Smelling salts are a type of chemical compound that has a very strong smell, usually ammonia, and are used to arouse consciousness because the release of ammonia gas that accompanies their use irritates the nose and lungs. They have been used since the 13th century to prevent or remedy fainting, and are still effective for this use. When held up to someone’s nose, the fumes irritate the interior of the nose, causing the lungs to quickly breathe deeply to clear the nasal passage, which can help revive someone who has fainted.

Athletic trainers have found alternative uses for smelling salts, such as treating head injuries or as a temporary self-treatment for a "cloudy mind" from a head injury. However, there is no evidence that they have any benefit for athletes, and smelling salts have even been banned by some leagues.

When used properly, smelling salts don’t have adverse effects. However, some uncommon side effects include coughing, difficulty breathing, headache, and temporary increases in heart rate. Prolonged inhalation may result in permanent lung damage or death.

In summary, smelling salts are a type of chemical compound that can help revive someone who has fainted by irritating the nose and lungs, causing the lungs to quickly breathe deeply to clear the nasal passage. They are generally safe when used as directed, but prolonged inhalation may result in permanent lung damage or death.