I'm really worried about my brother. He's been dealing with a nasal blockage for months now after recovering from Covid. He tested negative and was fine for a bit after vaccination, but now he can't breathe properly through his nose and has to breathe through his mouth, especially at night. It's affecting his sleep a lot. He quit smoking recently after a long habit, and his PFT results were okay, but he's still not feeling well. What could be causing this ongoing congestion?