The room is filled with a deadly gas. Either it is built so that it vents into the room naturally from deep under ground, or a diffuser is periodically refilled by whoever controls access to the room.

The McGuffin is the antidote to the gas, which you must obtain and administer before entering.

A homeless man held the door open for me at a pizza place. He didn’t ask for any money or have a sign or anything. Just made himself useful, presumably in hopes that some of the customers would show him an act of kindness in return. It was the first time I gave someone money. He also took a q-tip out of his pocket and somehow used it to relight a half-smoked cigarette out of the ashtray.