The threshold of a chemical compound is determined in part by its shape, polarity, partial charges, and molecular mass.
Some substances can be detected when their concentration is only few milligrams per 1000 tonnes, which is less than a drop in an Olympic swimming pool.
A 2014 study found no significant differences between men and women, and between non-pregnant and pregnant individuals, despite the existence of anecdotal reports of hyperosmia among the latter.
Olfactory function is more impaired in patients with primary progressive MS than that in relapsing-remitting MS.[8] Some species can detect odors that others cannot.
It is widely believed that animals such as dogs and rodents have a superior sense of smell overall, however a 2017 paper disputed that, saying that "the absolute number of olfactory neurons is remarkably consistent across mammals".