Friday, August 28, 2020

Headline Bingo

Once in a while, I end up with a whole bunch of headlines and not much inspiration. Here’s to clearing out the queue – 

Oldest material on Earth discovered

Jan 2020, BBC News


Dust grains within a space rock that crashed on Earth are found to be really old -- older than the Earth itself in fact. But I only came here to post this:


In order to analyze the rock, it has to be prepared... "Once all the pieces are segregated, it's a kind of paste, and it has a pungent characteristic – it smells like rotten peanut butter."


Asking yourself what rotten peanut butter smells like? Go find a tree-of-heaven, snap off a leaf, and smell it. It has to be a slightly older tree, not one just shooting out of the ground, or it will smell too mild and too much like not-rotten peanut butter.



Watch that smell! Scents can regulate fat storage

Apr 2020,


Honestly I can't even follow this, but it is a story that features the indispensable C. Elegans.


Ayse Sena Mutlu et al, Olfactory specificity regulates lipid metabolism through neuroendocrine signaling in Caenorhabditis elegans, Nature Communications (2020). DOI: 10.1038/s41467-020-15296-8



New sleep method strengthens brain's ability to retain memories

Mar 2020,


"By triggering consolidation processes in only one side of the brain during sleep, we were able to compare the activity between the hemispheres and isolate the specific activity that corresponds to memory reactivation," -Prof. Yuval Nir of TAU's Sackler Faculty of Medicine and Sagol School of Neuroscience


While exposed to the scent of a rose, research participants were asked to remember the location of words presented on either the left or right side of a computer screen. Participants were then tested on their memory of the word locations, then proceeded to nap at the lab. As the participants were napping, the scent of roses was administered again, but this time to only one nostril.


With this "one-sided" odor delivery, the researchers were able to reactivate and boost specific memories that were stored in a specific brain hemisphere.


"Our findings emphasize that the memory consolidation process can be amplified by external cues such as scents," she concludes. "By using the special organization of the olfactory pathways, memories can be manipulated in a local manner on one side of the brain. Our finding demonstrates that memory consolidation likely involves a nocturnal 'dialogue' between the hippocampus and specific regions in the cerebral cortex." -Ella Bar, Ph.D. student at TAU and the Weizmann Institute of Science


Local Targeted Memory Reactivation in Human Sleep, Current Biology (2020). DOI: 10.1016/j.cub.2020.01.091



Study shows major gender differences in the human social brain

Mar 2020,


I bring this one up because it’s a general fact that women “smell better” than men, meaning they have a better sense of smell. Personally I think it’s because women tend to be better at communication and language, and smell has such a poor affinity for language. If women can use language better, then it will seem like they’re better at identifying and detecting smells, when really they’re just better at talking about something that most people never talk about in the first place.


The Data:

·      U.K. Biobank database, background data and MRI scans of the brains of 10,129 male and female participants.

·      36 parts of the brain involved in processing social cues and behavior.

·      Lifestyle factors such as how many people were in a household, whether a person was married, how much they enjoyed their relationships and degree of social support.


The Results:

·      Neuroanatomical associations in the amygdala that were predominant in socially stimulated women, but barely present in most of the males.

·      Differences in volume of the ventromedial prefrontal cortex of men who lived alone versus those who were socially stimulated—variations not generally seen in the brains of the women under study.


Hannah Kiesow et al. 10,000 social brains: Sex differentiation in human brain anatomy, Science Advances (2020). DOI: 10.1126/sciadv.aaz1170



Smelling a rat - How rodents sniff out fake beggars

Mar 2020,


·      Lead author is Karin Schneeberger of the University of Potsdam in Germany.

·      They took rats that were either hungry from fasting overnight, or well-fed, and placed them in a room separate from the "focal" rat whose generosity they wanted to test.

·      Air from the rooms of the hungry or well-fed rats was pumped into the chamber of the focal rat.

·      They found that the focal rats were much quicker to provide help -- by pulling a food tray within reaching distance of another rat -- when the air was pumped from a hungry rat's room.

·      The authors then analyzed the air around the rats and found seven different organic compounds that differed significantly in their abundance between hungry and satiated rats.

