For those looking to manage their indoor CO2 levels, I found videos by this HVAC expert pretty helpful: https://www.youtube.com/watch?v=CkGDN85I29U

There are caveats like:

- opening windows and doors may not change much if the air is not moving i.e. with fans

- and it may not be the worth the tradeoff of introducing outdoor irritants e.g. pollen

- indoor plants unfortunately do not make a big difference in CO2 levels

- consider focusing on more important facets of indoor air quality, like VOCs

Personally, the unnerving fact is not that ambient carbon dioxide is harmful in current concentrations (it almost certainly isn’t), but more that the average baseline concentration outdoors (which we have to live with and cannot really escape much) is rising seemingly drastically. It’s probably not going to be an issue in our lifetimes, but because it’s a global rise we can hardly even have a control group to test for any subtle health effects from a 100 ppm increase. Also, most advice and regulations about indoor concentrations rely on the fact that we don’t exclusively live indoors, we do get regularly exposed to baseline level outdoors, and rarely account for the fact that that level is rising.