Phantom smells, like burnt toast and burning hair, can be a sign of a stroke, but they can also be a sign of other conditions, explains NBC News. A tumor can also cause people to experience phantom smells, as can an infection in the brain.
Though it's possible that people will detect phantom smells for no reason, smelling them is often due to a neurological issue. Mayo Clinic refers to this phenomenon as phantosmia, or olfactory hallucinations. The odors that are detected will vary by individual, but typically they are unpleasant and described as being chemical-like or burning.
In addition to stroke, people will often experience phantosmia as a symptom of other conditions including head injury, brain tumors, epilepsy and Alzheimer's disease, according to Mayo Clinic. A handful of cases have been reported where people experience phantosmia for no known reason. Some of these people may have had issues with scent for many years, and the symptoms of their phantosmia can come and go.
It's estimated that 2.7 million people have some type of problem that affects their ability to smell, reports NBC News. Because smell and taste are so closely connected, olfactory problems can cause issues with eating, as food can taste like the scent the person picks up, including like burnt toast.