Excessive dust and debris in the furnace area will be pulled into the.
Attic furnace filter.
Attic replacement and deep pleated air filters utilize 100 synthetic fibers high initial efficiency and will not promote the growth of bacteria mold mildew or fungi in normal operating environments.
However both your furnace and a c share a single filter.
Attic whole house replacement filters.
Merv means minimum efficiency reporting value merv and is an industry standard that measures the overall effectiveness of air filters.
The filter is probably at the air intake not in the furnace blower unit up in the attic.
Astro guard attic replacement filters air filters inc.
Keep the area around your attic furnace clean.
You say you changed the two in the living areas suggesting that you have a dual zone system 2 floors.
As the merv rating increases the finer the filtration becomes.
The most common airborne contaminates that these filters are tested against tend to include pollen dust mites mold spores dust pet dander.