Heading the ball has nowhere near the same impact force as the standard collisions in the NFL. There are the occasional unintended headbutts when to players go for the same header and that could be an issue. Again though, these are few and far between compared to the every down collisions that occur in the NFL. 

Don't get me wrong, I've been an NFL fan for 45 of the 50 years I've been on the planet and I'm hoping the NFL spends some serious dough on collision research to design helmets to mitigate the CTE risk.