A smell is a sparse cloud of molecules read as a pattern
An odor is one or more volatile chemicals, often at low concentration, that noses translate into scent. Millions of olfactory receptor neurons in a rear nasal patch bind odorants, fire when thresholds hit, and relay through the bulb into limbic and cortical circuits that tag memory and emotion.
An odour is caused by one or more chemicals that have turned to vapour, usually present in tiny amounts, which humans and many animals can pick up through smell. The word covers pleasant and unpleasant smells alike, while scent, aroma and fragrance tend to be kept for nice ones, especially by the food and cosmetics trades. Most odorants are organic compounds, but a few carbon-free molecules, such as hydrogen sulfide and ammonia, also register.
Detection happens in the olfactory epithelium, a small patch at the back of the nasal cavity holding millions of receptor neurons. Each neuron pokes hair-like cilia into the air; an odour molecule that binds a receptor protein on a cilium starts an electrical signal. Once that signal crosses a threshold the neuron fires, sending the message to the olfactory bulb in the limbic system, which relays it to the olfactory cortex and links it with past experience.
The code is combinatorial. One odorant is typically recognised by many receptors, and different odorants trigger different combinations, so the brain identifies a smell from the overall firing pattern. It reads the whole blend rather than any single ingredient, and the perceived smell need not track the strength of one component. Because recognition happens in the brain after the nose has done its chemical work, there is no fully objective way to measure an odour, although people's reactions tend to line up and vary with age, sex, health and personal history.
Smell ability declines with age. Overall sensitivity starts falling in the second decade of life and drops appreciably after 70. Pregnancy can heighten it, sometimes producing cravings or aversions. Studies disagree on sex differences: some find women better at telling odours apart, others report a male edge, and a 2019 meta-analysis concluded the gap is extremely small but does slightly favour women.
Source: Odor
