WHILE UNLOADING A PNEUMATIC BULK TRAILER WITH A TRUCK TRACTOR, HE FELT DEHYDRATED AND ASKED OTHE EE TO FINISH UNLOADING THE TRAILER WHILE HE SAT INSIDE OF THE TRUCK CAB. OTHER EE COMPLETED THE UNLOAD WITHIN 15 MINUTES. UPON CHECKING ON DRIVER IN CAB, HE FOUND HIM LAYING ON FLOORBOARD UNCONSCIOUS. EMTSWERE SUMMONED FROM MINE, THEY COULD NOT REVIVE HIM.