brain injury (TBI) – will have ongoing symptoms, however it highlights that concussion can be a really life-altering event to those people who don't just get better after a couple of weeks.

"It is estimated that more than 10 million people experience a TBI each year, with 70–95 percent of these being classified as mild. TBI has widespread impacts, and whilst the effects of moderate and severe TBI are well documented, much less is known about the long-term outcomes of mild TBI," the researchers – led by University of Waikato psychology researcher Nicola Starkey – wrote in their new paper.

"Over a third (36 percent, n = 54) of the participants with mild TBI reported that they thought they were still affected by the brain injury they had 8-years ago."

A concussion can occur anytime your head collides with something, such as a steering wheel, a knee, or the ground, or even when your body rapidly changes direction.

But it's not the collision that causes the damage: the forces from the sudden, extreme changes in movement act on the brain tissue, stretching out the ends of neurons called axons. This can cause myriad symptoms such as loss of consciousness, headaches, nausea, blurred vision, and mood changes.

The researchers found participants through the BIONIC study, which looked at all cases of TBI in Hamilton and Waikato, New Zealand over a year-long period between 2010 and 2011.

Although the team identified 1,298 people with mild TBI, they ended up with 346 that completed the original survey and 151 that completed the follow-up survey eight years later. The team then matched the 151 people with an equal number of controls who had never had a concussion.

Unsurprisingly, those who'd had a concussion reported more problems. What is surprising, however, is just how long these symptoms can stick around for – a third of the participants believed they were still impacted by their concussion eight years later. These long-term symptoms weren't mild either, with the questionnaires suggesting some people had PTSD or anxiety, issues with work, and post-concussion syndrome.

The worst outcomes seemed to be in women, particularly those who'd had multiple concussions.

"The mild TBI group reported significantly greater post-concussion symptoms compared to the TBI-free group. Females with mild TBI were twice as likely to exceed clinical cut-offs for post-concussive and PTSD symptoms compared to the other groups, and reported their health had the greatest impact on time-related work demands," the team wrote in their paper.

"Twice the proportion of women with repetitive mild TBI exceeded the clinical cut-offs for post-concussive, anxiety and PTSD symptoms compared with males with repetitive TBI or women with single TBI."

The researchers suggest better injury management and treating any mental health issues that might come up could help lower these long-term consequences. And in the meantime, it's one heck of a reminder to protect your head.

The research has been published in PLOS One.