Humidity enhances smell, threshold concentrations of odours are lower in humid air. See Eur Arch Otorhinolaryngol. 2008 Mar;265(3):299-302. Epub 2007 Sep 25.

There is also anecdotal evidence that tracker dogs perform better in humid conditions.

Ambient air always contains a proportion of humidity so our nose (and globally detection of mammals) is based on detection in humid conditions. For example, that's the reason why, in CG-MS-O analysis, the air is humidified to keep epithelium in humid conditions and to avoid dryness of sensorial cells.

Does anyone have a reference to research tracking the celular/molecular basis of this phenomenon?

The olfactory mucosa, as most of mucosae, needs to be damp to perform real well. This might sound strange, considering the fact that most of the component of olfactive active products are not soluble in water (essential oil are a good example).

But actually the mucus layer protecting the olfactory mucosa from aggressive environment and dryness is in fact a medium containing some very special proteins. those molecules are carriers. They are named Olfactory-Binding Proteins. And they carry the small olfactive-active molecules through the mucus,and "deliver" them to their respective recognition sites : proteins on the wall of olfactory neurons. So, humidity is important for a good active olfactory mucosa in different ways :

Proteins, like the recognition sites on the olfactive neurons, and the olfactory binding proteins, are, like most proteins, denaturated by change in humidity. Meaning they don't work as good.

The movements of the olfactory binding proteins are impeached by dryness of the mucosa

The fact that the mucosa is dry also slows down the cleaning process : there is a flow of mucus washing your olfactory mucosa all the time. Sometimes is more evident : peeling onions makes you cry, meaning that your lacrimatory glans emit a lot of tears to clean your eyes from aggressive molecules. And you will also notice that when the onions are very strong, your nose will also cry! Same with dust or allergens.

Please, excuse me for my poor english language. Hope this might have help you.

Dr Moisseeff

PS : remember to wash your nose everyday, like you do with yur teeth. Use physiological serum, the best is the dilution used to wash babies noses.

This is specially important in our modern days apartment as they are well heated and extremely dry. Remember this if you are a professional in this area of research. Take good care.

Dr. Moisseeff, thank you for your input on this issue. Based on your comments, I assume that the proper function of the olfactory mucosa of vertebrates would depend on external water supply (even though it provides mucus) and would not perform equally well at "all" environmental conditions that can be found in nature. Would this be right? Then, another question would be whether this is similar in insect sensila, which are well isolated from the external environment?

Sorry Christopher for my "invasion"!

Thanks,

The increase in smelling ability is also related to what happens in the environment, not just what happens in the nose. In short, it's hypothesized that water molecules fit with scent molecules for binding to soil particles. More water in the air means more water is displacing molecules that are smelled and more smell is released.

Vander Wall, S. B. (2003). How rodents smell buried seeds: a model based on the behavior of pesticides in soil. Journal of Mammalogy, 84(3), 1089-1099.

Dr. Downs - we observed this displacement of scent VOCs from soil by water and subsequent enhancement of analytical extractions (sample recoveries) from soil headspace. This 'flooding' effect is used to improve method detection limits for odorous VOCs from soil.

Wright, D., et al. 2005. Multidimensional GC-MS-olfactometry for identification and prioritization of malodors from confined animal feeding operations. Journal of Agricultural and Food Chemistry, (22), 8663-8672.

as any signals, there is a source and a receptor.

The olfactory mucosa is the receptor.

Concerning the source, as it is always the case, the environment is also acting its part.

But the rôle of humidity in the atmosphere is much difficult to modelize :

considering emissions from vegetal entities, some close their stomates when it is damp, thus reducing the emission, and some open their stomates. But even considering evaporation from a source, there are antagonist effects. It depends mainly on the particularity of the molecules and their relationship with the support : electric fields, vapor tension, surface tension, separation coefficients, etc.

For the human beings, i have been told more than often : "it is going to rain, the smell from the factory (biscuit, jam, chocolate, yeast....) is stronger.

I don't know for the insects, the receptor localized on the antennae present also a liquid, the hemolymph, very similar in their function, it seems, to the mucus found while studying mammals (look at the dog, you know that when it is healthy it's nose is wet and shiny). It is inside the channels inside the sensillae...

But there is also receptors specialized in the detection of humidity and it is quite possible that there are some interactions at the signal treatment between the two kind of receptors (also think about the captors dedicated to the detection of CO2).

Also, there is an older, but fascinating, chapter on the influence of environmental factors, including humidity, on the release of volatiles from point sources. Although the focus is on vertebrate pheromones (because that was the theme of the conference), I believe that the material is applicable to numerous sources. It is: Regnier, FE & Goodwin, M (1977) On the chemical and environmental modulation of pheromone release from vertebrate scent marks. In D Muller-Schwarze & MM Mozell (eds) Chemical Signals in Vertebrates. New York: Plenum Press.

I forgot to include the pages for Regnier & Goodwin. They are 115-133.

@ Jerry: Thank you very much for the reference. This was what I've been looking for. Regarding anecdotal evidence with canines: Here is one indicating a negativ correlation between canine olfactory sensing of tortoise and humidity (Mary E. Cablk ,John C. Sagebiel, Jill S. Heaton and Cindee Valentin; "Olfaction-based Detection Distance: A Quantitative Analysis of How Far Away Dogs Recognize Tortoise Odor and Follow It to Source"; Sensors 2008, 8, 2208-2222.

The canine detection communities (users) base most of their work on anecdotal evidence with good success. However, this is scientifically frustrating since one can plainly see the sucesses of canine detection without understanding how the dogs do it.

@ Michael: Thanks for delivering the reasons behind the influence of humidity on olfaction.

