Taste and Smell
Aging And The Olfactory System
Odors are detected through some fifty million olfactory receptor cells. These cells die and are replaced every sixty days. They are located in the mucus membrane located at the top of the nasal cavities, each of which occupies about 1 square inch. Sniffing concentrates the odors, since less than 10 percent of the air entering the nasal cavity reaches the olfactory epithelium. Odor molecules must dissolve into the mucus membrane surrounding the olfactory receptors, which lines a piece of porous bone called the cribiform plate. Olfactory receptors send axons though the cribiform plate to the olfactory bulb, which lies at the base of the brain. Studies of olfactory epithelium indicate that it becomes scarred and abnormal with age. The scar tissue may block the pores in the cribiform plate, thereby preventing the olfactory receptors from sending axons to the olfactory bulb.
Older people need a higher concentration of a given substance in the air to detect a smell than do younger people. This rise in threshold may reduce the ability to react to the presence of harmful chemicals in the environment and thus may pose a danger to older adults. A study comparing the ability to detect ethyl mercaptan, an ingredient in liquid petroleum gas, showed that adults age seventy-four required ten times more of the chemical to detect it than did twenty-year-olds. However, the ability to detect odors varied across older adults, with some performing as well as younger subjects.
Older people find concentrated odors less intense. A 1986 survey conducted by the National Geographic Society involved a scratch-and-sniff test using six different odors. Respondents rated their intensity on a five-point scale. Intensity ratings declined with age, more so for some odors than for others. Studies normally find a broad loss in olfactory ability for a wide range of smells from pleasant to unpleasant. A more comprehensive scratch-and-sniff test using forty different odors, showed that a person’s ability to identify odors by name starts to decline around the age of sixty. Sometimes that may be due to cognitive impairments or memory loss. The ability to detect an odor—without being able to name it—may remain unchanged.
Losing the ability to smell may affect the flavor and the enjoyment of food. One study compared the ability of young and older subjects to detect the herb marjoram in carrot soup. Older adults performed less well than younger subjects, and some were unable to detect marjoram at all. The deficit was related to smell as opposed to taste. When tested using nose clips, young subjects also were unable to detect or identify marjoram in the soup. Middle-aged and older adults were also less able to detect a woody alcohol odor, and sometimes failed to recognize such common odors as baby powder, chocolate, cinnamon, coffee, and mothballs.