Hyposmia is a reduced ability to smell and to detect odours. A related condition is anosmia

Anosmia

Anosmia is a lack of functioning olfaction, or in other words, an inability to perceive odors. Anosmia may be either temporary or permanent. A related term, hyposmia, refers to a decreased ability to smell, while hyperosmia refers to an increased ability to smell. Some people may be anosmic for one...

, in which no odours can be detected. Some of the causes of olfaction problems are allergies, nasal polyps, viral infections and head trauma. It is estimated that up to 4 million people in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 have hyposmia or the related anosmia

Anosmia

Anosmia is a lack of functioning olfaction, or in other words, an inability to perceive odors. Anosmia may be either temporary or permanent. A related term, hyposmia, refers to a decreased ability to smell, while hyperosmia refers to an increased ability to smell. Some people may be anosmic for one...

.

Based on latest scientific evidence, hyposmia might be a very early sign of Parkinson's disease

Parkinson's disease

Parkinson's disease is a degenerative disorder of the central nervous system...

. Hyposmia is also an early and almost universal finding in Alzheimer's disease

Alzheimer's disease

Alzheimer's disease also known in medical literature as Alzheimer disease is the most common form of dementia. There is no cure for the disease, which worsens as it progresses, and eventually leads to death...

 and dementia with Lewy bodies

Dementia with Lewy bodies

Dementia with Lewy bodies , also known under a variety of other names including Lewy body dementia, diffuse Lewy body disease, cortical Lewy body disease, and senile dementia of Lewy type, is a type of dementia closely allied to both Alzheimers and Parkinson's Diseases...

.