From a New York Times article on (what else?) COVID-induced anosmia, this interesting comment on smell research:
...A much-discussed unit of measurement in smell studies is the J.N.D. — the Just Noticeable Difference, or the degree to which chemicals have to differ from one another in order for us to tell them apart. In November, a new paper in Nature advanced the quest for a map of olfactory perception by creating a model that can predict what odorants will smell like by contrasting their chemical makeup with that of other smells.
The work relied on volunteers comparing hundreds of different odors and found them to be almost frustratingly good at it. “The failure to reach an absolute J.N.D. provides for yet additional evidence of an exquisite sense of smell in humans,” the authors said. “To rephrase this result: It is simply very hard to generate two multicomponent odorants that humans cannot discriminate.”
Good article, lots of basics about the sense of smell in general.