A designer proposes using the space above a suspended ceiling as a return-air plenum for a hospital administrative wing. Under what condition, per Section 407.4.1.4, would this be permissible?

• It is never permissible under any condition.
• If the plenum serves only nonsensitive areas.
• If the plenum is used for exhaust air only.
• If the design is specifically approved by the enforcing agency.