Just replace them after every series[1], that way while the offense is coming off the field they are getting their helmets swapped out and the defense is going

Just replace them after every series[1], that way while the offense is coming off the field they are getting their helmets swapped out and the defense is going in with fresh helmets, rinse/repeat. [1] A series being after the offenses/defenses swap out, this could be as little as one play, but rarely more than 15-18 and I would guess on average about 8 or 9. Edit: grammar