[valenterry]

Well, unless you stop breating, showering, cooking, keeping plants indoors, etc. you will have to somehow moderate/control the humidity as it will grow over time until the dew point close to walls/windows or other cold surfaces is reached.

The reason why houses in Sweden can be mold free is because people get taught that they need to air out their house on a regular basis. And if you look it up, it tells you to air out not during noon (even though it's warmer so you lose less energy) but in the morning and late evening - specifically for humidity control.

So...

> maybe at the cost of letting more air and energy be swapped

Yes, exactly that. But since we are in the context of air pollution, opening windows to air out isn't a great idea. That's why I mentioned an automatic ventilation system (which pretty much always contains filters).

> Older houses have local vents at the baths which are humidity sensitive and will blow out excess humidity quickly, again at the cost of wasted energy.

Not only that, fresh air will also be sucked in from somewhere else outside - which then again brings the air pollution inside.

[2rsf]

All new houses here have mechanical ventilation, usually with heat recovery and heat pumps but without filtering (excluding the heat pump's coarse filter for dust).

I got curious and read a little, it is generally accepted here that higher pressure in the house can actually lead to mold, maybe because there is no moisture control and it is expected that some of the moisture generated in the house will be driven into the walls, probably assuming the walls are not perfectly sealed. On the other hand air pollution in Sweden is a non-issue, except maybe for a couple of streets in Stockholm.

[valenterry]

Yeah that makes sense - if you have clean air then why would you need to filter. :)