Good points. And it probably isn’t formaldehyde. The only thing I’ll add is that formaldehyde can inhibit or kill bacteria. And I also recently learned the hard way that limonene or other terpenes (from fruits or cleaning products or air fresheners for example) can react with ozone and produce formaldehyde even in the absence of combustion. And I’ve a strong opinion now that science and society ignores the dangers of formaldehyde and VOCs about as much as we used to ignore germs and other things we can’t usually see. Until heat pumps with fresh air exchanges are considered standard or specified by housing code, we will probably always have to deal with VOCs as we don’t have an accurate way to measure them and identity their sources except in industrial contexts. Saying this because while an open window is the cheapest way to get fresh air, it often isn’t the temperature or humidity we expect.