For people who cannot form a mental picture of "what is a MICROgram, I have told this explanation here at NAT before:

When I was a freshman in college, I was originally a chemistry major. Our first semester chem lab project was a quantitative determination of a chemical from weights.

We had these small, ceramic crucibles in which we'd put the chemicals we were weighing. We'd put the crucibles into an oven to bake out any moisture.

If you handled the crucible with your bare hands, that would add weight you could measure with the extremely expensive Mettler balances we had, which could weigh things down to a resolution of 0.1 micrograms. These balances had a closed box in which you'd put the sample crucible to be weighed, because air currents would weigh over 10 micrograms.

I found that a single fingerprint of mine weighed about 1 microgram, and a thumbprint on a crucible was about 2 micrograms. That's the weight of the oil left on the surface by my fingerprint - one microgram.

Fentanyl dosages can be thought of in "fingerprints."