Sense of Smell
Smell is one of our 5 senses. It works by sensing chemicals in the air. All smells are combination of different molecules. Different molecules, and different combinations, result in different smells.
Although we usually don't realize it we are constantly assessing our environment by smell. In some cases, e.g. with smoke, we even assess for danger.
The smell of food is vital to how good the food tastes. Without smell taste is severely hampered, sometimes to the point where there is no taste. Someone who does not have the sense of smell is called an anosmic.
Interestingly enough we seem to be much more sensitive to bad smells. One theory is that we only sense good or bad smells. There is no such thing as a neutral smell. Think about this, where ever you are right now what do you smell? Chances are you don't smell anything. But, there are many chemicals in the air yet we do not sense them. If all of the sudden there was a foul smell you would detect it almost immediately.
Some experts also believe that smell can affect mood and emotions. Maybe this is why so many enjoy candles with pleasant smells. Some also believe that we choose mates and friends based on smells. This is hard to prove, but for sure smell has strong influences on our actions and choices.
olfactory epithelium are areas in the nose that contain olfactory sensory neurons. As molecules pass over this area they diffuse into the tissue, or may bind to various proteins. As receptors are stimulated signals travel along the olfactory nerve to the olfactory bulb (a small area underneath the front of the brain above the nasal cavity. From here, signals are sent to various parts of the brain which is interpreted based on previous experience of that smell. For example, you know know that someone is baking fresh bread simply by the smell. This is because we can remember smells and their associations.
Besides being an anosmic, no ability to smell, there are other conditions that one can suffer from. Hyposmia is a decreased ability to smell. Hyperosmia is an abnormal acute sense of smell. Phantosmia is hallucinating unpleasant smells. And dysosmia is sensing a different smell than what it actually is.