The perception of smelling bleach, chemicals, or other unpleasant odors when no source is present is called phantosmia, or an olfactory hallucination. This condition involves the brain perceiving an odor that does not exist in the environment. Phantosmia is a malfunction in the system that processes smell, where the brain misinterprets or generates signals along the olfactory pathway. Phantom smells are often described as chemical or metallic, like bleach, because unpleasant, foul odors are reported more frequently than pleasant ones.
Causes Originating in the Sinuses and Olfactory System
The most frequent causes of phantosmia begin in the peripheral olfactory system, located within the nasal passages. This system consists of the olfactory epithelium and sensory neurons that detect odor molecules and transmit signals to the brain. Inflammation or damage in this localized area can cause the olfactory nerves to misfire, leading to the perception of a phantom odor.
Chronic sinus infections (sinusitis) are a leading cause, as persistent inflammation and swelling can irritate or damage the delicate olfactory receptors. Nasal polyps, noncancerous growths, can also disrupt the normal flow of air and the functioning of the olfactory system. Acute events, such as a severe head cold, a viral infection like COVID-19, or allergic inflammation (rhinitis), can temporarily injure the olfactory neurons.
These localized issues cause damaged nerves to spontaneously generate a signal, which the brain then interprets as a smell, often foul or chemical. The resulting phantom smell is a form of sensory noise created by the irritated nerve endings. Because the problem is contained within the nose, phantosmia caused by sinus issues may sometimes be noticed only in one nostril. Conditions within the sinuses and nasal cavity account for a majority of all olfactory disorders.
Neurological Conditions That Trigger Phantom Smells
Phantosmia can also originate in the central nervous system (the brain), where the signals are processed and interpreted. The olfactory bulb and the olfactory cortex, particularly in the temporal lobe, are the brain regions responsible for smell perception. A malfunction in these areas can generate a smell sensation without any external trigger.
The perception of a sudden, strong, and fleeting phantom smell can sometimes act as an aura preceding a migraine headache. Phantosmia can also be a symptom of a focal seizure, especially those originating in the temporal lobe. During a seizure, neurons in the olfactory cortex may become overactive, causing the brief but intense perception of a smell like burning rubber, gas, or chemicals.
Head trauma, even a mild concussion, can cause temporary or permanent damage to the olfactory nerves as they pass through the skull base. Rarely, a lesion or tumor located near the olfactory bulb or cortex can directly stimulate the nerve tissue, resulting in a persistent phantom odor. Unlike the chronic smell from sinus issues, smells caused by seizures or migraines are sudden in onset and short-lived.
Systemic Health Issues and Chemical Changes
Phantom smells can sometimes manifest from systemic health issues where chemical compounds build up in the body and affect the olfactory system. These causes involve the bloodstream and breath, distinct from localized nerve damage or direct brain activity. The perception of a chemical odor is relevant in the context of advanced kidney disease.
When the kidneys fail to filter waste products from the blood, uremia occurs, leading to a buildup of toxins. One toxin is urea, which can be excreted through the breath and saliva. This process results in “uremic fetor,” a breath odor often described as fishy, metallic, or urine-like, which may be perceived as a chemical or bleach-like scent.
Certain prescription medications can also trigger phantosmia by altering the body’s chemistry or directly affecting the olfactory receptors. Medications such as antibiotics, antidepressant drugs, and anti-seizure medications have been linked to changes in smell perception. In older adults, the use of multiple medications, including antidiabetic agents, has been associated with experiencing phantom odors.
Recognizing Warning Signs and Seeking Help
While many cases of phantosmia are temporary, especially following a cold or viral infection, certain accompanying symptoms warrant prompt medical consultation. A healthcare professional can help differentiate between a peripheral issue (nose/sinuses) and a central issue (brain). The diagnostic process often begins with an ear, nose, and throat (ENT) examination to look for inflammation, polyps, or signs of infection.
If the phantom smell is accompanied by neurological symptoms, immediate investigation is necessary. The sudden onset of a smell followed by a brief blackout could indicate a seizure disorder. Diagnostic tools like Magnetic Resonance Imaging (MRI) or Computed Tomography (CT) scans may be used to look for brain lesions, tumors, or signs of stroke near the olfactory pathways.
Neurological Symptoms Requiring Investigation
- Severe, sudden headaches
- Unexplained vision changes
- Confusion
- Memory problems
- Loss of consciousness
Persistent phantom odors that do not resolve within a few weeks, or any smell accompanied by weight loss or changes in overall health, should also prompt a medical visit. The goal of seeking help is to identify and treat the underlying cause, whether it is a chronic sinus condition, a medication side effect, or a systemic or neurological issue.