A long-term study of nearly 3,000 older adults found that those who could not identify at least 4 out of 5 common odors were more than twice as likely as those with a normal sense of smell to develop dementia within five years. About 14% could name just 3, 5% percent could identify only 2, and 2% could name just 1. One percent of the study subjects were not able to identify a single scent.