The filters meant for chemical contaminants usually have some kind of active absorber ingredient that get used up which is why the replacement intervals are mor

The filters meant for chemical contaminants usually have some kind of active absorber ingredient that get used up which is why the replacement intervals are more defined. With particulate (which includes viruses) the only thing you have to worry about is the filter getting clogged or the filter breaking down due to old age but that isn't going to happen with indoor use unless you are talking years. That's why the filter replacement guidelines seems less specific.