American Football NCAA Rule 3-3-9

a. If a player’s helmet comes completely off through play, other than as the direct result of a foul by an opponent, the player must leave the game for the next down. The game clock will stop at the end of the down. The player may remain in the game if their team is granted a charged timeout.
b. When the helmet coming off is the only reason for stopping the clock, other than due to an injury to the player or their teammate (Rule 3-3-5), the following conditions apply (A.R. 3-3-9-I-III):
1. The play clock will be set at 25 seconds if the player is on offense and at 40 seconds if the player is on defense. With one minute or more remaining in either half, the game clock will start on the referee’s signal.
2. Ten-Second Runoff. If there is less than one minute in the half the opponent has the option of a 10-second runoff, unless the helmet comes off as the direct result of a foul by the opponent.
3. If there is a 10-second runoff the game clock will start on the referee’s signal. If there is no 10-second runoff the game clock will start on the snap.
4. The 10-second runoff may be avoided by the use of a charged team timeout, if available.
5. There is no option for a 10-second runoff if helmets come off opposing players. (A.R. 3-3-9-V)
c. If the ball carrier’s helmet comes off as in paragraph a (above) the ball is dead (Rule 4-1-3-q). If the player is not the ball carrier the ball remains alive, but that player must not continue to participate in the play beyond the immediate action of which they are engaged. Prolonged participation is a personal foul (Rule 9-1-17). By definition such a player is obviously out of the play (Rule 9-1-12-b).
d. A player who intentionally removes their helmet during the down commits a foul for unsportsmanlike conduct (Rule 9-2-1-a-1-i).