Human olfaction begins at the epithelium in the roof of the nasal passage, where hair-like cilia connected to receptor cells transmit odorant detection to nerve (mitral) cells in the olfactory bulb, where initial scent processing is performed. These filtered olfactory signals are then sent to various parts of the brain associated with emotion, behavior, and memory.
Olfactory receptor cells wear out rather quickly, and so continually regenerate, with total turnover every 2 months.
Like dogs, humans can track scents using the difference in odor intensity between the 2 nostrils. This is not to say that human sense of smell is up to snuff to that of dogs.
Dogs have 50 times the number of olfactory receptors cells in their noses, which sit on long snouts situated close to the ground. Whereas humans have about 10 cm2 dedicated to olfaction reception, some dogs have 170 cm2. Further, the nose of a dog is 100 times more densely innervated (packed with nerves) than a person.
7 categories of human-detectable odors have been suggested, corresponding to the 7 types of smell receptors in the nasal cavity: camphor-like, musky, floral, minty, ethereal (dry-cleaning fluid is exemplary), pungent (vinegar-like), and putrid. But this bears no relation to the complexity of odorant receptor production, nor even sufficiently characterizes the subtleties of human olfaction.
Excepting some hunter-gatherer tribes, humans generally struggle to explicitly name specific smells. Even experts are readily fooled by an odor’s context. Human ability to vividly imagine a smell is hardly worth a snort.
Olfaction has long been considered the least significant of the human senses. Biological anthropologists have suggested that vision supplanted olfaction as humans became upright.
~ Dutch psycholinguist Asifa Majid & Swedish linguist Nicole Kruspe
The limited terminology for scents and tastes suggests that these are not the strong suits of human sensory perception: contrast these vernaculars to the vocabularies associated with sight and sound. It may instead be that smell simply does not carry the cultural currency that vision and audition do: that we do not commonly converse of odors so specifically as to merit a verbose nomenclature.
Though dogs may have better sniffers, humans are able to sift through billions of different odors. We use scent to detect fear, stress, and sickness, and to assist in selecting mates.
There’s a true underappreciation for the way we use our sense of smell that contributes quite significantly to our overall well-being. ~ English physiologist Johannes Reisert
Women pack 40% more olfactory receptors in their heads than men, giving them a superior sense of smell.
Individuals have decidedly different senses of smell, owing to genetic and lifestyle differences.
Health determines smell sensitivity. An early sign of dementia and Alzheimer’s disease is loss of smell (anosmia; hyposmia is diminished or deficient sense of smell).