An Open Face Booth has no intake filters or doors and is used when quality of finish is not a high priority or dust in the air is not a factor.
An air make-up unit (AMU) can be added and either ducted to dump air into the same room as the open face spray booth or directly over the front of the paint booth.
Without an AMU, code requires some provision for a passive system to allow air to replace the air being sucked out of the shop by the paint booth. This can take the form of a fixed louvre or back draft damper mounted in the wall or some form of inlet mounted in the roof. It can also be switch mounted to a bay door that is interlocked to the booth to ensure that the door is open an appropriate amount before the booth can be fully operational.