We all know that we can be infected by the fear and stress that we see on other people’s faces. We unintentionally imitate them, and then feel what they feel. This transfer is what psychologist Brené Brown calls the work of gremlins—little tricksters that bring us down. The big surprise is that the infectious agent may be in the air. When volunteers at Stony Brook University sniffed pads that had been in the armpits of anxious first-time skydivers, their amygdala—the brain region associated with emotion and danger—"lit up" in an fMRI (brain scan) in a way that it didn’t when they smelled "exercise sweat." At Rice University, the smell of "fear" sweat biased women toward interpreting ambiguous facial expressions as negative. And in another experiment, people who smelled "fear" sweat made fearful faces, and people who smelled "disgust" sweat made disgusted faces. Their gasps or grimaces were completely unconscious.
This might help: Picking up on others’ moods is nothing new, but smell contagion is a surprise. The science here is still young; there’s no saying that the chemicals in another person’s sweat will definitely make you feel the same way they do. But there is mounting evidence that, on a subconscious level, exposure to their stress sweat might make you more vigilant and cautious about potential threats. (An upside to stress sweat is that it might also make you sharper temporarily.) The standard advice for avoiding emotional contagion applies: Reset yourself by going on a long walk. Oh, and do it in a well-ventilated place.