Around 50% of air in the cabin is recirculated air drawn out of the cabin at floor level and through ducts goes through filters then the recirculator fans before joining with new air from Pneumatic supply in the mixing duct. The filters prior to this Covid problem were replaced as routine maintenance, now though I think the regulators ought to change and enforce replacing these filters more often, either by a reduced number of flight cycles or reduced number of flight hours. Contaminated recirc filters and the whole aircraft is then contaminated, need more than small masks to stop the spread as the virus can linger in the eyes for a fair number of hours.