46 CFR §108.185

1. (a)The ventilation system for each enclosed classified location must be designed to maintain a pressure differential between the enclosed classified location and each non-classified location adjacent to the enclosed classified location, so as to prevent the discharge of ignitable gases into the non-classified adjacent locations.
2. (b)Each air intake must be outside of enclosed classified locations.
3. (c)Each unit must have alarms that are powered independently of the ventilation motor power and control circuitry and sound at a continuously manned station when—
   1. (1)Gas is present in an enclosed classified location; or
   2. (2)The ventilation system for the space is not working.
4. (d)Each ventilation system for enclosed classified locations must provide a complete change of air every five minutes.