Smelling the Past: Understanding Odour Molecules

[Graeme M]

A quick question about smells. I understand that smells are the result of so-called odour molecules given off by physical objects/processes binding to receptors in our noses. A quick Google tells me these molecules are light and "volatile" (quick to evaporate).

That leaves me uncertain. For example, how can it be that someone walking into a room that has been used for cooking some time ago can smell the odour of cooking? More generally, how can any animal smell odours from some time ago?

After all, an object or process that has been present but is no longer in place is no longer emitting these molecules. If the molecules are light (moved by air currents) or quickly evaporating, exactly what is later detected? Surely only a finite number of molecules are emitted and they are unlikely to remain simply floating around for any length of time?

I assume in most cases, some residue is involved, eg fat splatters from the cooking, but even then would enough molecules continue to be released by that residue and be available to any inquisitive nose? Is there some kind of "decay time" for the availability of smells from long past events?

 

[phinds]

Is there some kind of "decay time" for the availability of smells from long past events?

Pretty clearly there HAS to be since bloodhounds, to take an extreme example, can smell things that are days old and that's outdoors where wind is likely to disperse any airborne molecules. I have no idea what the chemistry/physics of it is though.

 

[Bystander]

(quick to evaporate).

"Evaporation" is not the same as "disintegration;" there are neurotoxic odors like hydrogen sulfide/selenide that fatigue the sense of smell and are extremely hazardous while being detectable, having threshold values/concentrations, far below more lethal gases, hydrogen cyanide, almond smell, that do not fatigue/tire the sense of smell; threshold lethal concentration is ppt for H₂S and ppm for the cyanide, or thereabouts, yet "olfactory fatigue" renders/pushes the hazard rating for sulfide higher than for cyanide. ppb detection of sulfide quickly "fatigues" to "I don't smell anything," when exposed to lethal concentrations, ppt, while tens ppm for cyanide are "persistent."

 

[russ_watters]

"Evaporation" is not the same as "disintegration;"...

Indeed, if not for evaporation, you wouldn't smell many chemicals at all!

[I believe smelling solids suspended solids and liquids works the same, but would not persist as long as vapors.]

 

[jim mcnamara]

If you refinish a wooden cabinet with drawers - previously in a house with smokers - you have to completely coat all of the interior with a barrier (e.g., shellac) because the residual smell will make clothing placed in a drawer smell "smokey". Drawers included. All surfaces interior.

There are even products made that specifically remove the stinky tar, e.g., Nilodor, because some pieces have been smoked on for years and merely covering may not work. The stink takes multiple years to dissipate otherwise.

It is a yellowish "tar".

 

[Graeme M]

Thanks for the comments. I was more getting at what causes smells to remain available over time. In Jim's example above, the surfaces have some kind of residual "smell". So that's exactly the kind of case I am thinking of. Originally, the smell was certain molecules in the smoke that can be inhaled. But later, say months after the house has been vacated, what is it that then "smells"? Presumably those molecules are no longer freely drifting around? Do some deposited materials (eg the tar) continue to emit those molecules?

What I am getting at is that we can't smell anything if there are no molecules of the right kind in the atmosphere. So at some later time after an event, those molecules have to continue to be available to be inhaled. I assume that means that there has to be some actual material deposited that continues to evaporate off the odour molecules. Or is it the odour molecules themselves that remain? Do bloodhounds detect molecules being evaporated off some kind of deposited material, or do they detect the original molecules that remain for some time on the surface?

 

[BillTre]

Yes there have to be some pile of molecules around that are slowly released if you can smell them.

That probably happens best in more complex environments, with a variety of micro-environments.
Evaporative extremes in a complex environment might be found at places like:

• quickly evaporating areas (such as dark impervious tile in the sun)
• other areas where it is cooler, shaded, perhaps more damp, and isolated by breezes (such as at the base of a clump of grass).

Different areas of a complex environment will be able to preserve some of the molecules for later release.
Isolation from much air exchange will prolong how long a molecule stash can last.

If you watch a dog as it is sniffing around its environment, you'll notice it will stick its nose in places like the clump of grass.
This is sampling the local air at the base of the clump that is not ungoing rapid exchange due the wind.
It might also also exhale into that space and then inhale (evaporating more molecules in the warmer exhaled air), and then sniff in.