Sadly, as people age, almost all the senses decline to varying degrees, including sight, hearing and, less obviously, the sense of smell. But in recent years, researchers have found that dramatic declines in olfactory function can be an early sign of dementia or Parkinson's. But a new study shows reduced sense of smell is also linked to an overall increased risk of death.

Nicola Davis at The Guardian reports that an international team of researchers looked at smell tests taken by more than 2,200 people between the age of 71 and 82 years old in 1999 and 2000 as part of the National Institute on Aging's Health ABC study. Each participant smelled 12 common scents and were asked to choose the smell from a list of four possibilities. The sniffers were then graded as having either good, moderate or poor olfactory function. The health outcomes of these individuals were then followed up for 13 years incluidng yearly phone surveys.

After compensating for other health factors like age and smoking, the team found that those elderly people with a poor sense of smell had a 46 percent higher chance of dying a decade out from the test than those with a good sense of smell. Even more, the sense of smell was a particularly good predictor of death for those who were in good health at the beginning of the study. Among those with a sniff score rated poor who were in decent shape, the chance of dying by year 10 was 62 percent higher than those with a good score.

Looking at the causes of mortality for the 1,211 participants who died by year 13 of the study, about 28 percent of the increased risk can be explained by dementia, Parkinson’s disease, and possibly to cardiovascular disease. Respiratory disease and cancer did not appear to be linked to the sense of smell. The research appears in the journal Annals of Internal Medicine.

That means, points out Stephanie Pappas at LiveScience, that 72 percent of the risk linking impaired senses of smell with death is unexplained.

“We don't have a reason for more than 70 percent of the increased risk,” study senior author Honglei Chen of Michigan State University says in a press release. “We need to find out what happened to these individuals.”

It’s possible, he says, that a deteriorating sense of smell is an early warning sign for health conditions that are not picked up during routine medical visits. To figure it out, Chen says he hopes to dig even deeper into the data.

In the meantime, he suggests physicians should start paying attention to olfactory problems. “It tells us that in older adults, impaired sense of smell has broader implications of health beyond what we have already known,” he says. “Incorporating a sense of smell screening in routine doctor visits might be a good idea at some point.”

Currently, however, there is no sniff test available for clinical use, and the U.K's National Health Service says there is no "smell test for dementia" on the horizon. In fact, the NHS points out that since this is an observational study, it cannot definitively explain why sense of smell and mortality could be linked.

Still, some researchers are confident enough in the link between dementia and sense of smell that they are developing scratch and sniff tests to screen for the disease. Columbia University’s Irving Medical Center reports that researchers hypothesize that the olfactory bulb is one of the first parts of the brain to suffer damage from Alzheimer’s and other neurodegenerative disorders. Early, small-scale experiments by neurologist William Kreisl show that a strong sense of smell can often rule out Alzheimer’s, but impaired smell may be related to many diseases, including Parkinson’s, Alzheimer’s and Huntington’s disease.

The biggest benefit of a smell test if and when it is developed, Kreisl argues, could be flagging those patients who should be referred for more invasive and expensive tests like PET scans.