It's the theoretical key question. The practical one is more like "how's the cost benefit curve looking like, especially in the beginning?".
My personal completely unscientific musings on the topic ended with buying and cutting up a couple of vacuum cleaner hepa filters, thus obtaining a shocking quantity of filter cloth, which then can simply be taped over the standard AC mesh filter. AFAIK, "hepa" ACs don't even cover the full surface - they just have a patch of hepa, which just ensures the air inside gets circulated through it eventually - being recirculated, there's not that much of a hurry. I'd recommend low-end filters, you don't want to remove virus particles but much larger clusters of virus+saliva.
So personally I'm guessing a rather high impact for very low cost / low effort changes.
My personal completely unscientific musings on the topic ended with buying and cutting up a couple of vacuum cleaner hepa filters, thus obtaining a shocking quantity of filter cloth, which then can simply be taped over the standard AC mesh filter. AFAIK, "hepa" ACs don't even cover the full surface - they just have a patch of hepa, which just ensures the air inside gets circulated through it eventually - being recirculated, there's not that much of a hurry. I'd recommend low-end filters, you don't want to remove virus particles but much larger clusters of virus+saliva.
So personally I'm guessing a rather high impact for very low cost / low effort changes.