It started with the need to wear a mask when sanding and an alternative to wearing a mask. A search of the internet showed a simple 20" box fan with a 1" furnace filter taped to it would work when sanding small pieces of wood. So I built one and it has expanded from there.
Flow tests shows ideal distance from the filter is less than 18" when fan is on slowest setting. Merv tests shows most particles common to wood shops, home air and pottery studios will be trapped with a Merv 8 filter although some fine ball clays will escape capture. Virus with a water envelope are easily trapped by the fabric of most filters.
I have made 3 simple filters and a double fan filter system for the radial arm saw. It is amazing how much is trapped by the filters. After 90 days the filters are usually as dirty as the furnace filter.
Change filters every 90 days with a tap on hard surface cleaning at 45 days.