46 CFR §108.175

1. (a)An enclosed space that has direct access to a Division 1 location is not a hazardous location if—
   1. (1)The access has self-closing gas-tight doors that form an air lock;
   2. (2)The ventilation causes greater pressure in the space than in the Division 1 location; and
   3. (3)Loss of ventilation overpressure activates an alarm at a manned station;
2. (b)An enclosed space that has direct access to a Division 1 location can be considered as a Division 2 location if—
   1. (1)The access has a self-closing, gas-tight door that opens into the space and that has no hold-back device;
   2. (2)Ventilation causes the air to flow with the door open from the space into the Division 1 location; and
   3. (3)Loss of ventilation activates an alarm at a manned control station; and
3. (c)An enclosed space that has direct access to a Division 2 location is not a hazardous location if—
   1. (1)The access has a self-closing, gas-tight door that opens into the space and that has no hold-back device;
   2. (2)Ventilation causes the air to flow with the door open from the space into the Division 2 location; and
   3. (3)Loss of ventilation activates an alarm at a manned control station.