Dmorse, at least you can see or smell what is causing those high readings. My counter was telling me that smoke was there long before I could smell it and, unlike in you case, I never did get to see the source. It could have been miles away.

Bob makes a good point, these counters are picking up fine particulates which we can't sense ourselves. If we could see or smell the problem ourselves we wouldn't need them... }

The take home message for me is to check the base level reading before dust making starts so that we can know to what extent any external high levels of 'low threat' particulates are confounding our readings inside the workshop.