I ask this question out of ignorance due to having never lived any length of time in COLD country, like below -10 for weeks at a time.

When people in the house routinely take long, hot, humid showers how do you keep the bathroom from becoming moldy?

Is the answer a big a** bathroom fart fan complete with seatbelts on the toilet? Or keep the window open and risk bad things from the really COLD air (I would presume.)

The air is so dry in cold country, you WANT to humidify the air.
Many install humidifiers on their furnaces. It helps avoid
nose bleeds.

Ventilation, a fan or partially open window, will work but some people like their showers steamy. The way around that is to take your shower and then to leave the fan on, or window cracked, for some time to clear the moisture out. A timer type switch helps control a fan.

Bathrooms need ventilation to dry out.
If you don't have a bathroom exhaust fan then leaving the bathroom door open when nobody is in it will help a lot.

Opening the bathroom window for a short while to remove the moist air helps but I think the risk of somebody forgetting to close it is a bit too much worry.

We crack the window, and the bathroom door. Our bathroom is the definition of TINY, and the water will drop from the ceiling if we do not keep it vented.

Also the mold is actually quite a rare beast in the cold countries (in Russian Siberia at least).

I agree with Alex and Clearwater. I live in WY where the relative humitiy ranges from 5% - 70% but not in the same season. In the winter here, there may be 2 feet of snow on the ground but it is dry enough to sneeze and get a nose bleed. That or you walk across the room and you are able to arc weld with the static electricity that you generate (ok a bit of a stretch, but...).

Seriously the climate is dry enough out here when it is cold, I have to humidify my house. Mold could be a problem in the spring / summer but not usually.

I have a central humidifier built into my furnace - it's dry here too (Colorado). I have to run it several hours a day to get it up to 25 - 30%. We just open the bathroom door after the showers, and enjoy.

The only place I see mold is on old food in the fridge.

Another thing, is you don't routinely take long hot showers. Costs too much to do that. I mean, you can take a hot shower, but usually dont' lounge for 20 minutes.

That being said, you really shouldn't do that in SoCal either cuz of the perpetual drought!

long cold winter here,we have a glass block window in the bathroom that has a small vent.we crack that a bit if needed.we also have lots --lots--of house plants and a couple fig trees in pots and watering those seems to kept the house just right,no mold problems but not to dry.