·      These might result from recently ingested food sources, the metabolic processes involved in digestion, or possibly even a pheromone that indicates hunger.

·      Taken together, these signals form a "smell of hunger" for rats that serves as a reliable cue of need, said Schneeberger.


Schneeberger K, Röder G, Taborsky M (2020) The smell of hunger: Norway rats provision social partners based on odour cues of need. PLoS Biol 18(3): e3000628.



Sweat sensor detects stress levels; May find use in space exploration

Mar 2020, phys.og


It detects cortisol in sweat. Another similar device measures uric acid in the blood.


Rebeca M. Torrente-Rodríguez et al, Investigation of Cortisol Dynamics in Human Sweat Using a Graphene-Based Wireless mHealth System, Matter (2020). DOI: 10.1016/j.matt.2020.01.021



Scientists discover a new class of taste receptors

Apr 2020,


Our bodies, all bodies will be perpetually a source of wonder. Here we see how receptors meant for detecting light are used also to detect very small amounts of a bitter-tasting chemical: Much like rhodopsins turned on by very dim light, the chemically-activated opsins then initiated a molecular cascade that amplified the small signals. This enabled the flies to detect concentrations of the compound that would otherwise be insufficient to trigger a response in their sensory neurons.


Could you imagine if every pore on your body was an eyeball? Because it kind of already is.


Nicole Y. Leung et al, Functions of Opsins in Drosophila Taste, Current Biology (2020). DOI: 10.1016/j.cub.2020.01.068


Thursday, August 20, 2020

Neuroplasticity and Olfactory Existentialism


New study reveals how the brain organizes information about odors

Jul 2020,


A study by neurobiologists at Harvard Medical School now provides new insights into the mystery of scent. Reporting in Nature on July 1, the researchers describe for the first time how relationships between different odors are encoded in the olfactory cortex, the region of brain responsible for processing smell.


The sense of smell allows animals to identify the chemical nature of the world around them. Sensory neurons in the nose detect odor molecules and relay signals to the olfactory bulb, a structure in the forebrain where initial odor processing occurs. The olfactory bulb primarily transmits information to the piriform cortex, the main structure of the olfactory cortex, for more comprehensive processing. [But] Often, subtle chemical changes—a few carbon atoms here or oxygen atoms there—can lead to significant differences in smell perception.


They use a database of physical and chemical features of tons of molecules, probably from the Dragon dataset, which is a relatively new set of thousands of chemical identities based on molar mass, chemical formula, chirality, boiling point, etc.


Then they choose three different synthetic odors, each one made of a group of different molecules, and each group having either very diverse features, intermediate, or minimal diversity. The minimally-diverse "odor" group could be made of molecules differing by only one carbon atom, for example.


Another way to think about these chemical combinations -- some are grouped close together in chemospace and some are spread out, where chemospace is a map of all the features in the dataset. It's like facespace, made of thousands of faces and mapping all the features against each other; eventually you will find the "eignenface" right smack in the middle of the map. These are both examples of an n-dimensional information space, where n is the number of features listed in the dataset. If your understanding of dimensionality stops at the common physical world of three-, then you will need to stretch for this one.


Mice then get exposed to each of these "odors" while their neural activity is observed using multiphoton microscopy. They found that low-diversity molecule-groups, ones that were very similar to each other in their chemical characteristics, were associated with clustered neuron patterns. For the inverse, an odor made of very different chemicals would excite neurons all over the place (chemospace x neuronspace). They also found that in general, certain chemical features could be matched with recognizable neuron activity patterns.


This sounds as if there is some universal olfactory attributes in the chemical compendium.


BUT, they also found that if they repeatedly presented the mice with any two chemicals paired together, then the corresponding neural patterns would become more strongly correlated, and despite how different those two chemicals were.


"The brain can rearrange itself."


And again, and in other words, olfaction is confusing:


"Part of the reason why things like lemon and lime smell alike, he added, is likely because animals of the same species have similar genomes and therefore similarities in smell perception. But each individual has personalized perceptions as well.


