How Good Is a Dog’s Sense of Smell? The Nose That Smells in Parts per Trillion

Take a dog for a walk and watch it read the world through its nose: every lamppost, patch of grass and gatepost is a newspaper full of stories. We live in a world of sight; dogs live in a world of smell. Just how good are their noses?

Hundreds of millions of sensors

Smell begins with olfactory receptors, special nerve cells high inside the nose that recognise odour molecules. Humans have about 5 to 6 million of them. Dogs have an estimated 200 to 300 million, depending on the breed.

The difference continues in the brain. The olfactory bulb, which processes smell signals, takes up a much larger share of a dog’s brain than ours. And dogs have a second smell organ, the vomeronasal organ, tuned to chemical signals from other animals.

How sensitive does that make them? In a 2006 study, researchers trained dogs to detect a fruity-smelling chemical and gradually lowered its concentration. The dogs could still pick it out at about 1 to 2 parts per trillion, comparable to finding a few drops of liquid in an Olympic swimming pool.

A nose built for sniffing

In 2010, a team led by engineer Brent Craven built a detailed 3D model of a dog’s nose from scans and simulated the airflow inside. They found several clever design features:

• When a dog sniffs, part of the incoming air splits off from the breathing path and flows into a chamber at the back of the nose, packed with smell tissue. There, odour molecules can linger.
• A dog takes several short sniffs per second, pulling in fresh scent each time.
• When it breathes out, the air leaves through slits at the sides of the nostrils. This creates a swirl that helps draw in more air from in front, and avoids blowing away the scent it is trying to follow.

Dogs can also smell with each nostril separately, which helps them tell which direction a scent is coming from.

What dogs can do with their noses

This super-sense makes dogs remarkable partners:

• Tracking: scent hounds such as bloodhounds can follow a person’s trail long after they have passed.
• Detection: dogs find explosives, drugs, banned foods at airports, bed bugs and missing people under snow or rubble.
• Medicine: in studies, trained dogs have detected signs of some cancers, low blood sugar in people with diabetes and some infections. Dogs do not replace medical tests, but they inspire research into “electronic noses”.

Why dogs sniff everything

For a dog, sniffing is reading. Urine marks, footprints and the scent glands of other dogs carry information about who passed by, when, and whether they were male or female, healthy or stressed. That is why sniffing walks are mentally tiring and satisfying for dogs, and why letting them sniff is good for their welfare.

In short

• Dogs have about 200–300 million smell receptors; humans have about 5–6 million.
• A large part of a dog’s brain is devoted to processing smells.
• In tests, dogs detected odours at about 1–2 parts per trillion.
• Sniffing sends air into a special smell chamber; dogs breathe out through side slits.
• Dogs can smell with each nostril separately to locate a scent.
• Their noses help track people, find explosives and even detect some diseases.