If your parents or grandparents cannot identify at least four out of five common odours - peppermint, fish, orange, rose and leather - from everyday life, then they are more than twice as likely as to develop dementia within five years than those with a normal sense of smell.
According to researchers, this simple smell test could provide a quick and inexpensive way to identify those who are already at high risk.
The olfactory nerve is the only cranial nerve directly exposed to the environment.
The cells that detect smells connect directly with the olfactory bulb at the base of the brain, potentially exposing the central nervous system to environmental hazards such as pollution or pathogens.
Olfactory deficits are often an early sign of Parkinson's or Alzheimer's disease. They get worse with disease progression.
Lead author Jayant M. Pinto from the University of Chicago said that the results showed that the sense of smell is closely connected with brain function and health.
"We think that smell ability specifically, but also sensory function more broadly, may be an important early sign that indicates the greater risk for dementia," Pinto added.
"Loss of the sense of smell is a strong signal that something has gone wrong and significant damage has been done," Pinto said.
The team analysed nearly 3,000 adults, aged 57 to 85.
They were asked to smell each item and identify that odor, one at a time, from a set of four choices.
The five odours, in order of increasing difficulty, were peppermint, fish, orange, rose and leather.
The results revealed that 78.1 percent of those examined had a normal sense of smell; 48.7 percent correctly identified five out of five odors and 29.4 percent identified four out of five.
About 18.7 percent were considered "hyposmic," who recognised two or three out of five correct.
The remaining 3.2 percent, labelled "anosmic," could identify just one of the five scents (2.2 percent), or none (1 percent).
Nearly 80 percent of those who provided only one or two correct answers also had dementia, with a dose-dependent relationship between degree of smell loss and incidence of dementia.
Losing the ability to smell can have a substantial impact on lifestyle and wellbeing, Pinto noted.
Pinto explained that those who cannot smell face everyday problems such as knowing whether food is spoiled, detecting smoke during a fire or assessing the need a shower after a workout, is closely associated with depression as people don't get as much pleasure in life.
The research appears in the Journal of the American Geriatrics Society.