The experience of smelling onions when no source is present, or when an existing odor is profoundly altered, can be confusing and alarming. The sense of smell, or olfaction, is a complex process where airborne chemical molecules are detected by specialized receptors in the nose and then interpreted by the brain. When this system malfunctions, the resulting odor perception can manifest in various ways, with an unpleasant, pungent smell like onions being a frequently reported complaint. Understanding the nature of this smell—whether it is a hallucination originating in the nervous system or a real odor caused by internal body chemistry—is the first step toward identifying the underlying cause. The pathways governing how we perceive smells are intimately connected to areas of the brain that manage memory and emotion, explaining why these olfactory disruptions can be psychologically impactful.
Defining Phantom and Real Olfactory Sensations
Determining the source of the onion smell begins with classifying the type of sensory experience. One common type is phantosmia, or an olfactory hallucination, where an individual perceives an odor without any actual smell molecules being present in the environment. This phenomenon is often described as smelling something foul, burnt, or chemical, but the phantom smell of onions is also reported. Phantosmia is essentially the olfactory system generating a smell signal internally without an external trigger.
A different experience is parosmia, which involves a distortion of an existing smell. In this scenario, a person smells something real, like coffee or perfume, but the odor is incorrectly translated by the brain, making it smell overwhelmingly like sewage or, specifically, onions. Parosmia suggests that the olfactory neurons have been damaged and are sending a corrupted signal to the brain’s olfactory centers.
The third possibility is that the onion smell is genuinely present, originating from the person’s own body or breath. This real odor is detectable by others and is caused by the body expelling volatile sulfur compounds through sweat or respiration. The distinction between a phantom smell, a distorted smell, and a real body odor is significant because each points toward a different category of underlying causes, ranging from localized nasal issues to systemic metabolic conditions.
Causes Originating in the Nasal and Sinus Passages
Many cases of phantom or distorted onion smells are rooted in the physical structures of the nose and sinuses, collectively known as the peripheral olfactory system. The olfactory epithelium, a patch of tissue high inside the nasal cavity, contains the specialized cells responsible for detecting odors. When these cells or the surrounding tissues become irritated or damaged, they can misfire, leading to a phantom smell.
Chronic sinusitis, a long-term inflammation of the sinus cavities, is a frequent cause of phantosmia. The persistent inflammation and swelling can damage the delicate olfactory nerve endings, causing them to send signals to the brain even when no odorant is present. Nasal polyps, which are soft, non-cancerous growths on the lining of the nasal passages, can similarly interfere with the normal function of the smell receptors.
Another common trigger involves upper respiratory tract infections, particularly those caused by viruses. Post-viral parosmia occurs when the olfactory neurons are damaged during the infection and improperly regenerate as the person recovers. This faulty repair process results in a distorted perception of odors, where once-pleasant smells are misinterpreted as foul, sometimes specifically like decaying onions.
Neurological and Central Nervous System Explanations
When the onion smell originates not in the nose but in the brain itself, it falls under the category of central phantosmia. This indicates a problem with how the brain’s olfactory cortex processes or spontaneously generates smell signals, distinct from peripheral damage. These neurological causes are often more concerning and require immediate medical evaluation.
One significant neurological cause is a temporal lobe seizure, where the phantom odor, known as an olfactory aura, can act as a warning sign preceding the seizure itself. The temporal lobe houses the primary olfactory cortex, and abnormal electrical activity in this region can directly trigger the perception of a strong, non-existent smell. Individuals often report these smells as unpleasant, such as burning rubber, metallic odors, or a pungent onion-like scent.
Other neurological events, such as severe head trauma or injury, can damage the olfactory bulb or the central pathways responsible for smell processing. This physical damage can lead to misfiring signals that the brain incorrectly registers as an odor hallucination. While rare, the sudden onset of a persistent phantom smell can also be a sign of a brain lesion or tumor located near the olfactory pathways. Conditions like migraines, Parkinson’s disease, and stroke have also been associated with phantosmia.
Systemic and Metabolic Conditions
In cases where the onion smell is a real, detectable body odor, the cause often lies in systemic or metabolic processes that produce volatile organic compounds. The primary mechanism involves the body’s inability to properly metabolize or excrete certain chemical compounds, which are then released through the breath, sweat, or urine. This category of causes is characterized by a real odor that others can perceive, unlike phantom smells.
Diet plays a direct role, as foods containing high amounts of sulfur compounds, such as onions, garlic, and certain spices, are metabolized and their volatile byproducts are released through the skin. When sweat, which is initially odorless, mixes with bacteria on the skin, these sulfur compounds are broken down into thioalcohols, which produce the characteristic onion or garlicky smell. Certain medications and dietary supplements can also contribute to this effect by influencing the body’s chemical composition.
A rare genetic disorder known as Trimethylaminuria (TMAU) is a metabolic condition that causes a strong body odor. People with TMAU lack or have a reduced function of the Flavin-containing Monooxygenase 3 (FMO3) enzyme, which converts the smelly compound trimethylamine into an odorless form. This unconverted trimethylamine builds up and is excreted through sweat, breath, and urine, often described as having a fishy odor, though it can also be categorized as a general unpleasant smell.
Impaired function of the liver or kidneys can also lead to the buildup of toxins that alter body odor and breath. When the liver is unable to filter waste products effectively, sulfur-containing compounds can accumulate and be released through the breath, resulting in a musty or sometimes sulfurous odor known as fetor hepaticus. Similarly, severe kidney failure causes the buildup of urea, which can be broken down into ammonia, leading to a bleach-like or urine-like smell on the breath and skin. The presence of a persistent, real onion or sulfurous smell warrants investigation to rule out these systemic issues.