The comparison people reach for is that dogs smell tens of thousands of times better than humans. The number varies wildly depending on who is quoting it, and it rather misses the point.
The interesting part is not that dogs smell more. It is that they smell in a fundamentally different way.
The hardware
A dog’s nose contains a vastly larger area of scent-detecting tissue than ours, folded into a complex structure of bony scrolls called turbinates. Estimates commonly put dogs at something like 200 to 300 million olfactory receptors, against roughly 5 or 6 million in a human.
The proportion of brain devoted to processing smell is also far larger.
But the architecture is what makes it remarkable.
They breathe and smell separately
When a dog inhales, the incoming air splits. Some goes down to the lungs for respiration. The rest is diverted into a separate chamber dedicated to olfaction, where it passes over the scent-detecting tissue.
This means a dog can keep analysing a scent sample while continuing to breathe normally — something we cannot do at all.
Exhaling out the sides
Watch a dog sniffing hard and you will see the side slits of the nostrils flare. Dogs exhale through those side slits rather than straight out the front.
This matters more than it sounds. Exhaling forwards would blow away the very scent they are trying to sample. Venting sideways and backwards actually creates small swirls of air that pull fresh scent towards the nose.
The rapid sniffing you hear — several sniffs per second — is this system running at full speed.
They smell in stereo
Each nostril samples slightly independently, and the brain can compare the two. This gives dogs directional information about scent: not just “there is a smell”, but “it is stronger over there”.
Combined with the ability to detect how a scent changes over a distance, it is how a tracking dog works out which way along a trail a person walked.
The extra organ
Dogs also have a vomeronasal organ, sometimes called Jacobson’s organ, in the roof of the mouth. It is specialised for detecting pheromones — chemical signals carrying information about other animals’ sex, reproductive state and identity.
This is a substantial part of what is happening when dogs investigate each other, and when a dog stands stock still at a lamp post reading what amounts to a community noticeboard.
Why sniffing is exhausting
This is the practical part.
Processing scent is genuinely hard cognitive work. A dog doing twenty minutes of concentrated sniffing is typically more tired afterwards than one who has walked briskly for an hour.
It also appears to be calming. Sniffing is associated with lowered arousal, and trainers routinely use scent work for dogs who struggle to settle, for reactive dogs, and for dogs recovering from injury who cannot exercise normally.
Let them sniff on walks
A walk where a dog is marched past everything without being allowed to investigate is, from the dog’s perspective, a fairly impoverished experience.
Build in sniffing time deliberately. A “sniffari” — a walk where the dog chooses the pace and route and you simply follow — covers less ground and does far more good than a purposeful march.
What dogs can be trained to detect
The range is genuinely remarkable, and expanding: explosives, narcotics, currency, agricultural contraband, missing people, human remains, invasive species and specific wildlife. In medical contexts, dogs have been trained to alert to certain cancers, to blood sugar changes in diabetics, and to seizures.
What is being detected in the medical cases is volatile organic compounds — chemical traces present at concentrations far below what any human nose could register.
The honest summary
Your dog’s nose separates air for breathing from air for smelling, vents sideways so as not to disturb the sample, works in stereo to give direction, and connects to a brain with a huge amount of capacity devoted to interpreting the result.
The most useful thing you can do with that is simple: stop hurrying them along, and let them read.
