The words “entire athmosphere” and the “plants” might be an exaggregation, but

1) I can readily imagine the _labs_ being contaminated by the reference samples.

2) If the compound is stable, not clumping and not hygroscopic - yes, certainly. “Microscopic” doesn’t do it justice, as there doesn’t seem to be any limit to how small the seed crystals can get.

Since we’re manufacturing literally tons of pharmaceuticals, and carefully manually spreading them over the Earth surface to the pharmacies, so for stable substances it doesn’t seem such an exaggregation to me.