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The smell of atmospheric air subconsciously affects our mood

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Povilas S
Povilas S Mar 28, 2021
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The title of this idea is a hypothesis that I came up with. In the following text, I'll clarify it and provide arguments to support it.


Do you notice the smell of the air when you go outside? If so, how often? Every time? Sometimes? Maybe when it's particularly apparent? By smell of the air I don't mean intense smells that happen to reach you in particular places and at particular times as you walk along, but the natural whiff of the atmospheric air itself. I've always noticed that atmospheric air smells. And that it smells differently on different days. Its smell changes pretty much like the weather. So does our mood.

Generally, atmospheric air never smells bad (unless you live near a cattle farm, a meat factory, a busy street, etc.), its smell is always natural and fresh. But the shades and intensity of that natural smell change. That change is what I want to focus on.

From experience, it's obvious that the smell of atmospheric air, when it's intense enough to notice, evokes positive emotional response and lifts up the mood - you walk down the street or in nature, suddenly catch a whiff of clean, fresh air and sigh joyfully. Sometimes that atmospheric smell is easily noticeable throughout the day, spontaneously or whenever you pay attention, sometimes it's very shallow or appears to be non-existent even if you intentionally look for it. The question is - what happens when it's unnoticed (which is most of the time), does it still subconsciously interact with your emotions? When its shades and intensity change, is this always reflected in the change (even if slight) of your mood?

Smells have long been considered to have an unconscious influence on human psyche. Partly because of their associations with pheromones, but also and mainly because of their ability to effectively trigger emotionally charged memories which is due to the specific neuroanatomy of the smell perception.

Why the smell of atmospheric air deserves special attention in contrast to all other smells when talking about emotional influences? Because atmospheric air (and whatever the smell of it) is always there interacting with our olfactory system. It's a background for all other smells, and when more intense, more transient smells are not there, the background smell is still there. This could be compared to our mood vs. our emotions, - mood is the slower changing background, emotions are more intense and quicker to pass.

I'm not trying to imply that smell of the air is the main or a very significant factor responsible for our mood swings, instead, I'm suggesting that it is one of many and the one that is easily overlooked and not considered in the context of emotional influencers.

A summary of arguments in support of this hypothesis:
  1. Smell perception is directly linked to our limbic system - olfactory bulbs are anatomically connected to this brain region, they are considered to be a part of it. Therefore odours are able to evoke memories and emotional responses before (or at least at the same time as) the electric signals reach neocortex, where cognition of the smell takes place. This is a unique feature among all sense perceptions that only the sense of smell has.
  2. Olfactory receptors are constantly interacting with atmospheric air. Atmospheric air for the olfactory system is like light for the eyes, it is a medium in which all the smells travel. Even when we are in closed spaces, the air which we breathe is atmospheric air reaching us through ventilation systems. Oxygen doesn't smell, but air, which is a mixture of gases, volatile compounds, and aerosols - does. Strong smells that we notice and react to usually come quickly and dissipate likewise, but the smell of the background air, which is subtle and lingering, is rarely noticed.
  3. Human olfactory system is extremely sensitive to some of the odorous compounds naturally present in the atmospheric air: ozone (smell of the air before, during, and after the storm) - olfactory threshold 7 ppb ; dimethyl sulfide (one of the chemicals responsible for the smell of the sea) - 1 ppb ; geosmin (contributes to the air smell after the rain) - 0.01 ppb .
  4. Environmental factors affect smell of the air, hence it changes often. Natural environmental factors influencing the smell of the air are - large water bodies, wind, rain, weather temperature, plants. I'll write more extensively about environmental factors influencing the smell of the air in a creative contribution below.
  5. Smell of the air likely had evolutionary importance for humans, especially having in mind water and plants. Humans were dependant on rain for survival (crucially important for harvest in agricultural communities, but also important for foraging). Additionally, the ability to smell water bodies from afar seems very important in the context of nomadic hunter-gatherer communities. This might explain why the smell of humid air blown with the wind is one of the most pleasant and most easily identifiable atmospheric odours (after plant odours). Wild plant odours lingering in the air are important in the context of foraging and keeping track of natural cycles.
  6. Smell of the air brings back personal, emotionally charged memories. In addition to universal responses that humans might have to atmospheric smells due to evolutionary psychology, their ability to remind of personal experiences is another, likely even more important factor affecting our emotions. Smell of the sea is likely to be associated with good times you spent at the seaside, flowery smell of warm summer night - of times when you went out late and had fun with friends in adolescence, the smell of clean winter's air - of sledding and playing in the snow in the childhood, etc. I've noticed from my own experience that atmospheric smells are especially good in bringing memories from the childhood. Smell of the air was the first thing you noticed when going outside, especially when spending time in less urbanized areas. It's somewhat synonymous with the atmosphere surrounding outdoor activities and events in the memory. And since the smell of fresh air is intrinsically positive, it seems to stick exclusively to positive memories. One might have specific, especially positive memories associated with certain shades of atmospheric air smell and therefore feel better (whether they realize it or not) when those shades of smell are present in the air.

Why does this matter?

Apart from contributing a piece to a larger scientific understanding, this also has an important practical side. That is - if this was proven to be true, it would be a good reason for people to become more conscious of their smell perception (which in turn is a step to being more conscious generally). When we know consciously why our emotions changed, what has influenced it, even if it's just one variable (in this case a smell) in a multivariable function, which is our mood, that variable is no longer subconscious, we now have an understanding instead of just blindly experiencing the cause.

