[teiferer]

Unfortunately it will be hard for you to know how much of that effect is placebo. Unless you tested this with some kind of double-blind setup.

[OutOfHere]

You're not wrong, but indoor CO2 at these sub-1000 levels is a useful proxy metric for bioeffluent VOCs which are an objectively tiring subset of total VOCs. Ventilation lowers both. This explains the observation better than nocebo theory. See https://www.aivc.org/resource/effects-carbon-dioxide-and-wit...