"The plasticity of the cortex may help explain why smell is on one hand invariant between individuals, and yet customizable depending on our unique experiences."

-Sandeep Robert Datta, associate professor of neurobiology in the Blavatnik Institute at HMS


(He also says the phrase "virtual olfactory world" in reference to a future that sounds very exciting, although still very far away.)


There's another study here which looks pretty similar. They offer a reiteration of this new explanation for how smells are perceived, and it sounds like this -- we all share a similar smell-space due to our shared genome. This is manifest in the olfactory bulb, and in the way that it codes individual odor molecules based on their physical and chemical properties.


They ask "Can distant odours be grouped together if their context is learned to be the same?" And that answer, because of studies like these, is becoming clearer by the day -- yes.


In fact, this is the basis of how olfaction works, why it works, and what it's supposed to be doing for us. The common association between chemical properties and neural activity is just a given. It's in that second layer, and over the course of a lifetime of being exposed to different combinations of chemicals, that our brain generates the end results, olfactory perception, which by its nature must be subjective, and because of the transient, vacillating and boundless parameter space of our ontogenetic chemical exposome. (Chemosome?) They call this pairing of chemical milieu and existential moment an odor "contingency," and they say these contingencies are what rewrite the higher olfactive network space.


Random fact: "The mouse initiates the trial by touching the lickometer." Yes, the lickometer.


Both of these are true; Yes there is a static, predetermined program that senses chemicals in the air in mostly the same way for all people (excepting genetic variation, which can be up to 30%). But there is another network layer found in the olfactory cortex where we all diverge from each other, and this next layer is based on our autobiography, for lack of a better word. Who we are and the decisions we make are changing this second layer with every new encounter.


Back to the study, their statement "Our results indicate that representations of categories emerge dynamically by mitral cells in a way that fits not only task demands but also categorical logic" is another way of saying that we can train our nosebrain to interpret any smell in any way we want. And again, "Once learned, these tasks allowed us to assign the same odour stimuli to different category schemes. If mitral cells activity reflect the learning rules, odour representations would change in a way that follows categorical information."


The point here is that our olfactory perception network is constantly changing, and that's why it's so hard to pin it down. And that's what it's supposed to do -- it's supposed to help us respond to our environment; but our environment is constantly changing. Society and culture, for example, can cause major shifts in both the chemical soup that surrounds us and in the way we semi-autonomously assign meaning to odor objects. See mint and analgesics in the postwar population of England, or licorice for the same age group, or the smell of "new car" in the East vs West.


Body odor, and especially the odor of others who do not eat the same food as you, or follow the same culturally inculcated hygiene rituals as you, will definitely smell "foreign" to you. Until you become more acquainted, of course, at which point it slowly becomes part of your identity also. You do not begin to smell that way, but because your olfactory translation machine no longer flags a smell as "foreign" if it's been around your neighborhood for the past ten years, or if it's been in your own bed for example. The brain can rearrange itself. 


So, smells are both objective and subjective. There is a common denominator to the perception of odor molecules, but depending on the infinite variations of exposure that accumulate over a lifetime, that common denominator is shredded, stretched, skewed and refit to match your personal experience in the olfactory multiverse.


And so to answer an age old question in olfactory research -- "Do odors have their own universal identity, or is our nosebrain a blank slate?" -- the answer is, "Yes."




Study 1 - Harvard Medical School

Stan L. Pashkovski et al, Structure and flexibility in cortical representations of odor space, Nature (2020). DOI: 10.1038/s41586-020-2451-1

[alt link]


Abstract: The cortex organizes sensory information to enable discrimination and generalization1,2,3,4. As systematic representations of chemical odour space have not yet been described in the olfactory cortex, it remains unclear how odour relationships are encoded to place chemically distinct but similar odours, such as lemon and orange, into perceptual categories, such as citrus5,6,7. Here, by combining chemoinformatics and multiphoton imaging in the mouse, we show that both the piriform cortex and its sensory inputs from the olfactory bulb represent chemical odour relationships through correlated patterns of activity. However, cortical odour codes differ from those in the bulb: cortex more strongly clusters together representations for related odours, selectively rewrites pairwise odour relationships, and better matches odour perception. The bulb-to-cortex transformation depends on the associative network originating within the piriform cortex, and can be reshaped by passive odour experience. Thus, cortex actively builds a structured representation of chemical odour space that highlights odour relationships; this representation is similar across individuals but remains plastic, suggesting a means through which the olfactory system can assign related odour cues to common and yet personalized percepts.


