An olfactory hallucination (phantosmia) makes you detect smells that aren't really present in your environment. The odors detected in phantosmia vary from person to person and may be foul or pleasant. They can occur in one or both nostrils. The phantom smell may seem to always be present or it may come and go. Phantosmia may occur after a head injury or upper respiratory infection. It can also be caused by temporal lobe seizures, inflamed sinuses, brain tumors and Parkinson's disease. Consult your doctor if you experience the symptoms of phantosmia, so that your doctor can rule out any serious underlying disorders that may be causing the detected smell. Parosmia is another smell disorder that's similar to phantosmia. But, in parosmia a smell that's present in your environment is distorted. Parosmia can occur with damage to the olfactory system, such as after a severe respiratory infection.