Brief Answer:
As below:

Detailed Answer:
Hi,

Thank you for your query.

1. Most of the times, the cause for this Parosmia / Cacosmia or an altered smell like smoke may never be found.

2. Some possible causes are:
a. Nasal infections and chronic rhino-sinusitis, oral infections, inflammation including gum and dental (any recent root canal or dental extraction?), acid reflux and Tonsilloliths are leading causes for a bad smell in the nose.
b. Damage to the olfactory system due to any severe infection, head injury, exposure to solvents and volatile chemicals.
c. Metabolic diseases such as diabetes, hypothyroidism, liver dysfunction.
d. Rarely, degenerative brain disorders such as Parkinson's and Alzheimer's. Have you ever suffered from Epilepsy?
e. Drug induced: Get a review of your current medications for RA (Rheimatoid Arthritis) and Cardiac issues.

3. CT / MRI of the Brain and Para Nasal Sinuses may be required. Have you undergone blood tests? Do you smoke or have alcohol? What is your occupational or household exposure to chemicals, extreme cold or constant air conditioning? Conditions like atrophic rhinitis, post nasal drip and acid reflux may also co-exist.

4. The bad smell that you get through your nose when you inhale is most likely due to a mild sinus infection or stagnant mucus including drying up of normal nasal secretions. Have you had a recent cold, cough or fever? The inflammation may be fungal. Foreign bodies are rare in adults but must be ruled out if it is one sided. A smokey smell usually means a sulphur containing compound usually produced by a bacterial infection. You may discuss the possibility of Biofilm formation in your sinuses with your physicians. Sometimes there are fungal infections and build up of waste products of inflammation, infection, immune system by-products and gases like nitric oxide and ammonia. This would explain the burnt smoke like smell which you experience.

5. Due to their unique design, airflow through our sinuses and their ventilation occurs gently and passively during expiration. This may explain why you get the smell off and on.

6. Get a general check-up done in addition to an ENT check-up as many diseases such as diabetes, liver diseases, nasal polyps, tumors and so on should be ruled out.

7. The sinuses may harbor silent infection. Fungal infections may exist only within the sinuses (as against invasive fungal infections) and yet cause allergic reactions. This constitutes Allergic Fungal Rhino Sinusitis (AFRS). Such fungal infections may be missed out on CT Scans and requires MRI Scans to detect. If there is nasal and sinus inflammation you may take a trial of treatment including antibiotics, anti-allergic decongestants, saline nasal washes, mouthwashes, tongue cleaning, anti-reflux medication and probiotics.

8. If no cause is found, it may be a an olfactory hallucination. This a commonly seen with damage to the olfactory mucosa or olfactory bulb and tract, esp. some years after a head injury. This will show up on imaging such as CT/MRI.

8. You may follow up with the answers to the above questions, results of the above investigations and treatment for further recommendations.

I hope that I have answered your queries. If you have any further questions, I will be available to answer them.

Regards.

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