The cultural reference to “smelling toast” describes the perception of an odor that does not exist. This experience, known as an olfactory hallucination or phantom scent, is a recognized medical phenomenon. This sensory disruption signals interference with the complex pathway that processes smell, either in the nasal passages or within the brain itself. The experience warrants investigation to determine the underlying cause.
Understanding Olfactory Hallucinations
The medical term for perceiving an odor that is not physically present is phantosmia, often called a phantom smell. Phantosmia arises when the olfactory system spontaneously generates a signal or misfires, causing the brain to interpret a scent. The odors experienced are frequently unpleasant, such as the smell of burning, metallic, or rotten garbage, which explains why “smelling toast” is a common description.
This condition is distinct from parosmia, a related smell disorder. Parosmia involves a distorted perception of an existing odor; a person smells something real, but their brain translates it into a different, often foul, scent. For example, a person with parosmia might smell a flower, but register the scent as gasoline or sewage. Phantosmia, in contrast, occurs without any external odorant stimulating the nose.
Phantosmia is categorized based on where the problem originates, which guides diagnosis and treatment. Peripheral phantosmia involves issues in the nose or the olfactory epithelium, the tissue containing smell-detecting neurons. Central phantosmia, which is often more concerning, results from a disruption in the brain’s olfactory processing centers. The phantom smell may be constant or intermittent, and perceiving it in one or both nostrils offers clues about its origin.
Non-Neurological Explanations for Phantom Smells
Many phantom smells originate in the peripheral olfactory system, meaning the cause is localized to the nose and surrounding structures. These non-neurological causes are generally more common and less serious than central nervous system issues. Chronic sinus infections (sinusitis) cause inflammation and swelling in the nasal passages, potentially leading to phantosmia. Irritation and damage to the delicate olfactory receptor neurons lining the nasal cavity can cause them to spontaneously fire, sending false odor signals to the brain.
Growths within the nasal cavity, such as nasal polyps, can also physically disrupt the flow of air and the function of the olfactory nerves. A common cold or other upper respiratory viral infections can temporarily damage the olfactory epithelium, the site of initial odor detection. As the tissue heals and inflammation subsides, these phantom smells typically fade away.
In some cases, the source is closer to the nasal passages, such as severe dental problems or abscesses, where local infection can trigger the sensation of foul odors. Certain medications, including some antidepressants and antibiotics, have also been reported to cause phantosmia as a side effect. When the cause is peripheral, the phantom smell is often experienced more strongly in one nostril than the other, which is a helpful diagnostic distinction.
When “Smelling Toast” Signals a Neurological Emergency
While many causes of phantosmia are localized, a sudden, persistent, or repetitive phantom smell can signal a serious neurological issue originating in the brain. The most specific neurological cause is a focal aware seizure, previously called a simple partial seizure, which begins in the temporal lobe. The temporal lobe contains the uncus and amygdala, regions deeply involved in processing smell and emotion.
When a seizure discharge begins in this area, the olfactory hallucination serves as an olfactory aura, acting as a warning sign before the seizure progresses. The phantom smell, often described as burnt, rubbery, or chemical, results directly from disorganized electrical activity in the brain’s smell center. An aura may be the only symptom, or it may be followed by a complex partial seizure, where the person may stare blankly, chew repeatedly, or lose awareness of their surroundings.
Other significant central nervous system events can also manifest as phantom smells, including strokes, brain tumors, and head trauma. A mass or lesion in the temporal lobe can irritate the olfactory processing centers, leading to the perception of a non-existent smell. The sudden onset of phantosmia, especially when combined with other neurological symptoms like confusion, vision changes, or motor problems, warrants immediate medical investigation.
If a phantom smell is new, repetitive, or accompanied by symptoms like confusion, jerking movements, or loss of consciousness, seek emergency medical attention immediately. Even in the absence of other symptoms, any new, persistent, or recurring phantosmia should be evaluated by a healthcare professional to rule out serious underlying conditions. A thorough medical workup is needed to differentiate between a relatively benign peripheral cause and a potentially life-threatening central nervous system disorder.