||It has been suggested that Spirits of hartshorn be merged into this article. (Discuss) Proposed since June 2011.|
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·H2O). 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.
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 Brett Favre, Peyton Manning, Carlos Boozer, 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.
Physiological action 
Smelling salts release ammonia (NH3) gas, which irritates the mucous membranes of the nose and lungs, and thereby triggers an inhalation reflex (that is, it causes the muscles that control breathing to work faster). Additionally, the irritant properties activate the sympathetic system as a response leading to elevated heart rate, blood pressure and brain activity. Specifically, many of the benign fainting events are caused by excessive parasympathetic and vagal activity leading to dilation of blood vessels, slowing of the heart and decreased perfusion of the brain thus leading to decreased muscle tone in general and postural muscles in particular, allowing the person to collapse. The sympathetic irritant effect is exploited to counteract these vagal parasympathetic effects leading to reversal of the faint. Similar action can be produced by cheek slapping which is often administered to fainting persons.
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, there have been no reports of any adverse health problems from their use.
There is a limited possibility of direct burning of the nasal or oral mucosa from high concentration of inhaled ammonia.
The use of ammonia smelling salts to revive people injured during sport is not recommended as it may inhibit or delay a proper and thorough neurological assessment by a healthcare professional, and some governing bodies recommend specifically against it. There is also a risk that as an irritant, use of smelling salts could cause reflex withdrawal from the source, thereby exacerbating any pre-existing cervical spine injury.
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