Study 2 - Hebrew University

Flexible Representations of Odour Categories in the Mouse Olfactory Bulb. Elena Kudryavitskaya, Eran Marom, David Pash, Adi Mizrahi. Hebrew University of Jerusalem. Mar 24 2020. BioRxiv. doi:


Summary: The ability to group sensory stimuli into categories is crucial for efficient interaction with a rich and ever-changing environment. In olfaction, basic features of categorical representation of odours were observed as early as in the olfactory bulb (OB). Categorical representation was described in mitral cells (MCs) as sudden transitions in responses to odours that were morphed along a continuum. However, it remains unclear to what extent such response dynamics actually reflects perceptual categories and decisions therein. Here, we tested the role of learning on category formation in the mouse OB, using in vivo two-photon calcium imaging and behaviour. We imaged MCs responses in naïve mice and in awake behaving mice as they learned two tasks with different classification logic. In one task, a 1-decision boundary task, animals learned to classify odour mixtures based on the dominant compound in the mixtures. As expected, categorical representation of close by odours, which was evident already in naïve animals, further increased following learning. In a second task, a multi-decision boundary task, animals learned to classify odours independent of their chemical similarity. BBBBBB Rather, odour discrimination was based on the meaning ascribed to them (either rewarding or not). Following the second task, odour representations by MCs reorganized according to the odour value in the new category. This functional reorganization was also reflected as a shift from predominantly excitatory odour responses to predominantly inhibitory odour responses. BBBBBB Our data shows that odour representations by MCs is flexible, shaped by task demands, and carry category-related information.

*In comparison to the above Harvard paper, for "mitral cells," read olfacotry bulb, and for "two-photon calcium imaging," read multiphoton microscopy.



Post Script:

Back to the "first layer" mentioned in the above studies, I need to mention what sounds to me like intensity being a big part of the universal "odor coder." It is certainly one of the chemical features in the corresponding dataset; odors tend to present to us in a known window of intensity, and each molecule has a detection threshold for humans measured in parts per million, billion or trillion. I'm not certain this is what's being implied and surely there's more complicated exclusions to it, but even prior research shows that intensity of an odor is a shortcut through a lot of these categorization and dimension-reduction attempts.


Next, this is nuts; pretty sure I've never seen this before -- "The general profile of excitatory vs. inhibitory responses by mitral cells changed with learning and task demands. In naive animals, most responsive cells (71%) responded by excitation to the odours (Fig. 6A,B). Following the learning of the 5-decision boundary task, the ratio of excitatory/inhibitory responses reversed. After learning, the majority (71.4%) of neurons now responded by inhibitory calcium transients to the odours (Fig. 6C,E). The ratio of inhibitory vs. excitatory responses reverted back to normal after retraining the mice on the 1-decision boundary task. Specifically, 73.3% of responsive neurons were again excitatory on day 18 (Fig. 6D,E). (The Hebrew U study)


So the "second layer," the one that adds the personal, subjective meaning to your odors, is all inhibition. It's dampening the neurons that were originally excited by this odor and based on universal chemical-genetic affinities, but now have to be quieted based on prior outcomes of interactions with it. It doesn't dampen everything, obviously, or else you wouldn't smell it at all (you mean like violets?), but it dampens enough that the overall pattern of the signal that finally does make it through can be dramatically different. You're literally evolving with every breath you take.



Compulsory Reference to the Greatest Work of Olfactory Philosophy Ever:

Hosek R J & Freeman W J (2001). Osmetic Ontogenesis, or Olfaction Becomes You: The Neurodynamic, Intentional Self and Its Affinities with the Foucaultian/Butlerian Subject. Configurations 9: 509–541.

