Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)
6. Things That Stink
People wonder about the unusual things in life, but they wonder even more about the annoying things. Though they might never ask why a flower smells so nice, they almost always wonder what is making that awful smell.
Humans’ main sense for detecting molecules is in their noses. Their tongues have only a few chemical sensors in them, and their other senses have even fewer. It is difficult to tell strong tea from vanilla extract by sight, touch, or sound. But the nose knows.
Why do things stink?
Why do chemicals smell?
There are two mechanisms that cause odors that smell or stink.
Some things strongly irritate mucus membranes in your nose, eyes, and throat. Caustic chemicals such as ammonia, chlorine, and acid vapors stimulate pain sensors and make you jerk away from the odor.
But most smells are more subtle and are detected by special cells in your nose. An odor molecule may stimulate more than one cell, and different odors can be recognized by combinations of receptors. Each olfactory cell in the nose has a nerve that sends the information to the brain. Humans have about 40 million of these cells. Dogs have 50 times as many—about 2 billion. Humans have about 350 genes for odor receptors and can distinguish about 10,000 different odors.
If you are exposed to a particular smell for a while, you become used to it, and your body no longer detects it. The brain seems to be interested in new smells more than common smells, such as your own body odor. Women tend to have more sensitivity to odors than men do. Pregnancy enhances this effect. Humans’ ability to detect odors decreases as they age.
Your ability to smell things helps you tell which foods are good to eat and which foods are rotten. It keeps you away from dangerous places that have high levels of bacteria or harmful vapors. Most of the things that smell bad are harmful to you or indicate that something may be harmful.