New guidelines restricting the amount of heading by children in football training are being launched.
The rules are being introduced following research that showed former footballers were three-and-a-half times more likely to die from brain disease.
What happens to the brain when you head a ball?
There are growing fears that regular heading of a ball increases the risk of footballers developing dementia, and dying from the disease.
A football weighs almost half a kilogram, and scientists have calculated that it can strike a player's head at speeds up to 128km/h.
When a ball strikes the head the brain, which floats within the skull cavity, bounces against the skull's back wall, causing bruising.
A 2018 study by the University of British Columbia found that blood levels of proteins associated with damage to nerve cells increase after heading the ball.
A single header is unlikely to cause any significant damage, but over an extended period the combined effect might lead to problems.
What evidence is there of an increased dementia risk?
The issue hit the headlines in 2002 following the death of former West Brom and England player Jeff Astle at the age of 59.
Mr Astle had been diagnosed with early onset dementia. A re-examination of his brain in 2014 found he had died from chronic traumatic encephalopathy (CTE). That is a brain condition normally linked to boxing which has been linked to memory loss, depression and dementia. It has been seen in other contact sports.
A coroner ruled that Mr Astle's brain had been damaged by years of heading heavy leather footballs.
In February 2017, researchers from University College London (UCL) and Cardiff University published a study based on post-mortem examinations of the brains of six former players which found signs of CTE in four cases.
Prof Huw Morris, of UCL, told the BBC at the time: "We saw the sorts of changes that are seen in ex-boxers, the changes that are often associated with repeated brain injury. So really for the first time in a series of players we have shown that there is evidence that head injury has occurred earlier in their life, which presumably has some impact on them developing dementia."
Then last year, a study by Glasgow University found former professional footballers are three-and-a-half times more likely to die of dementia than people of the same age range in the general population.
However, there is no definitive evidence that heading a ball does cause dementia. That would require long-term research.
Why is it difficult to prove a link?
The causes of dementia are complex and it is likely that the condition is caused by a combination of age, lifestyle and genetic factors.
Physical trauma to the brain is certainly a factor, but an unhealthy lifestyle, such as smoking, drinking too much alcohol and being overweight are also known risk factors.
Teasing out how all those factors might come into play is immensely difficult.
What research is under way?
The London School of Hygiene and Tropical Medicine (LSHTM), Queen Mary University of London and the Institute of Occupational Medicine have launched a study of 300 former professional footballers.
The plan is to put the ex-players, aged between 50 and 85, through a series of tests designed to assess their physical and cognitive capabilities.
Data will also be gathered on the players' careers in the game and lifestyle factors.
This will allow comparisons between defenders and centre-forwards and other players who tend to head the ball less often.
The test results will be compared with those from a general population study known as the 1946 Birth Cohort, which has monitored the ageing process in a group born in that year.
Lead researcher Prof Neil Pearce, from LSHTM, said: "We don't know much about the risks from concussion in football, and we know almost nothing about the long-term effects from heading the ball repeatedly.
"This study will provide, for the first time, persuasive evidence of the long-term effects on cognitive function from professional football."