Wednesday, August 12, 2020

Clean Air Beware


Just when you thought you knew what to do, you don't.


Ventilation is good (and a necessary part of controlling an airborne virus, by the way), but opening windows will not remove the chemicals from most homes, at least according to this work done with the HOME Chem model house.


Our indoor environments are filled with the chemicals that offgas from just about everything that surrounds us, from body care products to scented candles to building materials. Cleaning products are a major culprit here, and also an important consideration while trying to combat a global pandemic via the war against germs, aka chemical disinfectants, aka indoor environmental chemical warfare.


For almost any product you bring into your home, or any activity you perform there, chemicals are released into the air. Not all chemicals are bad, but some of them are. The problem is not even in identifying the "bad" things in the air, but in measuring how much things accumulate, no matter what they are -- everything is bad at the right dose.


Opening the windows is the most straightforward and effective way of reducing these concentrations. The problem is that at some point those windows must be closed. Experiments done in the "chem house" show that the concentrations of typical chemicals found indoors do drop precipitously when we open the windows.


But surprise -- a few minutes after they're closed, the concentrations go right back up to baseline.

The idea is that the chemicals cling to the walls, likely in the macro- and microscopic nooks and crannies on the surface. They act like reservoirs. Volatile organic compounds will offgas into the room, and especially during cleaning activities for example, where they settle onto those surface microtextures, and then re-volatilize into the air continuously.


In conclusion, opening windows is good for flushing chemicals out of the indoor air temporarily, but the real effort must be put into keeping them out in the first place. (Sorry Clorox wipes, I’m looking at you, don't act like you don't offgas.)


Image source: The Broom Maker, Victor on Flickr, 2019


Post Script:

What produces more voc’s, spraying a perfume on your wrist once a day or cleaning your house with a surface disinfectant once a week? *First of all, cleaning and disinfecting are not the same thing, and second of all, you’re probably over-disinfecting.



Opening the window in your home will not flush out the chemicals in the air

Feb 2020,


Chen Wang et al. Surface reservoirs dominate dynamic gas-surface partitioning of many indoor air constituents, Science Advances (2020). DOI: 10.1126/sciadv.aay8973


A good follow up:

Organic compounds in indoor air like to accumulate in paint

May 2020, Indoor Chem Blog


Measurements and modeling of absorptive partitioning of volatile organic compounds to painted surfaces. Algrim, L. B., Pagonis, D., de Gouw, J. A.,  Jimenez, J. L.,  and Ziemann P. J. Indoor Air, 2020. 00; 1-12


Further reading:

HOME Chem - House Observations of Microbial and Environmental Chemistry

University of Colorado and CU Boulder


ISIAQ - International Society of Indoor Air Quality and Climate


Saturday, August 1, 2020

The Tree of Heaven

The ghetto palm in its natural habitat

The Tree of Heaven* is also known by its international colloquial name, the Ghetto Palm,  because it thrives in the worst conditions. It's been intentionally brought to the New World from Asia for centuries, both for its exotic ornamental qualities, and its basic use as a fast-growing shade tree that reaches up to the vast ecosystem in the sky at an exceptional rate.
*Also called ailanthus, varnish tree, and chouchun (Chinese, foul-smelling tree).

Now it's considered an invasive species. Its unstoppable juggernaut root system crumbles anything made of concrete, from sewers to foundations to highways and bridges. It also crowds out indigenous species by releasing a toxin (ailanthone) into the soil via its roots and fallen leaves. It isn't affected by herbicide; it is the herbicide.

At this time of year, in the heat of summer, it's spreading like wildfire. Not only does it produce an extraordinary amount of seeds, but it sprouts from its indomitable lateral roots, and almost 100 feet away from the source tree. You don't even have to look for it; you will know it's there by its smell. If you don't know, you will in a minute.

The Tree of Heaven smells bad. Like what? "Bad," that's what.

Can you be a little more descriptive?

Not really. We are so bad at describing bad smells. In fact, the corpus of words used in olfactory science is skewed way to the good. We have many more words for good smells than we do for bad. There's a few reasons for this, one being that we don't like to think about bad smells long enough to generate descriptors. When it comes to bad smells, "objectionable" "disagreeable" "noxious" "offensive" and "smells like shit" will suffice.

