Smelling Sickness

Can you tell whenever someone around you is sick? It may not be a sixth sense, according to a new study published in Psychological Science.

Study author Mats Olsson of the Karolinska Institutet in Sweden says that there is both scientific and anecdotal evidence to suggest that certain diseases have a particular smell our noses can detect.

He said that he's never compared different scents based on different conditions, but he posits that certain diseases smell more strongly than others. For example, diabetes can smell like acetone or rotten apples on a person’s breath, he said.

Can Our Sense of Smell Help Keep Us Healthy?

Olsson wanted to test whether being able to sniff out these conditions could create an adaptation that would allow us to avoid potentially dangerous illnesses, especially during the early stages of the disease or condition.

"There may be early, possibly generic, biomarkers for illness in the form of volatile substances coming from the body," Olsson explained.

Olsson and a team of researchers injected eight healthy people with either a saline solution or lipopolysaccharide (LPS), which is a toxin that boosts the immune response. The volunteers were clad in tight shirts to absorb their sweat over a four-hour span. He found that those with LPS in their systems had higher body temperatures and increased levels of cytokines, which are inflammatory molecules in the immune system.

Then, Olsson recruited 40 people to sniff those sweat samples. They found that the shirts worn by people given LPS had a more intense, unpleasant smell than the others. This demonstrated that the more activated a person's immune response, the more unpleasant his or her sweat becomes.

Olsson noted that the precise chemical compounds that create these smells haven't been identified, but he said that humans definitely give off chemical signals when the immune system is activated.

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Can Smelling Sickness Make You Sick?

Sniffing won’t make you contract a disease though, Olsson said.

“Inhaling means that you can catch aerosols with pathogens, but it is the inhalation, not the attentive sniffing that makes you contract a disease,” he said. Plus, "the perceived odor may keep you at a distance.” 

Graeme Lowe, Ph.D., a member of the Monell Chemical Senses Center in Philadelphia, said he believes the most common way to catch a cold or the flu is to inhale aerosolized droplets after a person sneezes, or to come in contact with a contaminated surface and then touch your mouth.

Luckily, sickness odor signals may be detectable independent of the patient sneezing or coughing, Lowe said.

“[Sniffing] may provide a useful chemosignal that can help us avoid infected individuals before transmission occurs,” Lowe said.

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No Two People Have Same Sense of Smell

Don’t expect your friend to be able to sniff out sick people in the same way you do, however. Recent research shows that people have different sensitivities to smell. Certain odors are more appealing—or unappealing—depending on the person.

A study in the August 2013 issue of Current Biology found that there are genetic differences that affect how humans perceive smell. The genetic variants are in or near genes that encode so-called odorant or olfactory receptors that are located on the surface of sensory nerve cells in our noses.

“We were surprised how many odors had genes associated with them. If this extends to other odors, then we might expect everyone to have their own unique set of smells that they are sensitive to," said Jeremy McRae, a researcher with Plant and Food Research in New Zealand. "These smells are found in foods and drinks that people encounter every day, such as tomatoes and apples. This might mean that when people sit down to eat a meal, they each experience it in their own personalized way.” 

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