Antecedent events that precede the olfactory distortion havebeen described in the literature to include upper respiratoryinfection (viral?), headtrauma, allergic rhinitis and chronicrhinosinusitis (Mott and Leopold, 1991
; Duncan and Seiden, 1995; Apteret al., 1999; Quint et al., 2001). Whether they apply to stimulatedor non-stimulated distortions is difficult to determine becauseof the imprecise words used (parosmia, dysosmia, etc.). In myexperience, patients with stimulated distortions (troposmias)usually have a history of something that might decrease thenumber of olfactory neurons, such as an upper respiratory infection,head trauma or aging. The distortions seem to occur during eitherneuron death or regeneration.
In contrast to this, we have noticed, as has Zilstorff, thatmost phantosmias present with no history of upper respiratoryinfection, head traumaor aging (idiopathic) (Zilstorff, 1966).The typical history for a phantosmia is that it begins spontaneouslyin a woman between 15 and 30 years of age with an episode of odorperception the individual thinks is real but others do not appreciate. Thisinitial episode lasts 
5-20 min. It resolves spontaneously withno after effects. The next episode will occur the same way about1 month later. Gradually over the next year the episodes becomemore frequent and last longer each time. The perception maybe in one or both nostrils and may be blocked with occlusionof the nostril(s). Once the smell perception has started, itis usually not masked by foods and all foods have the flavorof the phantom smell. Typically the phantom smell resolves withsleep. Other activities that have been reported to stop thesmell include Valsalva's maneuver, forced crying, intranasalinstrumentation and gagging. After several months or years, theseall become ineffective in stopping the phantom smell perception. Althoughthere is nothing in the literature, I have found systemic corticosteroidsto be ineffective in helping these people.
I perceve a mild smoke smell from time to time.. a surprising number of people experience this.. before you dismiss this as psycho weird have a look !
http://chemse.oxfordjournals.org/cgi/content/full/27/7/611
thank you
Caliber