We already don't talk about smells much as it is; why waste that precious sensory indulgence on things that smell bad? We also don't like to talk about bad smells, because it can get socially complicated. You're not going to mention a bad smell while with your boss, and that's in case they are the source of it! We don't want to make them look bad. It's also too personal. It's just good social etiquette in general to not talk about bad smells, at all.

We also don't stop and take a second whiff when something smells bad. Flowers yes, the Tree of Heaven no. It will activate your disgust response, you'll reflexively twist your head back, flare your nostrils, and curl your upper lip. And you will not be going back for seconds.

Unless you're me. I've been trying to smell the Tree of Heaven for years now. It was introduced to me circa 2008 by a friend who lived on a farm; we were touring his property. "We call it the jizz tree," he said with a nervous giggle, him and his girlfriend. I don't remember smelling it, maybe it wasn't in season, who knows. I don't remember seeing it either. I don't remember anything except that there's a tree that smells like semen.

Later on I wrote a book about the language of smell, and shortly after that I came to the realization that I'm anosmic to putrescene, which means I cannot smell semen. Smellblind. People are smellblind to all kinds of things, bad things more often it seems, and roughly half of us are anosmic to something (natural gas and rotten fish will come up a lot, because they're talked about a lot, because it's a safety issue, right?).

I'll bet there's less people that know they're anosmic to putrescene. There are some people who are completely anosmic to everything, from birth, and don't realize it until they're ten years old. Kids don't even know that they have a sense of smell, and they certainly don't notice if it goes away temporarily, as in the case of those infected with the novel coronavirus of 2019 (the condition will present as them skipping meals and not being hungry, but it's likely because they can't smell).

Back to the tree. I started to notice the Tree of Heaven invading my extended neighborhood once I began taking the train for school. They grow along the train tracks really well, and they grow fast. After an extreme weather event, like drought, flood or fire, they are the first to pop back up, because they go dormant into their roots, conserve resources, and wait. They looked strange to me only because they were growing so fast. Within the first year of my taking the train, they had already grown from 5 to 20 feet. I started to notice them elsewhere, and once I started to look, there they came; these things are everywhere.

I looked them up, and found this notable characteristic -- this is the famed semen tree I've been hearing about, but never smelled. But it could be sumac, they look pretty darn close. In fact, the easiest way to identify it is to break off a leaf at the stem and smell it. If your head snaps back, it's the Tree of Semen, I mean Heaven. I went outside and grabbed a leaf, and as expected, couldn't smell anything. It could also be sumac; although it turns out I was wrong. 

Months later, I see one on the street and decide to try again, and rip off a leaf. It smells, kind of bad, maybe not, definitely not noxious, is it chocolate? Bad chocolate? What does that even mean? I must investigate further, because there's something there, although it's faint. Maybe it's just the beginning of the right season, who knows. Maybe it's still sumac.

A couple weeks later, it's getting hotter, a 12-day heat wave which is rare where I live, and I'm waiting in a parking lot for my laundry to finish (you can't wait inside, because of the virus). I'm in the back of the parking lot, against the train tracks, where there's shade, from the shade trees... There, I see it again... and now it's everywhere, I take a piece, break it off, and my goodness. My head snaps back. Terribly offensive.

I look it up again -- sumac makes red bunches of fruit, tree of heaven makes neutral colored and later in season pink to reddish-colored seed pods that kind of like maple tree "helicopter" seeds. I now confirm it is in fact the Tree of Heaven. And so is that what semen smells like? I call it pungent burnt rancid oily and dare I say, nutty, like peanuts?
Ailanthus samara seed pod, The Pennsylvania Flora Project of Morris Arboretum, 2011

How come when I look this up, in the more reputable sources such as the New Jersey Audubon Society or the Ecological Landscape Alliance, they use words like rancid peanut butter, burnt rubber, pungent, foul, and the the indispensable "smelly."

Is it because they're so "reputable" that they can't say "semen?" I think this may be half of it.

