[eythian]

It's a minor point, but it's interesting that they used having AC as a proxy for mechanical ventilation and conclude that it's rare in Europe. At least where I live (NL), mechanical ventilation is common - I think required in some situations - even though AC isn't. It's basically a fancy extractor fan that pumps air outside, so bringing fresh air in. That said, you'd need to reverse that flow to add filters.

[lm28469]

Yes, few people have AC but almost everyone living in a semi recent place has mechanical ventilation in, at the very least, kitchen and bathroom, which thanks to physics renew the air of the entire falt/house.