These anti-microbial replacement filters are for use with Friedrich ZoneAire Select and FreshAire Series PTACs. They are packaged in a 10 pack. Each Friedrich PTAC requires 2 filters. This 10 pack can be used to replace filters in 5 units. Filters should be changed fairly frequently but this also depends on the climate and frequency of use of the PTAC. Filters should at least be checked twice a month. Dirty filters affect the efficiency of your PTAC unit.