The loss of smell, known as anosmia, and distorted smell, called parosmia, became widely recognized symptoms during the COVID-19 pandemic. These changes affect a significant number of individuals infected with SARS-CoV-2. The impact of these smell disturbances extends beyond simply not being able to detect odors; it can profoundly affect daily life, influencing appetite, food enjoyment, and even safety by hindering the detection of hazards like smoke or gas.
How COVID-19 Impacts Smell
COVID-19 causes smell loss primarily by affecting supporting cells within the nasal cavity, rather than directly damaging the olfactory neurons responsible for detecting odors. These supporting cells, known as sustentacular cells, express the ACE2 receptor, which SARS-CoV-2 uses to enter human cells. When the virus infects and damages these sustentacular cells, it disrupts their ability to provide the necessary structural and metabolic support to the olfactory sensory neurons.
This indirect damage can lead to the retraction of cilia on olfactory receptor neurons and alter the composition of the mucus layer. While olfactory sensory neurons themselves are not infected, their function is temporarily impaired due to the compromised support system. This explains the rapid onset of smell loss in COVID-19 and the potential for recovery as these supporting cells regenerate.
Typical Recovery Timelines
Smell recovery after COVID-19 varies, though many improve quickly. Approximately 50% of patients regain their sense of smell within the first two to three weeks, with an additional 40% recovering within eight weeks post-infection. Studies indicate that 70% to 88% of affected individuals recover most olfactory function within one to three months. An estimated 95% of patients may recover within six months.
Recovery can continue even after longer periods, with some studies showing 88% of individuals who experienced smell or taste dysfunction reported complete recovery within two years. Factors such as the severity of initial smell loss, female sex, and older age may be associated with longer recovery periods. While most people recover, a small percentage, between 2% and 10%, may experience persistent or permanent smell loss.
Understanding Distorted Smell Sensations
As the olfactory system begins to heal, some individuals may experience distorted smell sensations. Parosmia involves familiar smells being perceived as unpleasant, different, or distorted, such as coffee smelling like burnt rubber or onions like sewage. Phantosmia refers to the perception of phantom smells, where an odor is sensed without any external source.
These distortions can be quite distressing. The prevalence of parosmia and phantosmia can increase as smell disorders lengthen, appearing weeks or months after initial infection. These phenomena suggest olfactory nerves might be regenerating abnormally, leading to misinterpretations of odors.
Approaches to Aid Smell Recovery
Smell training, also known as olfactory training, is a widely recommended approach to help regain a sense of smell after viral infections, including COVID-19. This involves regularly sniffing a set of distinct odors, typically four essential oils, for 10-20 seconds each, twice daily. Common scents include rose, lemon, eucalyptus, and clove, representing different fragrance categories. During training, individuals focus and visualize the scent to stimulate the brain’s ability to reorganize and repair neural connections. This practice is a low-risk, accessible method performed over several months.
Maintaining good nasal hygiene can be supportive. If smell loss persists beyond a few weeks or months, or if accompanied by other concerning nasal symptoms like congestion or discharge, consulting a healthcare professional, an Ear, Nose, and Throat (ENT) specialist, is advisable. They can rule out other causes and discuss management options, including specific therapies or clinical trials.