Researchers from the Royal University of Belfast collected samples of sweat and breathing from 36 people before and after they had performed a hard task of arithmetic in their minds, who reported on stress levels and who used sweat samples when the blood pressure and heart rate increased.
The results showed that people produce different smells through sweat and breathing when they're under stress, and dogs can distinguish this smell from normal smells when people are relaxed. It's remarkable that dogs recognize the stress of even strangers who they don't know.
The dog experiment, Treo, Fingal, Sage, and Winnie, taught them how to search for a man by "funny" smell, and then alert scientists to what sample meets all the parameters.
Four dogs were given samples of human odour in a relaxed state and in a state of stress. The researchers themselves did not know whether there was a difference in the odour that animals could distinguish. During the 36 sessions, each dog was given a sniff of specimens of participants.
"Our study emphasizes that dogs do not need visual or sound signals to capture human stress. This is the first study of this kind. The experiment has shown that dogs feel stress only by breathing and sweat. The results will be useful in training service dogs and therapists," scientists conclude.