Dogs can detect changes to human physiology through odour and are used for seizure scent detection in people who have epilepsy, and can detect blood-glucose levels in people with diabetes.

Researchers were interested in determining whether dogs could potentially do the same with stress.

They tested dogs' ability to distinguish between peoples' scents when they were at baseline, versus having been in a stressful situation, and found that dogs could accurately detect the breath and sweat samples from stressed participants 93.75 per cent of the time.

The results have been published in a new study in the journal PLoS ONE, and could have applications in training Emotional Support and Post Traumatic Stress Disorder service dogs.

"The findings show that we, as humans, produce different smells through our sweat and breath when we are stressed and dogs can tell this apart from our smell when relaxed - even if it is someone they do not know," explains co-author Clara Wilson, a PhD student in the School of Psychology at Queen's University Belfast in Northern Ireland.

"The research highlights that dogs do not need visual or audio cues to pick up on human stress. This is the first study of its kind and it provides evidence that dogs can smell stress from breath and sweat alone, which could be useful when training service dogs and therapy dogs."

"It also helps to shed more light on the human-dog relationship and adds to our understanding of how dogs may interpret and interact with human psychological states," says Wilson.

In humans, stress is associated with the release of the hormones epinephrine and cortisol into the bloodstream, increased heart rate, blood pressure and respiration, and the suppression of digestion.

Researchers collected samples of the breath and sweat from non-smokers who had not recently eaten or drank within the last hour, to prevent the impact of tobacco and food consumption on Volatile Organic Compounds (VOCs) in exhaled breath.

These samples were taken both before and after completing a stressful task - a fast-paced arithmetic task - alongside self-reported stress levels and heart rate and blood pressure measurements.

The scientists then used samples from the 36 participants who reported an increase in stress because of the task and also experienced an increase in heartrate and blood pressure.

Within three hours of collection these samples were presented to four trained dogs - Treo, Fingal, Soot and Winnie.

In phase one of the trials, the dogs were trained to identify a participant's stress sample alongside two blank samples (the same sample material but without sweat or breath) by performing an alert behaviour.

Then, in phase two the dog was presented with the same participant's stress sample, their baseline sample (taken before the task), and a blank, to see which sample the dog performed the alert behaviour for.

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If the dog was able to correctly alert on the stress sample while the baseline sample was also present, then this would suggest that the odours of these two samples are distinguishably different to the dog.

They found that overall, the dogs were able to find the participant's stress sample with 93.75 per cent accuracy, with individual dogs ranging in performance from 90 per cent to 96.88 per cent accuracy.

This suggests that dogs can detect an odour associated with the change in VOCs produced by humans in response to stress which emanate from our breath and sweat.

One of the pooches that took part in the study was Treo, a two-year old Cocker Spaniel. His owner Helen Parks says: "As the owner of a dog that thrives on sniffing, we were delighted and curious to see Treo take part in the study. We couldn't wait to hear the results each week when we collected him. He was always so excited to see the researchers at Queen's and could find his own way to the laboratory.

"The study made us more aware of a dog's ability to use their nose to "see" the world. We believe this study really developed Treo's ability to sense a change in emotion at home. The study reinforced for us that dogs are highly sensitive and intuitive animals and there is immense value in using what they do best - sniffing!"

Because this was a small study involving only four dogs the findings aren't necessarily generalisable to all dogs, but instead provide evidence that some highly trained dogs can differentiate between the samples. This could have applications in training anxiety and PTSD service dogs that are currently taught to respond mostly to visual cues of stress.