How scents travel

How can you tell when someone is wearing scent? You can smell it. How can you smell something that is on a person's skin a few metres away? The answer cannot be given in a short sentence so lets begin by thinking about what happens when you put scent on your skin and follow its path to your nose.

Scent is a liquid. It contains chemicals, which we can detect by our nose. When you put scent on your skin, it warms up a little. The scent, like all liquids, is made from tiny particles that are sliding over each other. The extra heat from the skin gives the particles extra energy. Those particles on the surface of the scent drop receive so much energy that they break free and enter the air.

The air is made from billions of tiny, rapidly moving particles. As they move so fast they crash into each other, bounce off each other and set off in a different direction. The particles of scent enter this crowd of bouncing particles as they move into the air. At first the scent particles keep close to the skin. We say that their concentration is high here. As the air particles bounce into them the scent particles are moved away from the skin and from each other. They bounce around with the rest of the air particles and spread out. As the particles get further from the skin there are fewer of them that are close together. We say that their concentration is getting lower.

The movement of scent shows us how all new gases behave when they enter the air. The particles form a region of high concentration just above the liquid they have escaped from. The particles are then jostled by the air particles (and also jostle the air particles back) until they spread out and become less concentrated. Eventually the particles become so spread out that they completely mix with the air particles. When scent particles are completely mixed with the air, we can no longer smell them.

In still air, scent particles form a layer above the scent drop, then gradually spread out in all directions. There are a large number of scent particles in the air close to the surface of the scent drop. They get in the way of particles trying to leave the surface of the scent drop but as the scent particles in the layer spread out, they make more room for more scent particles to escape from the drop. These two movements of particles - first from the scent drop, then from the layer of air around the scent drop - are slow.

When the air is moving the movements speed up. As the particles escape from the surface of the scent drop, they are pushed away by the air currents. This makes more room for more particles to escape so they leave the scent drop more quickly. As soon as the particles enter the air they are moved along more quickly than by the simple jostling of the air particles and so travel much further.

Next time a person wearing scent walks past you think about how the particles left their skin and reached your nose.

What substances were used to make the first scents?
Frankincense and myrrh were probably the first substances used to make long lasting scents. They are substances called gum resins and are made by certain kinds of tree. They contain large amounts of substances, which produce a pleasant smell.

Did Egyptians wear scents?
Yes, they did. They made cones of scented fat. On special occasions in the life of their city they wore a cone on their heads. The heat from the body melted the fat and let it run over the skin. At the same time the scent escaped from the fat and gave the people a pleasant smell.

Why do some people have bowls of flower petals in their home?
There are also spices mixed with the flower petals. The mixture is called a pot-pourri. Scents from the petals and spices move through the room and give it a pleasant smell. The pot-pourri was used in the days before air fresheners but some people prefer to use them today also.

Where do scents come from?
Scents are chemicals, which are made by plants and animals. We use the scents from certain plants such as the rose and lavender to make scents and perfumes for ourselves. The scents from many plants are brought together in a process called distillation. This produces a concentrated liquid of scent, which is then diluted so the smell is not too strong.

Why does food smell when it is being cooked?
The food contains a range of natural chemicals. Most provide us with nourishment but some escape when the food is heated and move through the air. They give the food its distinctive smell.

Why do people sniff when they smell a scent?
When you breathe in through your nose, the air goes through a space in your skull called the nasal cavity. In the roof of this cavity are special organs, which detect smells. When you breathe normally the air passes in along the floor of the nasal cavity and does not reach the smell-detecting organs. When you want to smell something you sniff and this action makes air flow up to the roof of the nasal cavity. The air flows by the smell-detecting organs and they inform your brain about the smelly chemicals in the air.

Why is the gas in a cooker so smelly?
The gas in a cooker is called methane. When this gas mixes with air it burns if it is heated. Methane does not have a smell so if pure methane were used you would not know if there was a gas leak. There are special chemicals called stenching agents, which are added to gas to give it a smell. The smell is strong and unpleasant so that you can tell quickly if there is a gas leak or if an unlit gas ring has been left on. If you smell gas you should not strike a match or even switch on a light. The spark from the switch can provide enough heat to make the gas and air mixture explode.

Why do animals use scents?
Many animals have a much stronger sense of smell than we do and use scents to communicate to each other. They use them to attract mates and to mark out the boundaries of their territories. The skunk squirts out a liquid with a very powerful smell if it is attacked. The strength and foulness of the smell makes the predator stop its attack and gives the skunk a chance to escape.