Kopfball...

It's safe to head a soccer ball
Twenty three amateur soccer players repeatedly headed a ball, submitting to a lumbar puncture 10 days later. Cerebrospinal fluid biomarkers of neuronal injury (neurofilament light protein, total tau, glial fibrillary acidic protein, the calcium binding protein S100B, and albumin) were all normal. Levels did not correlate with the number of headers (10 in 10 players and 20 in 13 players, and none in 10 healthy controls). The authors point out that head injury in soccer is more likely from head to head or head to goalpost collision.
References
Br J Sports Med 2007;41:574-7