1. Where in the Nose Odor Particles Are Received
Odor particles enter the nose with your breath. However, the place that receives them is not at the entrance of the nose, but high up in the back. This area is called the olfactory epithelium. Its size is approximately 4.8 to 6.4 square centimeters (about 0.7 to 1.0 square inches), with variations depending on the source [1]. This is roughly the size of a rectangle 2 cm (0.8 inches) tall and 3 cm (1.2 inches) wide, so it is smaller than you might think.
It is said that there are approximately 100 million olfactory cells here that sense smell [1]. This number comes from dictionary entries, and the figure varies depending on the dictionary, so it is best to accept it as an approximation. With 100 million cells, one might think they all do the same job. But in reality, each cell is limited to receiving specific things.
2. How About 400 Receptor Types Can Detect So Many Different Smells
Receptors that catch odor molecules are made from genes. It has been found that olfactory cells choose and use only one type of receptor gene. In mice and other animals, there are approximately 1,000 candidate genes [2]. In humans, analysis suggests that the receptor genes actually working are about 400 types [3].
If 400 types meant only 400 smells, we could not explain the smells in the world. However, one odor substance activates multiple receptors. The combination of which receptors reacted becomes the "pattern" of the smell [2]. This is similar to a piano chord. Pressing one key makes one note, but pressing several at once creates a different sound. Mathematically, choosing 2 from 400 types results in about 80,000 combinations, and choosing 3 results in about 10 million. This is just a calculation example, not the actual number of smells.
Axel and Buck reported the receptor genes in 1991. For this discovery, they received the 2004 Nobel Prize in Physiology or Medicine [2]. The announcement of the award explained that this mechanism is the foundation for distinguishing and remembering approximately 10,000 odors [2]. This "10,000" was a rough estimate at the time.
3. Why the "1 Trillion Smells" Number Does Not Match Other Estimates
The "approximately 10,000" mentioned in the Nobel Prize explanation and the "at least 1 trillion" claimed by the 2014 study differ by a factor of 100 million. The fact that the number in research moves this much suggests that counting types of smells is very difficult [2][4].
In 2015, Meister stated in a paper that the claim of "1 trillion types" was incorrect. He pointed out that there was a problem with the mathematical method used to estimate the total number from the results [4]. It is reported that other researchers also issued criticisms in the same year, but this article focuses on Meister's paper as a primary source.
So, what is the real number? Now that "1 trillion" has been criticized, a replacement number has not been decided. The actual number is unknown [4]. Just as we cannot say for certain that we can distinguish "1 trillion types," we also cannot currently say for certain that it is "about 10,000 types."
4. Why a Smell Fades Away When You Stay in the Same Room
The strong scent you feel when you first enter a bakery often becomes unnoticeable after a while. Many people have had this experience. The phenomenon where smelling something continuously makes you feel it less intensely over time is called adaptation [1].
Sources state that it becomes harder to feel the smell within a few minutes. However, the time it takes may differ depending on the odor substance. Adaptation is not the nose being "broken." It is a change in how you feel when a smell continues.
5. How the Nose's Smell Cells Are Replaced Every Few Months
Olfactory cells are replaced. Old ones disappear every day, and they are renewed approximately every 1 to 3 months [5]. The cells deep in the nose are always working while being renewed, like a construction site.
A study by the Department of Otolaryngology at the University of Tokyo showed that in mice, newborn olfactory cells needed odor input between 7 and 14 days after birth to mature. Without input, the immature olfactory cells were not incorporated into the circuit and disappeared [5]. This means new olfactory cells do not work immediately after being born; they become fully functional by receiving smells. However, this was an experiment with mice and cannot be directly applied to humans [5].
6. Why Pinching Your Nose Changes the Taste of Candy
The path for sensing smell is not only when breathing in through the nose. When you put food in your mouth, odor molecules travel from the mouth through the throat to the back of the nose. This path is also called the retronasal route [6].
The flavor of food is greatly influenced by smells that go from the mouth to the nose. According to Yale University news, the sweetness of mint candy can be felt even when the nose is blocked. However, the flavor is not felt until the breath returns to the nose [6]. Taste and smell combine to create flavor.
7. An Experiment: Test Smell Adaptation With Your Own Nose
You can try this today. Choose something with a clear smell, like an orange peel or mint candy in a bag. Do not put it in your mouth. Hold it slightly away from your nose and smell it once. Then, smell the same odor continuously for about 30 seconds. Did the intensity of the smell change compared to the first time? Step away, breathe fresh air, and try smelling it again.
If you want to know more, the question "how many types of smells can we distinguish?" still has no answer. Reading the original sources where the numbers were criticized is a clue to understanding the history.