Five facts about smell 20x20 slides

A video of me giving this talk can be seen here: https://skillsmatter.com/skillscasts/5050-five-facts-about-smell.

I went super fast because it was a 20x20 format meaning the slides auto progress every 20 seconds. Here are my speaker notes and also some links for more information and sources.

[Slide 1] Hello, my name is Alice, I work at GDS. Today I'm going to tell you five facts about smell, I'm doing this because we don't know very much about how the nose works and I find this fascinating since smell plays such a big part in our lives.

[Slide 2] Fact 1. Humans have three chemical senses: Smell, taste and trigeminal. Smell detects light volitile compounds, taste detects a couple of heavier compounds and your trigeminal receptors react to irritants. These are things like minty coolness, chilli, pepper etc. Your trigeminal receptors are found at the back of your tongue and down your throat.

[Slide 3] Fact 2. 1.5% of people have no sense of smell [1]. This condition is known as anosmia. In 1987 1.5million people were asked to smell six different compounds and report what they could smell. 1.2% of them reported not being able to smell anything. Some people can't smell certain things, this is known as parnosmia.

[Slide 4] I am one of the 1.5%. I can't smell anything and have never been able to. Until about seven I thought smelling things was a learned ability and so I didn’t tell anyone I didn’t know how to smell as I was embarrassed. It was only when I finally asked someone how to smell things that I realised that this was not a learnt ability but a sense. Even then I didn't tell people generally that I couldn't smell for quite a while.

[Slide 5] Some smells are smellier than others and nobody knows why. There are some compounds that humans can detect at vanishingly low concentrations and nobody knows why this is. The ability to smell these things presents no evolutionary advantage and does not seem to be related obviously to how the nose works.

[Slide 6] SUB-LIST TIME: The top five smelly smells are grapefruit juice, cork odour, butter, pepperoni and strawberry [3]. This is kind of weird for me, until I wrote this talk I had absolutely no idea that butter had a smell. Or cork odour. This does explain why people like grapefruits so much, because to me they are just extraordinarily bitter.

[Slide 7] But how smelly is grapefruit odour? Time for some science. Citral is a compound that is kind of averagely smelly. It is found in Lemons. Citral has a detection threshold of 32 parts per billion [2]. The less sexily named menthene-8-thiol is found in grapefruits and it's detection threshold is 0.00001ppb[3], 3.2 million times lower than citral.

[Slide 8] There are seven primary odours. These odour classifications are used in perfumery and not rooted strictly in science. They are kind of analogous to primary colours. Chemicals in these primary categories often share similarities in chemical shape.

[Slide 9] SUB-LIST TIME: Camphoraceous, floral, peppemity (not to be confused with the trigeminal sense of mint- a coolness) ethereal, pungent, prutrid, and musky. [5]

[Slide 10] Fact 5, this one really blows my mind: we don't really know how smell works. In 1914 Alexander Graham Bell wrote “if you are ambitious to find a new science, measure a smell”. Though we’ve made leaps and bounds in the 100 years since, there are still some pretty important things we haven’t yet figured out about smell

[Slide 11] In 1914 Alexander Graham Bell wrote “if you are ambitious to find a new science, measure a smell”. Though we’ve made leaps and bounds in the 100 years since, there are still some pretty important things we haven’t yet figured out about smell

[Slide 12] You have 347 different types of smell receptor. We know this because they are coded for in the human genome and so this was one of the things we discovered when we decoded it. Some people will have fewer receptors caused by mutations this is one cause of parnosmia. One type of smell receptor will detect several molecules and react with differing intensities to them.

[Slide 13] Your olfactory receptors seem to work in a combinatorial way. so instead of a 1:1 mapping of a smell to a receptor (meaning there would be 347 smells) its more like each receptor codes for a letter in the alphabet and in fact there could be up to 347x347 smells. In actual fact nobody seems to know how many smells the human nose can detect, guesses range from ~1,000 to 100,000

[Slide 14] The thing we haven’t worked out is, what is actually happening at the reaction site, when the compound triggers the olfactory receptor. what is happening? There are two competing theories and neither have been disproven.

[Slide 15] Shape theory is also called “lock and key” theory and states that molecules fit into receptors like lock and key. As an aside, When we talk about shape in chemistry we mean more than just three dimensional shape, we mean vibration levels, composition, etc. Shape theory makes sense intuitively and has historically been the preferred theory.

[Slide 16] You would expect to be able to isolate parts of a molecule and fit it to smell different, but it doesn’t. Nobody has successfully managed to predict what a new molecule will smell like based on its shape. Some compounds with the same shape smell very different, and some compounds with different shapes smell very similar.

[Slide 17] More science: Limonene is found in turpentine, it has two forms that are mirror images of one another, just like your hands are the same ‘shape’ but are in fact mirrors of one another too. The left hand molecule of limonene smells of lemons, but the right hand molecule smells like turpentine. There are a very small number of compounds that have these smell differences. Most ‘hands’ smell the same.

[Slide 18] The second theory is “vibration theory” and it states that it is not the shape but the vibration pattern of the molecule that determines its odour. This would allow molecules of the same shape to smell different, but there are molecules that seem to run counter to this theory too. And neither of these theories offer any explanation as to why grapefruits are smellier than lemons.

[Slide 19] Shape theory was first suggested in 1949 and vibration theory was proposed in 1938 and people have been scientifically poking at them ever since but here we are some eighty years later still scratching our heads.

Some references:
- [1]Gilbert and Wysocki 1987. Gilbert, A.N. & Wysocki, C.J. (1987) The Smell Survey results. National Geographic172:514-525.
- [2]Ohloff, G. 1990. Scent and Fragrances. Springer-Cerlag, Berlin.
- [3] Demole E., P Enggist and G. Ohloff. (1982) Helv. Chem. Acta, 65, 1785-1794
- [4] Buttery, R. G., R. Teranishi, L. C. Ling and J. G. Turnbaugh, J. (1990) Agric. Food Chem., 38, 336-340
- [5] Amoore, J. E. (1977). Specific anosmia and the concept of primary odours. Chme Senses Flav. 2. 267-281