Not a resident but I have friends there and stayed in a bunch of different apartments.

Bath rooms in smaller apartments, and also in hotels, tend to be, literally, whole-room plastic/resin structures, so it dries easier and if mold appears it's in the plastic surface and easy to get rid of it.

I have a friend with a very big fancy apartment, and his huge bath room is covered mostly in tile and has very good ventilation. The bath even has a waterproof TV so you can watch a movie meanwhile!

It's 98% humidity for months, you need to worry about the whole house!

Are you sure about that number?

I live in famously hot and humid Singapore, and it's usually around 70% to 80% humidity here during the day. I find it hard to believe Japan is even muggier?

I just randomly checked the weather for Osaka and Tokyo (on Google) and they have about 60% to 70% at the moment.

No, I exaggerated slightly; but we're past the wettest season now AFAIK.

These rooms have ventilation (and also usually a heater, cooling, dry mode). The ventilation can be set to 24/7 mode, which runs at a low setting, always on. Japanese houses/apartments also typically have a house ventilation system, meant to run when it's humid. Mold in general is a problem here, so ventilation is incorporated well.

It's fun in the winter when you're running humidifiers but forget to turn off your house ventilation and don't understand why you're still so dry.

> Japanese houses/apartments also typically have a house ventilation system, meant to run when it's humid.

Do they have Mechanical Ventilation and Heat Recovery (MVHR) or is this more like Aircon? I've recently moved somewhere more humid and I'm looking to get a grip on what other countries do.

From what I can tell it's a just a house fan. The house ventilation doesn't do heating, cooling, etc. It just moves air.

Unfortunately, it does build up from time to time, yes. However, there is another feature to most bathroom units, which is the room drying feature. It sends warm dry air into the bathroom to dry it. It is also a great tool for the 6 months of the year in which drying clothes outside is impossible. Even so, the appearance of mold can feel inevitable!