Our sense of smell is extremely complicated, but we rarely think about it. That is, until it's no longer available to us. COVID-19 did a job on many victims' sense of smell, sometime temporarily, sometimes permanently. Those who never caught the disease smelled fewer things, too, as we stayed home from work and wore masks in public. We don't realize how much flavor is based on smell instead of taste, and how much we rely on smell to warn us of noxious conditions, such as smoke from a fire.
While our vision relies on just three types of receptors, the human body has 400 or so different kinds of olfactory receptors. Often many receptor types work together to identify a certain smell. Not all of those receptors work in every person. Some people are sensitive to the distinctive smell of urine after eating asparagus, while others cannot detect any difference. And people either pick up or interpret some smells differently, like how cilantro is a lemony herb to some people while it smells like soap to others. And even though a normal human sense of smell is much less sensitive than, say, a dog's sense of smell, we still know relatively little about it. New research is ongoing about the genes that affect how our olfactory receptors work and don't work. Other questions include why we, as a species, have inactive genes for smells that were important to our evolutionary ancestors. Read about the new research into our sense of smell at Smithsonian.
(Image credit: Neeta Lind)