In article ,
dick lieber wrote:

I understand "close to the dust" but why higher?

Utimately the dust seems to settle down, not up; seems like lower would
catch more of it.

The primary purpose of these air filtration units is to catch the
floating dust that is so easily inhaled to the deep portions of the
lungs. The heavy stuff that falls fairly rapidly isn't a breathing
hazard like the very small single-digit micron-size particles. This very
fine dust can remain suspended in the air for quite some time - and
that's the material you want to filter out with your air cleaner.

Higher is better because the circulation of air at the 8'-ish level is
less obstructed and more thoroughly cycled through the filters. I would
think a second reason is that this is the level of room air that we
spend so much time breathing.

--
"Sure we'll have fascism in America, but it'll come disguised
as 100% Americanism." -- Huey P. Long