> if people don't focus on airflow and airchanges

Even basic airflow understanding is surprisingly not common knowledge. IE for air to leave a room occupied by humans, then air needs a path into that room too.

I commonly see ceiling exhaust fans in bathroom showers and toilets with windows sealed shut. The door is meant to be closed... so the only gap for air to come into the room is around the door somehow - while the exhaust fan struggles.

I even visited one host who implemented a seal along the bottom of the door because:

1. Shower creates steam and with the above mentioned little airflow, that steam stayed around.

2. So the exhaust fan had to run while they dried themselves with a towel.

3. But they didn't like getting cold feet from the draft of air coming in under the door...

I could not convince them they needed an air intake vent. :-(

Similar but different, a couple of times now I've been in hotel rooms with a bar fridge buzzing away entirely enclosed in a cupboard. Extrapolate that to the hundred rooms or more of the hotel and it is a colossal waste thanks to a "simple" understanding of airflow.

> I commonly see ceiling exhaust fans in bathroom showers and toilets with windows sealed shut. The door is meant to be closed... so the only gap for air to come into the room is around the door somehow

If the building has central air heating or cooling, having a vent into the bathroom is standard. That’s more than enough for the exhaust fan to pull from.

In areas where central air is uncommon, you need a decent door undercut. A surprising amount of air can flow around a door and modern exhaust fans can pull a relatively good vacuum.