In this article

1. Why stress might have a smell
2. Belfast: four trained noses and 720 choices
3. Bristol: can the smell change a dog's mood?
4. Fear and happiness: the Naples studies
5. What the studies can't tell us yet
6. What it means for you and your dog
7. When to get help

You are running late, the car keys have vanished and your heart is thumping. Your dog, flat out a minute ago, is suddenly at your heels, watching every move. It is tempting to think they can smell your panic, and the old saying that dogs can smell fear suggests plenty of people agree.

For a long time that was folklore. Now researchers in Belfast, Bristol and Naples have tested it with breath, sweat, trained noses and some very patient volunteers. Here is what they found, what it might mean for your dog, and what nobody knows yet.

Why stress might have a smell

Stress changes the body fast. Heart rate, blood pressure and breathing climb, and hormones such as adrenaline and cortisol are released into the bloodstream¹. Our breath and skin give off airborne chemicals called volatile organic compounds, and earlier studies found that these differ between people under stress and people at rest¹. That leaves two questions for the dogs: can they pick out the difference, and does it mean anything to them?

Belfast: four trained noses and 720 choices

In 2022, a team at Queen's University Belfast published what it calls the first controlled test of whether dogs can tell a person's stressed odour from their unstressed one¹. Volunteers, on campus or over a video call from home, wiped gauze on the back of their neck and breathed into a vial. Then they spent three minutes counting backwards aloud from 9,000 in steps of 17, without pen or paper, while two researchers urged them on sternly¹. A second sample was taken the moment they finished¹.

Samples counted only if the task had worked: the person's own stress rating had to rise by at least two points out of ten, and on campus their heart rate and blood pressure had to climb too¹.

Twenty pet dogs from around Belfast began training and four reached the test: a cocker spaniel, a cockapoo and two rescue crossbreeds¹. In each session a dog first learned that person's stress sample, set beside two blanks. Then the same person's before-task sample joined the line-up, and the dog had to keep choosing the stress one. The handler never knew which sample was which¹.

So stress changes how a person smells, at least straight after a short, stressful task, and trained dogs can tell the difference¹. What the study cannot say is how the dogs felt about it. The researchers did not score the dogs' behaviour, but noted in passing that none seemed upset by the stress samples. If anything they looked excited, because a correct choice earned a click and a treat¹.

Bristol: can the smell change a dog's mood?

A University of Bristol team took that question on, in a study published in 2024². Eleven young volunteers gave odour samples on two days. On one, they sat a stress test: a five-minute speech on why they would suit their ideal job, given to a stony-faced 'expert' and a camera that was not really recording, then subtracting 13 from 1,022 aloud, starting again after every mistake². On the other, they relaxed on a bean bag watching forest and seaside films².

Cotton cloths in the armpits, plus a breath on each, captured the odour². The team used samples from the three volunteers with the clearest stress response, including a lasting rise in the stress hormone cortisol².

The 18 dogs, aged eight months to ten years, included spaniels, Labradors, a whippet, a miniature poodle and seven crossbreeds². Each learned that a bowl in one spot always held a treat and a bowl in another never did, then met bowls placed in between². A dog in a hopeful mood tends to check uncertain bowls quickly while a gloomier one hangs back, so scientists use this test as a window on mood².

Each dog was tested three times, about a week apart, with a jar at the start line holding a blank cloth first, then the stress or relaxed odour in either order². Nobody in the room knew which smell was in the jar². With the stress odour, dogs were about half as likely to approach the uncertain bowl nearest the empty spot as they had been with the blank cloth². The researchers read this as caution, a dog less willing to gamble on a bowl that probably holds nothing².

The relaxed odour had no such effect². But the result was modest and patchy. It showed at only one of the three uncertain bowls, and on closer analysis only when the stress odour came in the third session, when repeated testing may also have taught dogs that in-between bowls stay empty². Nor did the stress and relaxed odours differ significantly at that bowl². Even so, the authors call it the first evidence that human stress odour alone may affect how dogs think and learn².