But as I go further into the informal investigation of the internet rabbit hole (this is called gray literature research, although you should probably call it reading urban dictionary entries, VICE articles and horticulturist blogs), and it starts to dawn on me. These descriptors are a mess; they're all over the place:
Rancid peanut butter, rancid peanuts, rancid cashews, cross between peanut butter and cat urine, well-used gym socks, yucky cooked meat, objectionable, disagreeable to humans, fetid-smelling, bitter, acrid, pungent, strong, and any word that refers to semen, and which can be described as a "chlorine musk," but is more directly associated with the molecule putrescene ... there's something in here about amines and ammonia also. You could also refer to it in your most prudent Victorian manner and call it simply "a man smell."
*Only the male flower smells, but both smell when their branches or stems are broken.
When I look at that list I am reminded of two things -- we are really bad at describing bad smells, and half of us are anosmic to something, and usually to bad smells.

So not only do we NOT talk about or think about the names to call bad smells, but for some of them, we can't even smell them in the first place.

When you don't talk about something, and you don't generate either a personal or a shared vocabulary for something, you will be really bad at identifying it, at discriminating it from similar things, or at categorizing it in your autobiographical database. This means you're more likely to mis-assign a name to the smell, calling plant-semen "dirty gym socks" instead. I may need some help here, because again, I'm smellblind to it, but does semen smell like dirty gym socks?

Furthermore, when you're presented with a cocktail of bad smells, as would be expected emanating from a living biochemical reactor*, you may be missing a major component of the mixture due to smellblindness to one of the molecules, and that could change your impression dramatically.
*A plant's essential oil is not the same as an isolated synthetic compound, because olfaction is a combinatorial affair, shown from about 2015 research and on.

If you combine all these factors together, you get one dirty mess of a database. There is no absolute, no discrete points. If you could manage to ask 10,000 people around the world  (or 1.5 million in the National Geographic Smell Survey) to describe 10 different bad smells, each a natural biologically-generated smell cocktail, what would that list look like?

That list would tell you something about the overall distribution of genetic diversity in the study population based on olfactory receptor genes, or it could tell you something about the cultural milieu of a sub-sample (like the Victorians!), but it won't tell you any better what the Tree of Heaven "really" smells like, or stink bugs for that matter.

When it comes to making sense of the world as an olfactory phenomenon, you're on your own. Olfactory reality is not a consensual reality. And that's unsettling, because in the Information Age, approximation seems like failure, no?

Post Script: 
Within days of my most recent experience with the potent odor of this tree, I can now smell it as a drive down the highway, from the trees on the side of the road. In my typical self-induced pseudo-hyperosmic fashion, I have become very sensitive to it.

This reminds me of an idea about regeneration of olfactory neurons and combinatorial perception, and as it relates to people recovering from the novel coronavirus of 2019. After some traumatic disturbance to your olfactory neurons, like from being attacked by a virus, you may experience changes in smell or taste. This is also called anosmia, partial anosmia, or phantosmia, the last referring to not a loss of smell but a change in the way things smell.

Phantosmia, like all phenomena in olfactory science, is not understood enough to say much from an evidenced-based point of view. I'm making a broad speculation here, not to explain, but to make someone interested enough that they will investigate further for themselves, and maybe even initiate more research into the topic.

The change in smell that comes from phantosmia is common, but its origins are often overlooked. It likely signals a change in the structure of neurons used to smell. These are the only part of your brain that pass the blood-brain barrier and rest outside your skull, in the mucous atop the epithelium skin way up in the top of your nostril canal (right where they swab that sample for your PCR test by the way, and not a coincidence). These neurons are thus both very vulnerable to damage, and able to regenerate indefinitely.

Combine this with another fact about olfaction -- it is combinatorial. That means when you smell "apple," there are a bunch of different receptors all lighting up in a pattern that means "apple." There is no Apple gene. For some there are, but for the most part, no. No single gene codes for any single cell. Olfaction is all gestalt. Take one piece out, and the entire picture gets weird as hell. Something's wrong but you can't tell what. So your brain misfires, it says "cigarette smoke" when it's really something else entirely. But after damage to your system, it is re-learning how to smell. Your nose-brain is a deep learning neural network that requires countless iterations to "learn" what a smell is. And while it's relearning after an infection, it gets confused.

