Football headers: 'Damage is covered up'
Professional footballers can perform dozens of headers during a match, and hundreds more over a week's training - adding up to tens of thousands of blows to the head over their career.
Scientists now say that repeated blows to the head from headers may be linked to long-term brain damage.
The research examined the brains of six players renowned for headers - all of whom developed dementia in later life.
Here, players and coaches share their reactions.
'Six of my teammates have Alzheimer's'
Ian St John, who played for Liverpool between 1961-71, says six of his teammates - from a group of about 16 players - now have Alzheimer's.
"For people of my vintage, I would say all the facts that we have got, stand up," he told the Victoria Derbyshire programme.
"And I don't know why the FA and the PFA have covered this up for years.
"I talked about it to the PFA a couple of years ago, and their answer was: 'Well, women get dementia, so therefore it's not an industrial injury'. Which is a load of nonsense isn't it?
"I don't know about today's light ball, but in our era, heading that heavy ball day in and day out - not just at matches but training as well - the lads know at this stage of their lives we're either dying or have dementia.
"The footballs that we've been heading for years are causing this, but they're [the FA] in denial about it.
"How many goalkeepers have got dementia over the years? If they did a survey, it would be interesting if the answer was none, which means that the goalkeeper - the only guy on the field who is not heading the ball on the field - is OK."
'It was a bit scary at the time'
Former England striker Kevin Davies says contact sportsmen know there's always going to be a risk, but more research needs to be done.
"I'm well renowned for heading the ball and we used to get all the stats back - during the game it can be between 25 and 40 times.
"When you start to look at the numbers in terms of professional players, the amount of contact they have with the ball - if you look at my career, 800 career games, and then you take into training, the numbers start to stack up and it could be anywhere between 10,000 and 50,000 times that you're heading the ball.
"The ball has changed a lot, the training methods have changed a lot now.
"[Previously] you'd see some sessions where you were hurling balls at defenders and it was a bit scary at the time - I think that has changed a lot and the style of football has changed a lot but there needs to be more research with the new footballs."
'Youth football will ban headers'
Gordon Smith, former Scottish Football Association chief executive, says he expects youth football to ban headers.
"Certainly, I know, I'm an ambassador of the Scottish Youth FA in Scotland and they're looking at it because they understand what's happened in America regarding the banning of kids heading the ball from 11 downwards.
"I see football at the younger level and because now the game has changed from 11-a-side up until they're 12 years old to seven-a-side games, the ball is very rarely ever in the air. The kids aren't heading the ball as much as they used to - there's no doubt about that.
"And the secondary aspect of it is that from Ian [St John's] day, the ball has improved a lot - they were very, very heavy. I think a lot of the damage was done because of those types of balls. And people were doing a lot of heading backwards - there's no doubt about it.
"I think that ban [on young players heading] will come to this country soon. I would say that from 12 years downwards there should definitely be no heading the ball at all.
"You can imagine sometime in the future they might be a scenario where each player has to sign a disclaimer…to say that if I'm playing football I am taking this risk."
'It depends on the training'
Nathan Sargeson, lead football coach with Little Lions football club in East Lancashire, says children rarely perform headers.
He says: "The latest footballs that are used have come a long way and the technology around the ball - where it's more synthetic leather, rather than the solid leather casing.
"But it's like Kevin [Davies] said; in grassroots football you don't see a lot of kids heading the ball - it's really how much is done at training.
"Obviously you've got to limit that amount and think about it - and obviously with the findings and the studies, you've got to take it all into account."