Microolfactoryfilter

September 16, 2006 8:32 AM

What's the smallest thing that can be smelled by a human?

Where the human can identify (accurately and reliably) what it is, such as, "Oh, I'm smelling a such-and-such!"

posted by Eiwalker to science & nature (21 comments total)

A molecule that triggers an olfactory receptor. Molecules are pretty small.
posted by Blazecock Pileon at 8:34 AM on September 16, 2006

OK, but what's the LARGEST thing...?
posted by Aquaman at 8:36 AM on September 16, 2006

Blazecock Pileon: So the question could be: "How many molecules does it take to trigger a conscious perception of smell?", right?
posted by bru at 8:42 AM on September 16, 2006

ethyl mercaptan
posted by caddis at 8:54 AM on September 16, 2006

Oh, we're going for smallest molecule? Then I submit
hydrogen sulfide

If you're asking what's the smallest amount of material we can smell, then I would think it would depend not only on our molecule-number sensititivity to the compound (and I would think this would be different for different compounds) but also on the ability of the substance in question to vaporize.

Have you looked here?
posted by solotoro at 9:00 AM on September 16, 2006

So the question could be: "How many molecules does it take to trigger a conscious perception of smell?", right?

Not sure. It could be asked how many receptors need to be triggered in order to send a signal, since the trigger is what registers the perception in the brain, not the odor molecules themselves. It could well be said that time is another important factor in smell perception.
posted by Blazecock Pileon at 9:03 AM on September 16, 2006

A Connisseur's (sic) Guide.
Should be Connoisseur, but a worthwhile read, especially for the comments.
Has anyone else experienced the olfactory "illusion" of smelling skunk when the first traces of brewing coffee hit your nose? It may be that small amounts of mercaptans in the coffee are the first to register. Weird that a miniscule whiff of something that smells so good can smell bad.
posted by weapons-grade pandemonium at 10:06 AM on September 16, 2006

One of my favorite books is The Emperor of Scent: A Story of Perfume, Obsession, and the Last Mystery of the Senses by Luca Turin. It is largely about this.
Here's more from here if you're interested.

...auntbunny, who smells like white shoulders and lawn clippings today.
posted by auntbunny at 11:05 AM on September 16, 2006

It will vary with the substance in question and the individual. Try googling "odor detection threshold" (odour for the UK/Australian etc. scientists). It is a value that is always expressed as a concentration (generally ppm) and averages are calculated for purposes from perfumery to determining safety guidelines for chemicals.
posted by nanojath at 2:48 PM on September 16, 2006

Nice responses. Some good candidates, and also the good point that taking a big long whiff makes it easier to smell something than taking a quick little whiff. Similarly, how far away you are from the thing is relevant. I take it that if it were in your nose, that would be ideal. But if the smallest thing we can smell is a raunchy molecule, this rules out smelling a single atom, even if it were in one's nose. But if the smallest thing we can smell is a molecule, how close would we have to be in ordinary circumstances, such as in a 10 x 10 x 10 bedroom, or outdoors in a park? A foot, a meter? Apologies if this information is contained somewhere in one of the previously mentioned links!
posted by Eiwalker at 4:50 PM on September 16, 2006

I've suddenly taken an interest in Aquaman's question, "OK, but what's the LARGEST thing...?"

We can smell single molecules, and compounds, but not, I take it, entire countries or houses. What is the approximate limit, anyway? That is, what's approximately the biggest thing we can smell (and identify) with a single big long whiff?
posted by Eiwalker at 5:09 PM on September 16, 2006

Proximity matters only in that you can smell something when the molecule collides with a receptor in your nose. At this point you're talking diffusion in the room. The longer it's had time to diffuse or if there is more active air currents than mostly still air, the sooner you'll smell it.
posted by plinth at 5:09 PM on September 16, 2006

What is the largest thing?

Have you ever even gotten close to Gary, Indiana?
posted by caddis at 7:19 PM on September 16, 2006

I smell a misapprehension in Eiwalker's second-latest question.

But if the smallest thing we can smell is a molecule, how close would we have to be in ordinary circumstances, such as in a 10 x 10 x 10 bedroom, or outdoors in a park? A foot, a meter?

Smell isn't like sight or hearing, which act at a distance via assorted kinds of wave; smell involves physical contact between molecules originating in the smelly object and receptors in your nose. You can't smell a single molecule at all until it physically docks with a receptor inside your olfactory canal.

If you do perceive a single molecule as a smell, that gives you no information at all about how close you are to the object that molecule was originally part of, except that it was more likely to be far than near. You'll get literally billions of scent molecules up your nose from anything even remotely near you.
posted by flabdablet at 10:47 PM on September 16, 2006

This also means that when you can smell a fart you have molecules in your nose that have been in someone else's rear.

The more you know.
posted by null terminated at 12:08 AM on September 17, 2006

It's OK. Even if you can't smell a fart you have molecules circulating in your bloodstream that have been in someone else's rear.

It's a chemistry thing, it's a sharing thing, it's a beautiful thing.
posted by flabdablet at 6:02 AM on September 17, 2006

Possibly interesting side note:
A science exhibit I saw once said that the shape of the molecule is more important than the makeup of the molecule itself. So you can arrange a molecule made of other stuff that is shaped and sized like a strawberry molecule and it will smell like strawberries even if there are no actual strawberry molecules there.
posted by rmless at 1:17 PM on September 17, 2006

auntbunny had left a link for the Vibration Theory of Olfaction from Wikipedia: http://en.wikipedia.org/wiki/Vibration_theory_of_olfaction

I guess my question of how close you have to be to smell something only makes sense on that model.

Thanks again for the apt comments!
posted by Eiwalker at 2:27 PM on September 17, 2006

Well, no, actually it doesn't. The vibrational theory is about additional requirements for molecules that are already docked with receptors, and doesn't imply smell action at a distance.

Philip Callahan's interesting infrared model for insect pheromone detection doesn't apply to us, because we don't have anything like a working insect antenna; our only way to detect the molecules we perceive as scents is by physically sucking them up.
posted by flabdablet at 12:27 AM on September 18, 2006

I stand corrected again! Thanks again!
posted by Eiwalker at 5:11 AM on September 20, 2006

Oh, we're going for smallest molecule? Then I submit
hydrogen sulfide

Hydrogen fluoride would be notably smaller, and apparently has a "strong, irritating odor." But HF is nasty stuff and I have no intention of verifying that personally.
posted by DevilsAdvocate at 5:08 PM on September 22, 2006