As recently as 1987, British coal mines used caged canaries to act as sentinels, warning miners of toxic gases. Birds are more sensitive to them than we are, so they can suffer before the gas reaches dangerous levels for humans, allowing miners to evacuate and avoid suffocation.

The sense of smell is the canary in the coal mine of human health, according to new research. Losing your sense of smell predicts death within five years, according to research published today in the journal PLOS ONE, suggesting that the nose knows when death is coming, and that smell can be a bellwether for a person’s overall state. as a marker of exposure to toxic substances in the body or environment.

The study included more than 3,000 participants ages 57 to 85 from the National Social Life, Health, and Aging Project (NSHAP), a longitudinal study of factors that influence well-being among older Americans.

In 2005–2006, Jayant Pinto of the University of Chicago and his colleagues asked all participants to perform a simple test to identify five common odors (rose, leather, fish, orange, and mint) using the number of misidentified odors. by odor severity score.

Five years later, the researchers sought out as many of the same participants as possible and asked them to take the same odor test a second time. During the five-year interval between the two trials, 430 of the original participants died (that’s 12.5% of all participants). Of these, 39% of those who failed the first olfactory test died before the second test, compared to 19% of those with moderate loss of smell at the first test and only 10% of those with a healthy sense of smell.

In other words, participants who failed the first sniff test were four times more likely to die within five years than those who correctly identified five odors. This was true when other factors that affect smell were taken into account, such as race, gender, mental illness, and socioeconomic status, and mild smell loss was associated with a slightly increased likelihood of imminent death.

Loss of smell was a more accurate predictor of death than diagnoses of cancer, heart failure, or lung disease, with the only common cause predicting it more accurately being severe liver damage. But the researchers point out that this is unlikely to be the cause of death in itself, arguing that it is merely a signal of what is to come, and offer two reasons why this might be the case.

The olfactory nerve endings, which contain olfactory receptors, are the only part of the human nervous system that is continuously renewed by the primary cells. The production of new olfactory cells declines with age, as our ability to detect and distinguish odors gradually declines. Loss of smell indicates that the body is in a damaged state and is unable to repair itself.

The olfactory nerve is also the only part of the nervous system exposed to open air. As such, it allows poisons and pathogens to enter the brain more quickly, so a loss of smell can be an early warning of something that will eventually lead to death.

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