The air contains an incalculable number of volatile compounds, which can be detected only by one sense: the smell.
There are about 110,000 smells in nature.
The human being perceives only about 100-200.
But not everybody detects them in the same way. The olfaction of children is much finer. While those working in the perfume industry can detect 600-800 smells!
In women, the sense of olfaction is strongest around the ovulation period, significantly stronger than during other parts of the menstrual cycle and also stronger than the sense in males.
The left nostril catches smells better than the right one.
The man is an audiovisual animal, with a rudimentary olfaction, but some animals can even communicate using olfaction through some chemicals called pheromones.
Birds, too, have poor olfaction, except kiwi, condors and their relatives (black and turkey vultures), and albatrosses with their relatives (petrels and shearwaters).
Amongst mammals, cetaceans (whales, dolphins) present anosmia and monkeys present a weak olfaction.
Insects, especially the colonial ones (ant, termites, bees), keep a real olfactory conversations using pheromones.
In people, pheromone communication is almost inexistent. People secret pheromones, but in extremely small doses and their psychological effects are hard to detect.
In humans, the olfactory surface is about 10 square centimeters (in dogs 150!), compassing about 1 million olfactive cells. In dogs and other mammals, the olfactive mucosa is extemely folded and much more densely innervated.
For an olfactive signal to emerge, it is necessary that 8 molecules of a specific substance reach an olfactive cell. And for the olfactive sensation to appear, at least 40 olfactive cells must be stimulated.
For 80 % of the people, the nose is not exactly on the middle of the face, but to the right. This will help people detect the source of a smell, as the slight difference permits a comparison. (the same happens with the owls, which have their ears disposed asymmetrically to locate the source of a noise in the dark). 70 % of infections are propagated through the nose.
Try to smell a handful of salt. If it's pure, you won't sense anything. For people, quinine is inodorous. But a dog will feel them even dissolved at 1:10,000 (a gram in a cub of water). However, we will detect them better in food.
When we have a cold, we do not feel the taste of the aliments. But the cold affects the nose, not the taste receptors. But we associate the taste of the aliments with their smell. If you try to taste open binded and nose tapped butter or grease, tea or water, they will seem impossible to identify, appearing tasteless.
Our nose is the location of an amazing sense that the modern humans lost it because they don't use it anymore. But indigenous populations and animals do have it and use it. It's the biological compass, made of many magnetite (an iron oxide) crystals, located at the base of the nose's sinus. This organ enables the organism to detect the north.
That's why - unlike the explorers - the people of the jungle tribes never get lost in the forest. The first Europeans in contact with these tribes thought they had a keen sense of the details but the same thing happened with the nomad African tribes from the deserts, where marks change quickly.
The Limits of the Human Nose
How much can a human smell?