In a very mild manner, and for reasons I will attribute to having been infected myself, my Tom Ford Italian Cypress lost its depth and presented as cinnamon and bubblegum, for about three days. If you've ever smelled Italian Cypress (and if not good luck it's discontinued since 2014), you would know that it does NOT smell like cinnamon and bubble gum. That's happened to me once before, and I will now assume it was because I was then also infected with a virus. But I got lucky, it wasn't bad, just weird.

And another thing -- when faced with new smells, we tend to call them bad. After repeated exposure, we can start to see them as good, but it's more likely we call them bad at first. So if your system is relearning how to smell, then lots of typical odor exposures will present as "bad" to you, simply by their being new, that is, new to your newly developing system. And all of the sudden, anything that doesn't compute properly on your new system will become "cigarette smoke," for example. Rotten meat is another good one for this, but it could be anything really (and it could also be really debilitating, just imagine.)

Eventually, the system will recalibrate and relearn how to smell, and you'll be back to normal. But that doesn't always happen. Blunt trauma can kill those neurons forever. Really bad infections too. Sometimes it doesn't come back because you're not using it, like therapy after a stroke, it only comes back if you try really hard to use it.

And some of us, well, we're just getting older. Things don't work like they used to. And not only that, changes in smell can predict all kinds of neurological diseases decades in advance (Parkinson's, Alzheimer's, etc.).

All this being said, my system has apparently, and finally, learned how to smell the Tree of Heaven.

Some good recent research on genetic variation in olfactory receptors:
Did You Smell That No I Didn't
Jan 2020,

And the most relevant among them for today:
Any two individuals differ by ∼30% of their olfactory receptor subtype genome.
Mainland JD, et al. (2014) The missense of smell: Functional variability in the human odorant receptor repertoire. Nat Neurosci 17(1):114–120.

The human olfactory genome contains 418 intact odorant receptor genes and their 912,912 intact odorant receptor alleles.
The 1000 Genomes Project (2008-2015), the largest public catalogue of human variation and genotype data.

Pop Search Trivia:
"Tree of heaven smells" like peanut butter, #1 search result. (July 2020)

What is the tree that smells like dead fish?
(Callery pear trees)

What is the tree that smells like peanut butter?
(Butterfly tree or peanut butter shrub)

What are other trees that smell?

  • Callery pear, Bradford pear tree - flowers emit dead rotten fish semen trimethylamine dimethylamine
  • Maidenhair, Ginkgo bioloba tree - female fruit produces putrid rotten eggs vomit
  • Chinese chestnut tree - male flowers emit "off-putting" smell; again I think this smells like semen and it's just not said that way because it's uncouth!
  • Linden tree - smells like semen? How could it smell like semen and yet someone else says it smells like the most powerful fragrance in the plant kingdom, of honey and lemon peel?


Best source of information on this topic:
Ecological Landscape Alliance - Tree of Heaven, An Exotic Invasive Plant Fact Sheet - May 2014

Identify and Disambiguate:
New Jersey Audubon Society - How to Correctly Distinguish Invasive Tee-of-Heaven from Native Sumac - July 2018

Post Post Script:
This gal is trying to decode the bad smell network; I made an odor descriptor-molecule network graph of her research with a regional air quality odor complaint database, interesting work, under-explored territory.

In word-searching the list from the Curren's study, "amine" turns up "fishy" and "pungent" via trimethylamine, and "pungent" via ammonia; no mentions of semen ever.

"Pungent" then brings up pentanal, 2-pentanone, formaldehyde, ammonia, trimethylamine. And  "rancid" connects to butyric acid. "Rotten" brings the expected hydrogen sulfide and dimethyl trisulfide from "rotten eggs" and "rotten vegetables."

A Case Study of Odor Nuisance in the South Coast Air Quality Management District 
Curren, J. 2012. Characterization of Odor Nuisance. UCLA.

What the Hell Does a Stink Bug Smell Like?