The link between head injuries and dementia has just grown stronger: A new study of 2.8 million people in Denmark, including those who had and had not experienced a traumatic brain injury (TBI) in their lifetime, found that those with at least one TBI had a 24 percent increased risk of dementia, on average. The average diagnosis age was 81.

TBIs occur whenever an external force causes physical damage to the brain, and they range from the mild and moderate to the severe. The dementia risk was slightly higher for severe TBIs. As expected, the number of TBIs is positively correlated with risk.

The NHS emphasize that this is about increased risk after receiving a TBI, however. The absolute risk of developing dementia throughout your lifetime is very small. Of this sample, only 5.1 percent of those that had a TBI developed dementia, compared to 4.5 percent of non-TBI people that also developed dementia.

Writing in The Lancet: Psychiatry, the team explain how they tracked the health of these people between 1999 and 2013, and marked off those who were diagnosed with dementia in that period. They adjusted for other health factors to try and rule out confounding variables that may have significantly contributed to the dementia diagnosis.

The researchers – from the University of Washington, Copenhagen University Hospital, and Aarhus University Hospital – also found that getting a TBI in your 30s is associated with a 37 percent increased risk, compared to a 2 percent increased risk for those in their 50s. A TBI in your 20s is associated with a 63 percent increased risk.

Professor Jonathan Schott, a neurologist at University College London (UCL) not involved in the study, said that this enormous sample size-based cohort study “provides perhaps the best evidence yet that traumatic brain injury is a risk factor for dementia.”

He cautioned, however, that we still don’t know what types of head injury – such as sports-based concussions or traffic accident-related injuries, for example – can be included in this risk assessment. (It’s worth noting that contact sports are not that popular in Denmark.)

Additionally, the population studied all came from Denmark, which means the results can’t be applied to other countries with very different population groups. It's also an observational study, so no direct cause-and-effect relationships between TBIs and dementia can be definitively described.

Dementia, a chronic or progressive syndrome associated with a decline in brain functioning, has a variety of causes, including an abnormal build-up of particular proteins in the brain. Genetics and environmental factors are also thought to play a role.

Alzheimer’s is the most common form of dementia, contributing to 60-70 percent of the 50 million people living today with the condition.

Although dementia is associated with aging, it’s not a normal feature of getting older. However, as people are living longer, the number of people with dementia is increasing, and 10 million more people are diagnosed with the syndrome every year.

There is no cure and limited treatment methods available – unsurprisingly, the World Health Organization has made it a public health priority.

Concussions and other TBIs have long been linked to dementia, but earlier studies have found mixed results, and their methodologies have often been flawed. This study, arguably the largest of its kind, certainly does a lot to corroborate the hypothesis, even if the underlying biological mechanisms remain somewhat elusive at present.