BTW Americans learned metric in school as kids (I did late 70s) and didn't learn the "Imperial" system, which I still don't know, but it seems that signs and so forth never got switched over. I have wondered for 40+ years if it's a laziness thing, and I don't know.
Either that, or else I have a problem with metric of my own.
If it was cubic mils in a cubic furlong, I'd never have made that typo.
Metric makes you careless and lazy. :)
So right, a one micron particle is a billion (10^9) times lighter than a millimeter (1000 micron) particle of the same density and shape.
It's a big spread.
The small particles will behave differently and get into different places from the large ones. The micron ones can pass through some membranes that the 1000 microns obviously cannot.