The Scientist: Joshua Zeichner, M.D., is the Director of Cosmetic and Clinical Research in the Mount Sinai Medical Center Department of Dermatology.

The Answer: Our faces can become flushed when we’re nervous or embarrassed, when it’s hot out or we’re exerting ourselves, in response to certain foods and beverages, after sex or as a result of rosacea.In all these cases, the outcome is the same: Blood vessels in your face dilate (open wider), allowing more red blood cells to flow through them. Because the capillaries in your face are very near the surface of your skin, the result of that increased blood flow is a ruddy glow we know as blushing.

This skin reddening serves an important physiological function when your body temperature rises. Vasodilation brings more hot blood near the skin’s surface, where excess heat can be released outside the body, cooling the blood—and you. (On the flipside, when it’s cold out, your peripheral blood vessels constrict to maintain blood flow to your vital organs. That’s why your hands and feet get cold first.)

When we blush for emotional reasons, the same thing is happening, but there isn’t a physiological function (that we know of). Social discomfort, like being embarrassed, can activate the sympathetic nervous system, which responds in times of stress. The activation signals blood vessels to dilate. Cue that telltale red face. Some studies suggest blushing is an evolutionary adaptation; in experiments, people are more likely to trust and forgive wrongdoers who appear to be blushing with shame or remorse.

Some of us get flushed because of sensitivities to particular foods or beverages. For example, many Asians turn red from drinking because they are deficient in a key enzyme that breaks down alcohol. MSG and sulfites are also common triggers. Vasodilation also causes reddened skin in people with the genetic condition rosacea, which is exacerbated by extreme temperatures and certain foods as well.

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