Smelling salts are a combination of ammonium carbonate and perfume. They were traditionally used to revive people from faints, and are often associated with frail 19th century women. Some athletes use them before competition as a pick-me-up, or coaches may use them to revive an athlete who has been injured. Medically, smelling salts certainly will make someone more alert, but they may not be as beneficial as was once claimed. Indeed, some of the ingredients in them are actually potentially quite dangerous.
Older names for smelling salts include sal volatile and salt of hartshorn. The horns of male deer, or harts, were a common source for ammonia before chemical synthesis allowed scientists to make salts in a laboratory. The name presumably came about because fumes from the mixture were intended for inhalation. Novels set in a certain period seem to involve an alarming amount of sal volatile being wafted under the nostrils of ladies of delicate sensibilities.
The base of this substance is ammonium carbonate, a salt with a white crystalline structure. When ammonium carbonate is mixed with water, in the case of “aromatic spirits of ammonia,” or perfume, the reaction creates fumes which rise from the salts. When placed under someone's nose, the fumes irritate the mucus membranes of the nose, throat, and lungs, stimulating the body to breathe more quickly. The mixture of perfume with ammonium carbonate could have resulted in a rather interesting odor, which is why smelling salts were kept tightly bottled when they were not needed.
The acceleration of respiration brought about by using these salts will make someone more alert, and could potentially revive someone from a faint. It may even improve headaches, which can sometimes benefit from an increase of oxygen to the brain. Headaches and fainting were the two treatments for which smelling salts were routinely recommended. However, ammonia fumes are not very good for the body, and prolonged exposure should be avoided.
Fumes from ammonia can be toxic if they are allowed to build up. Therefore, smelling salts should only be used in small quantities, and ideally in a well ventilated or outdoor area. When not in use, the salts should be sealed so that the vapors cannot leak out. Discard old salts with care, because ammonia is extremely toxic to fish.