Smelling salts, also known as spirit of hartshorn or sal volatile, are chemical compounds used for arousing consciousness. The usual active compound is ammonium carbonate, a colorless-to-white, crystalline solid ((NH4)2CO3). Because most modern solutions are mixed with water, they should more properly be called "aromatic spirits of ammonia." Modern solutions may also contain other products to perfume or act in conjunction with the ammonia, such as lavender oil or eucalyptus oil.
Smelling salts have also been known as 'sal volatile', for their ability to produce ammonia vapour.
The use of smelling salts was widely recommended during the Second World War, with all workplaces advised by the British Red Cross and St. John Ambulance to keep 'sal volatile' in their first aid boxes. Nowadays, their use and prevalence has dramatically decreased.
Smelling salts are often used on athletes (such as boxers) when they are knocked unconscious or semi-conscious to arouse consciousness and restore mental alertness.
They are also used in competitions (such as powerlifting, strong man and ice hockey) to "wake up" competitors to perform better. Famous athletes such as Alexander Ovechkin, Tyler Seguin, Brett Favre, Peyton Manning, Carlos Boozer, Samuel Eto'o, and Tom Brady have been seen using smelling salts on the sidelines.
They are also still used for people feeling faint, or who have fainted, either administered by others, or self-administered, with some at-risk groups, such as pregnant women, sometimes advised to keep them close to hand.
Smelling salts release ammonia (NH3) gas, which triggers an inhalation reflex (that is, cause the muscles that control breathing to work faster) by irritating the mucous membranes of the nose and lungs. 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. The sympathetic irritant effect is exploited to counteract these vagal parasympathetic effects and thereby reverse the faint.
Ammonia gas is toxic in large concentrations for prolonged periods and can be fatal. Since smelling salts produce only a small amount of ammonia gas, no adverse health problems from their use have been reported. However, a high concentration of inhaled ammonia might burn the nasal or oral mucosa.
The use of ammonia smelling salts to revive people injured during sport is not recommended because it may inhibit or delay a proper and thorough neurological assessment by a healthcare professional, such as after concussions when hospitalization may be advisable, and some governing bodies recommend specifically against it. The irritant nature of smelling salts means that they can exacerbate any pre-existing cervical spine injury by causing reflex withdrawal from them.
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