From FireFighterCloseCalls.com:
The following is in the new revision of NFPA 1901, effective for apparatus contracted on and after January 1, 2009.
"14.1.8.4* The following statement shall be included in the operator's manual: 'Fire helmets shall not be worn by persons riding in enclosed driving and crew areas. Fire helmets are not designed for crash protection and they will interfere with the protection provided by head rests The use of seat belts is essential to protecting fire fighters during driving.'
14.1.8.4.1 A location for helmet storage shall be provided.
14.1.8.4.2 If helmets are to be stored in the driving or crew compartment, the helmets shall be secured in compliance to section 14.1.11.2. (This relates to the g-force restraints.)
14.1.8.4.3 A label stating 'DO NOT WEAR HELMET WHILE SEATED' shall be visible form each seating position.
A.14.1.8.4 The minimum seat head height values in this standard assume that the occupants are not wearing helmets. The use of helmets puts the occupant at greater risk of neck or back injury during a rollover or during a severe road event.
http://www.firefighterclosecalls.com/fullstory.php?60623