In addition to concerns relating to carbon dioxide levels, when I contemplate the push for ever-more-tightly-sealed homes and particularly smaller homes with lower ratios of internal air volume to internal surface area (of walls, built-ins like cabinetry, and furniture), the issues stemming from outgassing of volatile organics from the adhesives and sealants as well as from the materials (solid wood became plywood with wood veneering, became sawdust-and-binder with plastic faux-wood veneer, etc.) now composites made of multiple substances and multiple binders and glues.
Some years back, "sick building syndrome" was a prominent topic of discussion and there was a push for fresh air, windows that opened, etc. but this seems to have largely dropped off the radar, afaict.
