The sense of smell in humans is often underrated. On the score card of scents and senses, the sense of smell hits the bottom.

Even scientists in the 19th century espoused this view that our sense of smell is comparatively weaker than the other senses. Nonetheless, researchers today have dismissed this claim as mere myth—claiming that those scientists had never measured our ability to detect odours.

Neuroscientist John McGann, who has been studying the science of smell by working with rats and mice at Rutgers University, remarked, “I was taught in school that human olfaction isn’t a great sense.”

“It’s taught in introductory psychology courses and it’s in the textbooks. But this whole thing is a crazy myth,” he added.

The truth is humans, too, can trace smell trails and distinguish between similar odours; as well as detecting a wide range of substances – sometimes more sensitively than rodents and dogs can. According to McGann, we live in a rich world of scents and sensibility, where odours deeply influence our emotions and behaviour.

"Actually, we have a really excellent sense of smell," he said. "There are quite a lot of experiments showing that the human sense of smell is pretty similar to what you can find with a rat or a mouse or a dog."

Examining the work of 19th century scientist Paul Broca, McGann said, "He was interested in free will and he had this idea that smell was this very animalistic sense and that it compelled animals to have sex and feed."

"And humans – having free will – could choose how we responded to smells and presumably had a less strong or less special sense of smell than other animals,” he added.

He and his team started with an experiment that involved taking two odours that humans can’t tell apart—and we couldn’t find any,” he says. “We tried odours that even mice can’t tell apart; yet, humans were like: No, we’ve got this.”

In his study, he found that humans have about 400 distinct smell receptors in our noses, compared to like 1,000 receptors in rats. "But, in fact, 400 is an awful lot," said McGann, "and, quite honestly, there are very few odours that are volatile enough to get into the air that humans can't smell."

In theory, we can distinguish tens of millions of unique smells, and maybe more. There have been a few nose-to-nose comparisons between humans and other mammals in the lab, but McGann believed there's no consistent winner.

"Humans are best at some, and dogs are best at some, while mice are best at some. It just depends on what the chemical is."

Alexandra Horowitz, a psychology professor at Barnard College, expressed that "we are better smellers than we think."

"Without smell you can't taste, and that's a real loss," she said. "I will acknowledge that's something we're great at – maybe even better than dogs."

The human brain helps detect early-stage diseases in others

In a study published recently, it is shown that our sense of vision and smell alone are enough to make us aware that someone has a disease even before it breaks out.

A Swedish study suggests that people are able to smell if someone is ill. They say humans can detect when a person's immune system has gone into overdrive after exposure to bacteria.

The researchers from the Karolinska Institute in Sweden say there is even anecdotal and scientific evidence to suggest that different diseases have particular smells.

For example, scrofula, an infection of the lymph nodes, is reported to smell like stale beer. And a person who suffers from diabetes is known to sometimes have breath that smells of acetone.

“Being able to detect these smells would represent a critical adaptation that would allow us to avoid potentially dangerous illnesses,” claimed Professor Mats Olsson, the lead researcher.

By injecting harmless sections of bacteria, the researchers activated the immune response in the participants, who developed the classic symptoms of disease — tiredness, pain and fever —for a few hours, during which time smell samples were taken from them and subsequently photographed and filmed.

The injected substance then disappeared from their bodies, and so did the symptoms.

The researchers then exposed another group of participants to these smells and images – and got them to rate how much they liked the people, while their brain activities were measured in a magnetic resonance scanner.

They were then asked to state – just by looking at the photographs – which of the participants looked sick, which they considered attractive, and which they might consider socialising with.

"Our study shows a significant difference in how people tend to prefer and be more willing to socialise with healthy people than those who are sick, and whose immune system we artificially activated," Olsson said.

"We can also see that the brain is good at adding weak signals from multiple senses relating to a person's state of health," he added.

Woman who could smell her husband’s disease

Joy first noticed a “sort of woody, musky odour” in her husband, an anaesthetist, 12 years before the onset of his disease. She did not realise that was the smell of something pressing that threatened to disrupt their lives. A brain scan confirmed that Les Milne, 45, had symptoms indicating a diagnosis of either a brain tumour or Parkinson’s.

Seventeen years on, while attending a Parkinson’s UK awareness lecture in 2012, she revisited the particular odour and linked it with the disease. Dr Tilo Kunath, the charity’s Senior Research Fellow, recalled, “I tracked her down and we did a test where she smelled 12 T-shirts – six having been worn by Parkinson’s sufferers and six without,” he explains. MIMS

Read more:
3 new research breakthroughs in neurodegenerative diseases
Artificial neurons built, that work just like the real thing
Science Bites: 4 biotypes of depression found using fMRI, Fructose conversion in brain linked to neurological problems
Parkinson’s disease a growing concern among aging Asian populations