The sense of smell in numbers
About the sense of smell in numbers
Humans can discriminate between more than one trillion smells (1).
There are an estimated 21,000 (2) genes in the human genome. Of these, approximately 900 are Odour Receptor genes (OR genes), our largest set of genes and over 3% of the genome; this was unexpected. Around half the OR genes are thought to be non-functioning, with around 400 believed active. This large number of genes helps explain why we are so good at odour differentiation. As a comparison, colour vision depends on just 3 types of cone cells.
More recently, the OR gene has been found in other places in our body. It’s in human sperm and is thought to help with navigation towards the egg (3). The OR gene is in our skin, our liver, our heart. In fact it seems it could be all over our body (4). It seems the OR gene is mis-named, it’s really a chemical receptor gene. That the OR genes are our largest set of genes makes more sense now – it’s pretty busy.
One nanometer is the approximate average size of an aroma molecule. Aroma molecules are tiny by comparison to say, a grain of salt, a whopping 100,000nm and airborne pollen at around 10,000nm to 100,000nm.
Some aroma molecules are super strong: Thiomenthone, naturally found in blackcurrant and cat’s urine can be smelled at around 0.05 parts per billion, that’s the equivalent of a tiny drop in an Olympic sized swimming pool!
We take 20,000 breaths per day, give or take a few thousand depending on age, fitness and activity. That’s 20,000 opportunities to sample our environment, the air. This includes the air in our mouth, the aromas of the food and drink we consume. It includes the air we breathe with the smells of our home and loved ones, the smells outdoors which help orientate us as to the time of day, the season and the weather, and of course it includes the smell of perfume.
More than 800 different aroma-molecules are found in the smell of coffee (5) and as we get better at analysis, we find more. Compared to coffee, perfume might be considered simple! Some may contain a mere 60 aroma molecules, although perfumes which use high levels of natural essential will contain many more.
One last thing, when we smell we can ‘feel’ the size of a molecule, relative to others. We instinctively know whether the molecule is big or small. We also have a sense of complexity on a molecular level (6). Our noses are amazing!
1. https://doi.org/10.1126/science.1249168
2. https://www.genome.gov/About-Genomics/Introduction-to-Genomics#one
3. http://vosshall.rockefeller.edu/reprints/VosshallCB04.pdf