@ Marcelo: Your invasion is very much welcome! :-)

@Cynthia, Jasek, Micheal and Charles: There appears to be quit a lot of evidence that humidity is influencing the odor source. The Geneva International Centre for Humanitarian Demining did some work with regard to explosive vapors above buried mines. They claim, that incresing the humidity from 60 to 90 % above dry soil increased the explosive vapor concentration by an faktor of 10. (GICHD; Remote explosive scent tracing - REST", Nov. 2011, Kai-Uwe Goss, page 54). More detailed information can be found in a GICHD-publication in 2003.

It was so nice to work with such a nice team. I learned a lot. Thank you to all.

I believe this is one of the very important problems in the dogs nose.

I tried this problem many years ago but could not get the good results because of the structure of the dogs nose.

If you are interested in, please check the following paper:

Olfactory Receptor Cell Responses of Dog and Box Turtle to

Aliphaticn-Acetates and Aliphaticn-FattyAcids

Keiichi Tonosaki and D Ticker BEHAVIORAL AND NEURALBIOLOGY 35，187-199（1982）

Charles, any chance of getting the pdf of :

Regnier, FE & Goodwin, M (1977) On the chemical and environmental modulation of pheromone release from vertebrate scent marks. In D Muller-Schwarze & MM Mozell (eds) Chemical Signals in Vertebrates. New York: Plenum Press

?

I would really appreciate that (I did not find it on the internet).

Thanks in advance!

Marcelo, we've started research for this publication on our net. Who knows?...

M,arcelo,

Please ask to Dr.Wysocki. he might be remembered the name "Don Tucker and Keiichi Tonosaki".

Marcelo, this came back as an answer. Is it any help to you?

Http://www.springer.com/life+sciences/ecology/book/978-1-4614-5926-2

also this :

http://link.springer.com/chapter/10.1007%2F978-1-4684-2364-8_8

Thank you very much Michael!

Marcelo, please contact me direct :

[email protected]

Dr. TonosakI,

I do remember. Indeed, Don Tucker was on my dissertation committee at Florida State. I completed my degree in 1978 and came to Monell to do a 2 year post-doc. I guess I haven't yet finished.

Marcelo, I should be able to scan the pheromone chapter and form a pdf. When I do I will attach it to another msg.

Chuck

Okay. It should be attached.

Charles,

thank you so much for your kind attention! I already downloaded your attached copy and look forward to read it asap.

All the best,

Marcelo

Dr.Wysocki,

I am happy to hear from you. If you interested in our dogs paper, I will send its PDF to you.

Yes, please do. My email is [email protected].

I conduct research on canine detection capability and train and deploy detection canines for law enforcement. I've seen several great references in the discussion thread regarding humidity and we also point to the pesticide research as well looking to the role of evapo- and transpiration in VOC availability for detection. Compiling these studies we conclude that water molecules compete with odor molecules for binding sites, and with higher rH comes more competition for those sites, thus increased "bumping" of odorants off of the binding sites. A molecule cannot simultaneously be bound to a surface and detected! It must be in the air column to be inhaled and then ultimately detected or smelled. So when there is greater rH, there are more water molecules in the air column and thus more molecules bumping odor molecules off of binding sites, which means more odor molecules in the air. This is why we see canines exhibit increased change in behavior related to their target odors along actively photosynthesizing vegetation, along streams and where there is water present. Some canine handlers will spray asphalt with water, wait a few minutes, and then the canines can sometimes appear to re-establish the trail. This has NOT been scientifically studied.

In terms of observational data, we find that this (increased VOC availability along increasing rH gradients) is particularly evident where there is a relatively low background rH, say less than 30%-40%. At the same time it should be remembered that environmental variables correlate, such as temperature and rH, and temperature plays a role in vapor "production" and availability. Just because there exists an rH gradient doesn't mean that sufficient odor concentration exists that is above the canine's detection, or trained, threshold for that target odor.

Our data over six years for canines deployed in the Mojave desert show that canines can be trained to detect targets in very low rH, like single digits, as well as when it's raining. We deploy canines in Death Valley, the Great Basin and Mojave deserts with detection success. These same canines deploy in northern CA and OR also with success. However, when I bring canines over here from the more humid environments, those canines require additional training to adjust their recognized signature, their trained thresholds, or a combination of both before they are generally successful.

Hello Mary,

thank you very much for your response. Do you have any documentation describing the results you had with canines trained at very low rH versus the dogs trained at high rH?

Hello Mary, can I also ask was there any component of acclimitisation in the dogs, or is the extra training necessary for them to function in the changed (drier) environment? Thanks

I agree with previous comments. Humidity is very important for olfaction. The olfactory sensory neurons are situated in the olfactory epithelium.  The sensory olfactory receptors are positioned in the olfactory cilia that are immersed in the layer of olfactory mucus.  Thus, odoriferous molecules need to travel through the layer of mucus to reach the olfactory receptors and to evoke the olfactory signals. Therefore, if the mucus layer is too thick (staffed nose) we cannot smell. Also, if the mucus layer is to thin, the structural position of cilia is compromised and we cannot properly smell. In human, the olfactory epithelium is protected by turbinates. It is interesting that primates are divided into Strepsirrhini, species that have a curly “wet nose” rhinarium, and Haplorhini, those that have a dry “simple nose” structure. Strepsirrhini, generally have smaller brains relative to body size, larger olfactory bulbs indicating more reliance on smell.  In haplorrhine primates, the loss of the wet rhinarium and decreased number of turbinates are associated with their reduced reliance on smell. Therefore, the dry noses of humans place them under the Haplorhini conditions.[ Williams, B. A., Kay, R. F., & Kirk, E. C. (2010). New perspectives on anthropoid origins. Proceedings of the National Academy of Sciences, 107(11), 4797-4804.] I think that variations in humidity may affect the olfactory mucus layer and olfaction.