Where does all that foul air go when we switch them on? Is there a dedicated outlet duct for such purposes that discharges the foul air outside the building. Reason I ask is that there's a funny smell where I stay, not quite sewage stench. I wonder whether it's coming via the air ventilator from my neighbors.

Many places in Thailand have been constructed without proper traps in the drains to block out the smells from the sewage system. Go around and sniff your drains. That may be where the smell is coming from.

Speaking of bathroom ventilator fans, I've noticed that not many condo bathrooms have them. Building codes in the west (at least the USA) require them if there is no exterior window. I'd think that in the humid climate of Thailand it would be the same, but that'd certainly be naive, wouldn't it?

The other day I looked at a condo in a high-rise that had a ventilator fan in the bathroom and asked the owner where the extracted air went. He said, it enters the building's vertical plumbing shaft and would exit there.

I had lifted the ceiling tiles in the bathroom of a unit in a similar building recently and there was no clear path to that vertical plumbing shaft from the bathroom. So, I guess the extracted air (with all its humidity) just lingers in the cavity above the ceiling?