From The Guardian on the effect of headers:

"When wet enough, it was the equivalent of 10 standard bags of coal being dropped on a player's head," Birkbeck said. "A high-speed camera measured the velocity of a football being kicked against a player's head. We immersed a new ball in water and found the difference in damage compared to an old ball was negligible. We then got a boxer to hit a punchbag and you would rather be hit by a good amateur boxer."