I installed a HRV system in my (well insulated) house, and I love it - air quality indoors is noticeably improved.

Only downside is that there’s lots of wood burning in my particular (and densely populated) area of Europe, so I was quickly drawn into the rabbit hole of particulate matter sensors and how to automatically shut the ventilation system off when the neighbors turn on their fireplace.

And when it comes to mold, our bathroom used to have black spots on the walls after a while, but with improved ventilation it stays amazingly clean and everything dries much more quickly. Almost no condensation on surfaces like mirrors and walls anymore either.

I see that they're saying 24% of the population, not the 1-in-6 I recall reading from some other source. Also, I seem to have been wrong about the test's needed to be ordered by a doctor (except in Pennsylvania).