D.W.
Apologies for my slow reply, I have been consumed by other matters this week.
The counter sits about 6ft away from the grinder and positioned just a bit above head height where I can see it when I'm working on the woodturning lathe. Primarily used for measuring fine wood dust levels and in particular for when I'm power sanding. The fine dust from that, which is a serious risk for respiratory health, can remain suspended in the air for many hours if not efficiently extracted.
With the tools I'm grinding the risk is more from their toxic metal content, which I also want to avoid. With some of those I'm dry grinding up to #1,000 and that produces very fine metal particles that remain airborne for longer. Until I implemented some control measures I was getting quite high numbers like you.