[greybeard]

Mcgyver - good point about the time factor.
More background re the actual setup I'm trying to improve.

He does wear a full face air stream mask during work, and it is the belt sander which is the major problem, which might be in use for several hours at a time.
His isolated location allows him to run a filter all night, so we reckoned that would take care of the real time aspect. I've made several suggestions to him regarding the design of his air ducting system to increase the actual air flow and air speed.
As I've not made any measurements of the particle sizes, I don't want to be pedantic on that score.
But certainly any commercial equipment I've seen in other workshops, is always covered in fine dust, along with all other surfaces in the shop. I've made extensive reading of a site that was set up by someone who devotes himself to this topic, and he makes a good case for more work to be done(sorry I can't find a link).
However his approach is to redesign the cyclone extractor to cope with a wider range of particle sizes. I would be more interested in a secondary system, separate from the main filter.