Researchers at the University of Chicago say a decreased sense of smell is one of the strongest predictors of five-year mortality — stronger even than a diagnosis of heart or lung disease. According to the study, published in the journal PLOS One, people with no sense of smell were more than three times as likely to have died within five years than people with a normal sense of smell.

The study looked at 3000 people between the ages of 57 and 85. Survey researchers went to the subjects’ homes to assess their overall health and measure their senses, including their sense of smell. They found that a test subject’s sense of smell was a “very strong predictor” of who would be alive five years later when the researchers came back to examine them again.

At first, the researchers thought the results were complicated by other "confounding factors," including other risk factors of death, says Jayant Pinto, associate professor of surgery at the University of Chicago and one of the authors of the study. But when they controlled for these factors, the finding was basically unchanged.

The researchers used a version of a European smell test in which five felt-tipped markers with an odor embedded in them are presented to the subjects. Responses were scored on a scale of 1 to 5 according to how many the subjects got right. The odors were rose, leather, orange, fish and peppermint.

A score of 4 or 5 correct answers was considered normal. Below that, each additional error corresponded to an increased risk of 5-year mortality. Despite the strength of the finding, the researchers can still only speculate why this is the case.

“Smell is an ancient part of our nervous system,” Jayant says. “We know that people at risk of brain diseases like Alzeimers and Parkinson’s Disease lose their sense of smell before they develop the devastating consequences of those diseases. So, we think that sense of smell might be an early indicator of brain problems.”

A diminished sense of smell could possibly indicate that the body’s ability to regenerate has declined, because there are stem cells in the nose that regenerate the olfactory nerves. It could also be a sign of damaging environmental exposures.

“[Our] olfactory nerve sits in the roof of the nose and is connected through little holes in the brain,” Jayant explains, “So it’s out in the environment, it’s out in the air — and things in the air can injure it.”

Jayant says it’s not time to panic if you find you have a decreased sense of smell. “I have been receiving emails non-stop from people all over the world who are worried. Let me reassure people by saying there are many risks of death for everyone in the world — including getting in a car, smoking a cigarette and eating a cheeseburger. So we have to put this in context.”

Nevertheless, he does suggest that older people who notice a decreased sense of smell should seek medical attention and their doctors should consider whether they need closer examination.

The recent study is part of a larger project called the National Social Life, Health and Aging Project, led by the University of Chicago. The project aims to better understand health and aging across a wide number of behavioral, social and medical circumstances.

This story is based on an interview that aired on PRI's Science Friday with Ira Flatow.