Ive had this since 2017 precovid and at its peak is very distressing.
It comes and goes for days or months and i went to GP who diagnosed olfactory hallucinations and referred me to ENT. The consultant checked mu sinuses with a scope and referred me for an MRI to rule out anything sinister. He said it was probably the result of a virus and peak allergies (easter bad cold and hay fever bomb) damaging the olfacotrh receptor cells in my nose. He prescribed some strong nasal steroid drops.
HE ALSO SAID GET A CO2 ALARM AND CHECK FIRE ALARM AS YOUR REACTION TO SMOKE SMELLS REDUCES!!!! Intentional capitals. I thought this was incredibly insightful
I still have I but i can manage it now. Ht it has tsken a while. I had some allergy tests done and was alergic to house dusdust mite and various plants but not the dog! and the worst place was in the living room. With huge sofa and brick inglenook (dust gatherer)and dog bed!
I found if i covered my face (often found with nose under t shirt!) the problem lessened so knew it was something i was breathing in. I bought a nasal rinse from Amazon (water pulse). I asked for fexofenadine and a stronger steroid spray. Fluctisone or somwthing and rhis combintation manages it.
Usually i smell smoke (it started as cigarette smoke smell) now its a dusty musty smell and after using the nasal spray and a fexofendine or two it goes aftet 24 hours.
Sometimes it comes on very suddenly and intensely without an environmental change ie can be sat in the living room fkr hours then it comes on strong, which makes me think it also somethings in my body, not just simple response to allergens.
Once after a bad cold or covid the smell changed to bleach. I sent me a bit crackers that did - i thought id i haled bleach driving my car!
Its better in winter snd on holiday 80% i am symptom and drug free and the other 20% i manage with above methods.
Olfacatary hallucination and phantosmia are interchangeable. Paraosnia is smalling the wrong thing, so rose smell like decaying matter and so on.
Hope this help- it might be self limiting and go away.