On Monday, July 13, 2020 at 8:07:39 AM UTC-4, TimR wrote:
The purpose of the filter is to protect the equipment, not the people. You want to keep the coils clean.

The air that hits the filter comes from inside the house. You've already breathed it.

I thought it was actually both. And that to protect the eqpt, even a
minimal filter will do that. That's why they put a spot for a 1"
filter in the typical furnace. To filter out allergens, finer particles
requires the higher rated filters. I've liven for many years in homes
that just had the 1" filter, never had any eqpt problems.