Foam / felt filter replacements as the pre-filters.

Be for the prefilters to catch the big coarse dusts before hepa and ulpa filtersbe very good air permeability and low air  resistance. 

The filter foam to catch the dusts while filtering the air. 

Washable and reusable reducing infiltration of dust, pollen and other allergens into the air you breathe.

Replace pre-filter every 2-3 months.

The pre-filter can be washed and completely air dried before being reused.