The sudden, unmistakable odor of natural gas triggers immediate alarm due to its association with potential hazards. When this pungent, rotten-egg smell is repeatedly detected but no leak is found, the experience is a recognized medical phenomenon. This perception of an odor that is not present is a form of olfactory hallucination. The issue lies not in the environment, but within the body’s complex system for processing smell.
The Phenomenon of Phantosmia
The medical term for smelling an odor that does not exist is phantosmia, also called a phantom smell or olfactory hallucination. Phantosmia differs from parosmia, which is a distorted sense of smell where an actual odor is present but perceived incorrectly, such as coffee smelling like burning rubber.
The perception of a gas leak is common because natural gas is naturally odorless, so companies add a chemical odorant for safety. This additive is typically mercaptan, a sulfur-containing compound detectable at very low concentrations. The foul, sulfur-like smell is often described as rotten eggs, which is the specific chemical trigger the olfactory system is hallucinating. The phantom odor may be constant or intermittent and can affect one or both nostrils.
Common Causes Originating in the Nasal Cavity
Phantosmia causes are categorized based on whether they originate in the peripheral olfactory system (the nose and nasal passages) or the central system (the brain). Peripheral causes are the most common and relate to inflammation or structural issues within the nasal cavity that affect the olfactory receptor neurons.
Chronic sinusitis, which is long-term inflammation of the sinuses, can cause olfactory neurons to misfire and generate the phantom signal. Nasal polyps, which are soft growths in the lining of the nasal passages, can also disrupt the normal function of smell receptors.
Recent upper respiratory infections, such as a cold, flu, or COVID-19, may cause temporary inflammation of the olfactory epithelium. In these peripheral cases, the phantom smell may be stronger in one nostril, reflecting a localized issue. Severe dental infections can also cause referred phantom smells.
Neurological and Systemic Triggers
When the cause is not localized to the nasal cavity, the issue stems from the brain’s central olfactory system, which interprets the raw signal from the nose. These central causes are often more medically significant because they involve the neural pathways or the processing centers themselves.
One recognized neurological cause is a temporal lobe seizure, where the phantom odor, or olfactory aura, is the initial sign of seizure activity. These hallucinations may last only a few seconds and are caused by abnormal electrical firing in the brain’s smell processing area. Phantom smells can also occur as an aura accompanying a migraine headache, acting as a warning sign before the pain begins.
Head trauma, even from an old injury, can damage the olfactory bulb or cortex, leading to a malfunction in smell processing. Persistent phantosmia may also be a symptom of a serious underlying condition affecting the central nervous system.
These conditions include a stroke or a brain tumor located near the olfactory pathways. In central cases, the smell is often perceived in both nostrils and is more persistent, as the malfunction is in the interpretation center. Neurodegenerative conditions, including Parkinson’s disease, can also present with phantosmia as an early symptom.
Medical Evaluation and Management
If phantom smells persist, occur frequently, or are accompanied by neurological symptoms like muscle weakness, vision changes, or seizures, seek professional medical attention. Diagnosis typically begins with a thorough physical examination by an ear, nose, and throat specialist (otolaryngologist).
This evaluation often includes a nasal endoscopy, using a thin, flexible tube to look directly into the nasal passages and sinuses for inflammation, polyps, or structural issues. If a sinonasal cause is suspected, a Computed Tomography (CT) scan of the sinuses may be ordered to check for chronic infection or polyps.
If a central or neurological cause is suspected, Magnetic Resonance Imaging (MRI) of the brain is the preferred imaging method. MRI looks for abnormalities like tumors, stroke, or lesions in the olfactory pathways. Management focuses on treating the underlying cause, such as using antibiotics or steroids for a sinus infection, or anti-seizure medication for olfactory auras from epilepsy.