They are airtight in as much as they can effectively deliver the prescribed pressure without leaking air. Not truly airtight. The system HAS to have a safeguard

They are airtight in as much as they can effectively deliver the prescribed pressure without leaking air. Not truly airtight. The system HAS to have a safeguard against malfunctions, as power loss is always a possibility. They generally have a passive airflow built into the line somewhere.