The team also saw hints that the stress odour sped up the dogs' learning². Because the odours came from strangers, they suggest dogs may be reading a stress signal that people share, with no need to know the person².

If the odour from stressed humans affects a dog's emotional state, perception of rewards, or ability to learn, it suggests that stress may not just travel down the lead (as is often stated) but also through the air².

Fear and happiness: the Naples studies

At the University of Naples Federico II, researchers came at the question from another side, by watching what untrained pet dogs did^{1, 4}. Sweat from young men shown frightening or happy films went into a container in the middle of a room, where Labradors and golden retrievers explored with their owner and a stranger present^{3, 4}.

Dogs smelling fear sweat showed more stress behaviours and had higher heart rates than dogs smelling happy sweat or none³. In a later analysis of 84 dogs, fear odour lengthened both the time dogs spent with their owners and their stress signals, and drew more interest in the door, which the researchers suggest may mean wanting to leave⁴. Happy odour drew dogs towards the stranger, female dogs in particular^{3, 4}.

Puppies add a twist. In 61 puppies aged three to six months, fear odour set off the same pattern of behaviour seen in 80 adult dogs, but happy odour made no difference⁶. The researchers suggest the response to fear may be inborn and the response to happiness learned, and call the first of those a highly tentative conclusion⁶.

What the studies can't tell us yet

Each of these studies has its limits. Belfast's authors say their four dogs show that some carefully chosen, highly trained dogs can tell the difference, not that every dog does¹. They also point out that the Naples donors' feelings were not checked with any physical measure, and that their fear and happy samples were given a week apart¹.

Every sample came from volunteers given a short, set task or film, and the Naples and Bristol samples had been frozen, Bristol's for three to six months^{1, 2, 4}. None of these studies tested a dog with its own family's stress at home^{1, 2, 4}, and the Belfast team suggests real-world settings as a next step¹. It also names what is still unknown: which chemicals the dogs detect, how long the odour lasts and what happens with long-term stress, as well as how untrained dogs might read it¹.

On long-term stress there is a hint from Sweden. In 58 Shetland sheepdogs and border collies living with women owners, cortisol stored in the dogs' hair over months tended to be higher when the owner's was higher, and this was not explained by how active the dogs were⁷. The researchers think dogs mirror their owners' stress⁷. How the dogs pick it up was not examined, and the Belfast team suggests voice, sight and smell may all play a part¹.

What it means for you and your dog

None of this shows that your bad days harm your dog. The reactions seen so far came from strangers' odour in a lab, and the Bristol effect was modest^{2, 4}. It does suggest your dog may notice more than you realise, so on tense days it is worth keeping their world steady.

When to get help

Some dogs are on edge much of the time, whatever kind of day you are having. If your dog's behaviour changes suddenly or worry seems to be taking over, start with your vet, who can check for illness or injury^{5, 8}. If nothing is found, your vet can refer you to an accredited expert such as an ABTC-registered clinical animal behaviourist⁸.

Your dog spends their days learning you: your voice, your face, your habits and, quite possibly, your smell. That is no reason to bottle anything up, only one more reason to keep their days kind and predictable, the stressful ones most of all.

Sources and further reading8 sources

The small numbers in the article link to the source behind each fact. Sources open on their own websites.

1. Dogs can discriminate between human baseline and psychological stress condition odoursPLOS ONE
2. The odour of an unfamiliar stressed or relaxed person affects dogs’ responses to a cognitive bias testScientific Reports
3. Interspecies transmission of emotional information via chemosignals: from humans to dogs (Canis lupus familiaris)Animal Cognition (Springer)
4. Sex differences in the behavioral responses of dogs exposed to human chemosignals of fear and happinessAnimal Cognition (Springer)
5. Tips to identify and prevent stress in dogsPDSA
6. When are puppies receptive to emotion-induced human chemosignals? The cases of fear and happinessAnimal Cognition (Springer)
7. Long-term stress levels are synchronized in dogs and their ownersScientific Reports
8. Find a clinical animal behaviouristRSPCA