What is smell?

“Smell” is molecular particles interacting with receptors in your nose and mucous membranes.
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@syz So then how could we measure it?

I’m not sure I understand what you’re asking. Are you talking about measuring the strength of a smell?

@syz Yeah – like how we can measure how loud a concert is, could we measure if a rose smells stronger than a lilac?

Smell is not waves in the same sense as sight (electromagnetic waves) or hearing (sound waves). Smell is the diffusion of various molecules in air that makes its way into receptors in the nose. Each receptor is a protein complex that binds one or more specific molecules, which differ from receptor to receptor. The molecules in question are mixed with incoming air molecules in the vicinity of your nose, individual molecules in the gas phase or aerosols & suspensions of very small clumps of molecules (such as smoke). There are hundreds of unique olfactory receptors in the noses.

Here’s the exact mechanism, courtesy of the ever-dependable Wikipedia:

Rather than binding specific ligands like most receptors, olfactory receptors display affinity for a range of odor molecules, and conversely a single odorant molecule may bind to a number of olfactory receptors with varying affinities.Once the odorant has bound to the odor receptor, the receptor undergoes structural changes and it binds and activates the olfactory-type G protein on the inside of the olfactory receptor neuron. The G protein (Golf and/or Gs) in turn activates the lyase – adenylate cyclase – which converts ATP into cyclic AMP (cAMP). The cAMP opens cyclic nucleotide-gated ion channels which allow calcium and sodium ions to enter into the cell, depolarizing the olfactory receptor neuron and beginning an action potential which carries the information to the brain.

I would add that the gypsy moth—to name one creature with an exquisite sense of smell—has been shown to be able to detect individual pheromone molecules with its feathery antennae, allowing it to fly up the concentration gradient to mate with its source.

@TheLadyEve You can measure the volatility of a substance, which would affect how much “smell” we perceive it to have. When comparing two items (as in your example), I would probably compare volatility.

In general, I would guess that you could get a true measure of “strength” by measuring ppm (parts per million), but there are too many variables on the end of the perceiver (the person doing the smelling) to be able to correlate it to an absolute number (smokers have a reduced sense of smell, for example, and those with allergies, etc).

This is a very interesting question. In the Kaylie Anthony murder case, the lawyers are saying that they tested the air in her car trunk and can prove that choroform and dead body was in there. So on this same topic, how can they test the air for certain smells? How can they prove it?

@TheLadyEve You can measure it in the form of saturation. AKA how many particles in a given volume of air.

@Skaggfacemutt …how can they test the air for certain smells?
A certain smell means a certain molecule or mixture of molecules. Think molecular!—i.e. 20th century lol. Using mass spectrometry*, gas chromatography**, or any of a variety of other techniques of analytic chemistry, by which low concentrations of trace chemicals in the air can be detected and (hopefully) identified. This is as close to artificial smelling

In fact there are so many applications – from environmental testing to biological research to “artificial nose” development – you’ll probably want to google search it yourself for specifics.

* In a mass spec unknown molecules are ionized (& possibly fragmented in the process). The instrument precisely measures masses of these ionized molecules & fragments.
**In gas chromatography unknown gas molecules are separated according to their interactions with a series of solvents, precisely measuring a mix of components.

It’s a sense that I only rarely am able to experience.

I read this interesting article on this very subject just the other day

Thanks, @gasman . You’re so smart! :)

Smell is a sensation created by our brain. Olfactory receptors expressed in the cell membranes of olfactory receptor neurons are responsible for the detection of odor molecules.

http://en.wikipedia.org/wiki/Olfactory_receptor