Smelling salts release ammonia (NH3) gas, which triggers an inhalation reflex (that is, cause the muscles that control breathing to work faster[7]) by irritating the mucous membranes of the nose and lungs.[7] Additionally, the irritant elevates the heart rate, blood pressure, and brain activity by activating the sympathetic nervous system. Fainting can be caused by excessive parasympathetic and vagal activity that slows the heart, and decreases perfusion of the brain.[15] The sympathetic irritant effect is exploited to counteract these vagal parasympathetic effects and thereby reverse the faint.[16]