Thank you for your inquiry.
You are correct in that scientific information regarding the use of ammonia for “reviving” people who feel faint or have fainted is sparse. The implied mechanism of action, however, is the induction of hyperpnoea via a vagally-mediated reflex. The article abstracted copied below discusses this issue. Therefore, the rationale is to induce a deeper, faster breath.
Clearly, ammonia can be a respiratory irritant, however, in asthmatics and, in my opinion, there is a contraindication to using smelling salts in asthmatic patients.
Finally, I could find no good information regarding the true efficacy of smelling salts in terms of their “revival potential.” However, for the most part, I believe the evidence points to the lack of any true effect in this regard. There is a nice discussion of this in terms of the use of smelling salts in athletes which also mentions their use in general. The article is available free of charge online here.
Thank you again for your inquiry and we hope this response is helpful to you.
Studies on mechanisms underlying the reflex hyperpnoea induced by inhalation of chemical irritants
R. Buff, E.A. Koller
Volume 21, Issue 3, September 1974, Pages 371–383
Abstract Chemical irritants (cigarette smoke and ammonia vapour) as well as histamine-induced bronchial asthma attacks initiate analogous changes in the mechanical properties of the lungs, namely uneven and asynchronous ventilation, resulting qualitatively in the same reflex hyperpnoea. The reflex is mediated by the vagus nerves. The experiments presented suggest that pulmonary deflation receptors — probably identical with the endings termed pulmonary irritant receptors — are involved, and that lung deflation or compression may be the natural stimulus of these endings.
The return to normal breathing after irritant-induced hyperpnoea is discussed with regard to the generalized stabilization in lung mechanics and the rapid adaptation of the vagal receptors involved.
Phil Lieberman, M.D.