Germicidal UV could make airborne diseases as rare as those carried by water

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.

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.