So, how airtight does a building need for it to work?
Everything above 3 air exchanges is asking for trouble. This is the reason building regulations worldwide have quickly moved from 15, 10 or 5 air exchanges to 3 or lower.
In Australia, we are just at the beginning of implementing airtightness in the NCC. Ireland, England and many other countries started ten or more years ago.
Moreover, we need to ventilate our buildings mechanically; there is no way around it. I’ve been trying to avoid mechanical ventilation for the past 10 years. Trust me, there is no guaranteed solution for good air quality without some force.