(a) Buses and Farm Labor Vehicles. All buses and farm labor vehicles shall provide ventilation that is adequate for passengers in any weather. Openings for ventilation through the front of a vehicle shall be equipped with screens that prevent passage of insects, gravel, and other objects.

(b) School Buses. School bus bodies shall be equipped with a suitable ventilating system of sufficient capacity to maintain adequate ventilation during operation without the opening of windows except in extremely hot weather. Ventilation shall be adequate to assure a complete change of air at least once every 3 minutes while a school bus is moving.