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Sense of smell

posted Feb 2, 2015, 1:28 AM by   [ updated Jul 14, 2016, 1:33 PM by Upali Salpadoru ]

What is Anosmia ? 

  It is the inability to detect smell. 
Not much of a loss”, one would think. 
  • An affected person  would not  sense a fire in a hidden corner, 
  • Could never derive the pleasure of tasting food.  

Wood, rubber, plastic and toast can produce their  specific smells on heating or charring.  Strong specific flavors in food especially in fried stuff or the smell of ripe fruits  make them appetizing. 

In Wellington NZ, There is a good residential suburb called  Churton Park.  It has been now nicknamed as CURRY PARK.  

The reason?  

The place has a high proportion of Sri Lankans and Indians. The other commuters  in public transport get the smell of curries for which the carriers have become blunt. Their thick garbs of woolen carry volatile substances released in the process of making their favourite curries.    

A smell or odour is due to microscopic particles which are present in air at extremely low concentrations.  Most people can detect Hydrogen sulphide, H2S, the gas with a rotten egg smell, 1 part in a million.  

The minimum concentration that may be detected is known as the ‘threshold of odour’.  Just as this can vary according to individuals it can vary widely according to the chemical nature. Although no instrument has been discovered to measure this as yet , CIA gives some readings.

Vanillin .......A synthetic vanilla essence  C8H8O3.................... 10-13mg / ml.
Synthetic musk (kasthuri) .. Originally obtained from the male musk deer ……… 10-12mg/ml
Mercaptan ......Responsible for bad breath and the odour in faeces and flatus...... 10-1mg/ml Skatol....... C9H9N, has a strong fecal odor, found naturally in faeces..........10-13mg/ml    source:- CIA

The smell of a substance is solely on the chemical nature of it.  There is a small  area at the back of the nasal cavity which is lined with millions of neurons. These are always fresh and never older than 28 days. 

We have five to six million such neurons while a  dog  possesses  220 million 

Each neuron has a cilia capable of identifying a particular substance and send an electric pulse through the olfactory nerve to the brain. 

The brain associates this with past experience and decides to act accordingly. 

It has been  discovered that children below 5 years experience every odour as a pleasant smell. (The reason for them to touch dirt)  

Sense of smell seems to be the most memorable of all.  Even after a year people have  remembered a smell with a 65%   accuracy. 

Only 50% of Visual recall last after three months. The smell is the sense that mostly govern our emotional moods.
Search is on for identifying people by the smell of their sweat. . 

“Israeli chemists say the food we eat, drugs we take, gender and even state of mind, all combine to make each person’s sweat unique. each person has his or her own chemical fingerprint.”

Animal Senses.



Fig.2. The black bear of Sri Lanka. ( Kalu walaha.)

Most animals have a very acute sense of smell. The ability of  pets to recognise their masters is well known. Out of almost all animals the bears have the best sense of smell; over 2000 times better than a human. 

They can detect a rotting carcass 20 miles away. The olfactory bulb in their brain is 5 times larger than that of a human.


Elephants have a very special sense for water; even for underground sources. 

They have been found tracking on a scent 12 miles away. “A herd will use feet and tusks to dig waterholes for themselves to drink and bathe”


Insects have no nostrils.  

Yet  most of their activities, such as finding food, getting mates, and escaping predators are governed by the sense of smell. 

Their organs of smell are the antennae. a sophisticated system of odour detection. 

They possess sensilla, which have a pore for scent molecules to enter and two nerves to carry signals .  They can detect pheromones ,  secreted by the female at a distance of more than six miles away. 

An ant can smell and trace the path, a member has traversed earlier, Hugh Robertson,  Professor of entomology, University of Illinois  says, 

Honey bees use odor recognition for finding food.  Worker bees might encounter a bewildering number of flowers to choose from, but they can discriminate between them using subtle olfactory cues……….A large number of odorant receptors allows the bees to find food and communicate its location to other bees."


Among the birds New Zealand kiwi takes the pride of place. They are flightless and feeble creatures which depend on their sharp sense of smell for their survival. The placement of the kiwi's nostrils in the tip of its bill is an unusual adaptation that makes finding insects and worms on the ground easier.


Smell for a fish is much more important than vision

They land mark places not by pictorial memory but by the memory of smells. 

Sense of smell enables salmon  to sense the stream where they were born, so that they may return to it to spawn. 

Some fish recognise each other  by scent. Minnows release an alarm pheromone if their skin is damaged

According to Tricia Meredith, a biologist at Florida Atlantic University, the sense of odour in sharks is often over estimated.

“A shark zeroing in on the source of an attractive scent such as blood uses the same sort of zigzag approach that” ……. . "From what we know now, they can't smell a drop of anything in an Olympic-sized swimming pool," .  

It may be that their sense is more acute in open oceans where they swim fast.
In water smells do not spread as fast as in air.  For the fish to get a smell water has to flow in and out fast to get the smells. .