Sweat may give off stress signals, study says

(CNN) -- If you're working in a stressful environment, you and your colleagues may be communicating tension to one another without even realizing it.

A new study published in the online journal PLoS One reveals changes in brain activity when people are exposed to sweat from others who have been in a stressful situation. Researchers found that people may become more alert to potential threats when inhaling this "stress" sweat.

"The results suggest that we can detect others' stress just by breathing in their sweat," said Lilianne Mujica-Parodi, assistant professor of biomedical engineering at Stony Brook University in New York and lead author of the study, in an e-mail.

Researchers took sweat samples from 144 people who had put themselves in the somewhat stressful situation of tandem skydiving for the first time. Each participant was strapped to an expert skydiver, and each pair jumped from 13,000 feet. Control samples were taken from people who had run on a treadmill.

In the first trial, sweat samples from the experimental and control conditions from 40 donors were given to eight males and eight females while their brains were scanned using functional magnetic resonance imaging. This procedure was repeated with 40 more donors and a group of 16 different participants with the same gender ratio.

Imaging results indicated that the amygdala, an area of the brain associated with emotion, was more active when exposed to the skydivers' sweat than to the runners' sweat. An additional experiment showed that participants could not discriminate between the sweat samples based on smell alone.

Researchers then conducted a behavioral test in which participants separately breathed the "exercise" sweat and the skydiving "stress" sweat while judging the emotions conveyed in pictures of faces on a computer screen. People tended to discriminate better between neutral and angry faces when exposed to the "stress" sweat.

The results suggested that more activity in the amygdala is associated with more awareness of possible threats, Mujica-Parodi said.

"It would certainly make sense that if others around us are stressed, that we would enhance our vigilance to potential danger," Mujica-Parodi said.

The study does not, however, prove that the signals from sweat that the participants' brains responded to are "pheromones," chemicals found in animals that communicate information and influence responses in behavior and physiology.

Pheromones have been popularized as a factor in sexual desire -- namely, that certain odors that people give off attract potential mates -- but their role in humans is still controversial.

There is no hard evidence that the same kinds of pheromones found in animals also exist in humans, said Charles Wysocki, researcher at the Monell Chemical Senses Center in Philadelphia, Pennsylvania, and adjunct professor at the University of Pennsylvania. There are fragrance products marketed as "pheromonal" but which do not actually contain pheromones, he said.

Pheromones in certain animals have been found and classified according to their functions, including sexual attraction and puberty. Some animals can even learn about each other's sex, reproductive status, age and dominance status, just by smelling.

In humans, no one has identified analogous chemical compounds that serve as signals or prompt behavior in these ways, Wysocki said. However, humans may have their own kinds of pheromones.

One of Wysocki's studies showed that when women were exposed to male underarm sweat, they became more relaxed and less tense. That paper also found that male sweat may affect the length and timing of a woman's menstrual cycle.

Other studies have concluded that people give off different body odors that correspond to certain emotional situations, but what hasn't been shown is whether other people become a little stressed after smelling the body odor from stressed people, Wysocki said. The Stony Brook study goes in that direction, looking at the brain region that modulates emotion and finding that people responded differently to behavioral tests.

Wysocki questioned the procedures that the Stony Brook study used to isolate the sweat chemicals from participants and whether the actual compound that could have triggered behavioral patterns was preserved. But Mujica-Parodi said the researchers did not alter the sweat at all, just diluted it with clean water.

"The reason the sweat did not have odor is that, when you think of 'sweat odor,' you're not really smelling the sweat," she said. "You're smelling the bacteria that feed off of it."

Mujica-Parodi said humans do emit reproductive pheromones, but the extent to which they affect everyday social behavior is not established, causing the debate among scientists, she said.

Research such as this study may help people develop safety measures that take into account how people react to emergency situations: for example, on an airplane or a space shuttle, she said. They may also provide insight into behavior in stressful office environments and emergency rooms.

One theoretical application of research along this lines would be creating a substance that enhances or maintains vigilance, she said. This would be useful in people who have to make critical assessments under repetitive or sleep-deprived conditions, Mujica-Parodi said.

But Wysocki noted that the chemical signals animals give off are relatively fleeting. Thus, this idea could work for "someone who knew that they needed to be alert during a very short episode" -- and that person could perhaps get an effect for 10 or 15 minutes after a snort of this theoretical substance, he said.

"But you wouldn't want to let a long-haul truck driver try this," he said.

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