According to a new study published this week in the open-access journal PLOS ONE by Clara Wilson of Queen’s University Belfast, UK, and colleagues, the physiological processes associated with an acute psychological stress response produce changes in human breath and sweat that dogs can detect with an accuracy of 93.75%.

Body odors are chemical signals that have evolved for communication, primarily within species. Given dogs’ remarkable sense of smell, close domestication history with humans, and use to support human psychological conditions such as anxiety, panic attacks, and post-traumatic stress disorder (PTSD), researchers wondered if dogs could detect chemical signals to respond to their owners’ psychological states.

The new study collected breath and sweat samples from nonsmokers who had not recently eaten or drank. Samples were taken before and after a high-speed arithmetic task, as well as self-reported stress levels and objective physiological measures such as heart rate (HR) and blood pressure (BP). Within three hours of being collected, samples from 36 participants who reported an increase in stress as a result of the task, as well as an increase in HR and BP during the task, were shown to trained dogs.

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Four dogs of various breeds and breed mixes had been trained to match odors in a discrimination task using a clicker and kibble. During testing, dogs were instructed to locate the participant’s stress sample (taken at the end of the task), while the same person’s relaxed sample (taken only minutes before the task began) was also in the sample line-up.

Overall, dogs could detect and perform their alert behavior on the stress sample in 675 out of 720 trials, or 93.75% of the time, which was significantly higher than expected by chance (p0.001). The dogs correctly alerted to the stress sample 94.44% of the time when they were first exposed to a participant’s stressed and relaxed samples. Individual dogs ranged in accuracy from 90% to 96.88%.

The authors conclude that dogs can detect an odor associated with a change in Volatile Organic Compounds produced by humans in response to stress, a finding that reveals more about the human-dog relationship and could have implications for training anxiety and PTSD service dogs, which are currently trained to respond primarily to visual cues.

The authors continue: “This study shows that dogs can tell the difference between human breath and sweat taken before and after a stressful task. This discovery indicates that an acute, negative psychological stress response alters the odor profile of our breath/sweat, and that dogs can detect this odor change.”