High levels of carbon dioxide might be bad for your brain and stuff (https://slatestarcodex.com/2018/08/23/carbon-dioxide-an-open-door-policy/).

Has anyone found a way to scrub indoor CO2 besides buying a ridiculous number of plants?

I've done brief searches for products and come back empty handed except for this algae bioreactor-filter thing, which claims to be "25 plants worth" and not require sunlight (https://algenair.com/). I'm not yet convinced this is viable for e.g. a one-room office

Any tips appreciated!

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Indoor CO2 concentrations and cognitive function: A critical review (2020)

"In a subset of studies that meet objective criteria for strength and consistency, pure CO2 at a concentration common in indoor environments was only found to affect high-level decision-making measured by the Strategic Management Simulation battery in non-specialized populations, while lower ventilation and accumulation of indoor pollutants, including CO2, could reduce the speed of various functions but leave accuracy unaffected."

I haven't been especially impressed by claims that normal indoor CO2 levels are impairing cognitive function to any extent worth worrying about. Crack a window, I guess?

We have no solution better than opening windows, at the cost of losing control of the temperature.

Also the cost of sound, and possibly outside pollution (though that can be addressed with HEPA filters & ozone filters)

I've worked in indoor air quality for three years and I'm not aware of any products that capture CO2 below the industrial scale, unfortunately. If you come across anything, I'd love to know! 

I've tried the indoor plants approach and tested CO2 levels with Kitagawa tubes, and all the data showed was when the HVAC system was running... There are already lots of plants outdoors, and they're lower-maintenance there, so it's worth checking whether your HVAC system could be doing its job better of bringing outdoor plant-enrichened air to you.
- Are the filters clogged? Houses sometimes have a 5" filter hidden in the unit in addition to the return filters.
- Are the coils dirty? 
- Are the vents open? 
- Are the ducts insulated and in good repair? 
- Are the filters the right size and MERV rating for the unit? 
- Could the unit be upfitted with a filter that has more surface area, so that it restricts the airflow less? Going from a 1" filter to  a 2" or 4" filter can increase the airflow. If not, look for a filter with more pleats per inch.

If you've got great ventilation  - or no authority over the ventilation - and you're still experiencing poor indoor air quality, a HEPA air cleaner with carbon can't fix CO2 but it'll get most other contaminants. 

HVAC systems with outdoor air inputs (like DOAS) may be an effective way to reduce the buildup of CO2, VOCs, etc indoors in an automated and temperature-controlled manner.