I'll write about how this hypothesis could be experimentally tested in a creative contribution below.


[1]https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1600-0668.2007.00476.x

[2]https://www.tandfonline.com/doi/pdf/10.1080/00022470.1969.10466465

[3]https://sci-hub.do/10.1093/chemse/17.1.23

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Creative contributions

Different ways to experimentally test this hypothesis

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Povilas S
Povilas S Mar 28, 2021
A "domestic" approach: Just using subjective assessment scales for both the smell of atmospheric air (the intensity and pleasantness of its odour) and mood for each different day and then trying to correlate the values of the two could reveal certain trends. A group of individuals participating in that kind of study would be instructed to pay close attention to the smell of atmospheric air each day for a certain number of subsequent days. Or this could be done in the period of one year in different seasons to ensure the differences of atmospheric odours. The mood scores for each different day would also be subjectively evaluated by participants themselves.

There are numerous factors affecting individual's mood each day. However, if certain factor has any significant influence on it, during long enough test period such correlation can be revealed using specialized data analysis methods, such as fourier transform. The bigger problem with this approach is that the smell of the air would be evaluated consciously throughout the day and become intertwined with the process of mood evaluation, so this is rather unwise when trying to determine the level of subconscious rather than conscious influence. Nevertheless, the revealed correlations (if any) can be useful since conscious and subconscious aspects of the mind are related.

Using more rigorous methods: In order to approach testing the subconscious aspect of the emotional influence of atmospheric smells, it would be very useful to sample (preserve) such smells. Headspace technology is designed to take samples of odorous compounds directly from the air. However, it's usually used to collect the air surrounding odorous object (its headspace) and not to catch scents lingering in the air. The difference between the two is not big though. Something like a hybrid between a headspace device andatmospheric aerosol collector could be used to capture odorous compounds from atmospheric air the whole day.

Those odorous samples "of the day" could then be presented to the participants of the survey to smell at the end of each day. The subjective rates for intensity and pleasantness of the sample would be marked. The evaluation of their daily mood would be done separately, either by subjective evaluations of the participants or by monitoring their physiological parameters (best both). The latter could be easily done the whole day by wearing a device like this.

This way there would be no need for the participants to pay attention to the smell of atmospheric air each day. The samples (given they would be of high quality) would represent the baseline odour of the atmospheric air that day. Correlations would be done between the odour evaluation scores of those samples and mood evaluation scores. Participants wouldn't be informed why they are evaluating odorous samples and how this is related to their mood evaluation process.
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Here's how atmospheric "perfume" is made

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Povilas S
Povilas S Mar 28, 2021
Different environmental factors affect the smell of atmospheric air and change it in time. Those are:

Oceans, seas, and other large water bodies:
If you live not too far from a sea or an ocean, and wind is blowing your way you can feel the scent of fresh, humid air. This is due to sea aerosols which are a mixture of gases, tiny particles of water, and solids (sea salt) suspended in the air and blown with the wind. The smell of the sea has been largely attributed to dimethyl sulfide (DMS) which is produced by marine microorganisms. Additionally, many other odorous compounds have been noted to contribute to it, such as dictyopterenes, hydrogen sulfide, bromophenols, and chemical derivatives of DMS - all those are either directly or indirectly produced by macroalgae and various microorganisms of the sea. Gaseous iodine released from seaweeds is also likely to contribute.

Even though sea smell is most noticeable while being close to it, odorous compounds present in sea aerosols can potentially travel long distances with the wind. How far a certain volatile compound will travel with the wind depends on the movement of air masses and its reactivity with other chemicals present in the air. A certain concentration of it has to remain for human nose to be able to still sense it. But having in mind that oceans' surface area is huge, so is the produced amount of these compounds, also, human olfactory system is very sensitive to some of the odorous compounds, being able to sense them in few parts per billion or even few parts per trillion and lower, depending on the chemical. Dimethyl sulfide is one of them. Although living 300 km away from the sea, on some days the air smells very much like being close to the seaside for me. Have you experienced something similar?

Similar mechanisms (but different molecules) are responsible for the smell felt near smaller water bodies: lakes, rivers, etc. Moving air masses can carry the humid air with odorous molecules further away from them and mix it into the atmospheric background smell.

Rain:
Another factor causing humid air smell is rain. The smell of the air after (and often before and during) the rain has a characteristic odour that is very familiar to most people. This one is again caused by few different chemicals and microorganisms are again involved. Microorganisms present in the soil produce an earthy smelling compound called geosmin (lot. earth smell) which is released from the soil into the air during rain. Together with geosmin, oils from plants that were secreted during periods of drought and trapped in the dry soil are also released and contribute to the "after rain" smell known as petrichor. Those oils themselves are complex mixtures of chemicals. Last but not least, ozone contributes to the "clean" smell before, during, and after thunderstorms. It is produced in the lower atmosphere from oxygen by lightning.

Humid air often intensifies or reveals smells, because tiny water droplets present in the air pick up and carry odorous molecules from plants, soil, etc.

Plants:
Plants are generators of the strongest atmospheric scents that can linger in the air. In northern regions with expressed seasons and long winters plants influence the smell of the air predominantly during the warm season, while in regions with evergreen flora they affect it all year round. Essential oils are responsible for most of the odours of plant origin, but other compounds, e.g. non-essential oils as in the case of petrichor, green leaf volatiles, and especially cis-3-hexenal, that are responsible for the smell of hay, cut grass, etc., are also important in this context. Flowery scents lingering in the air are essential oils released by flowering plants during their blooming season. Different floral scents dominate the air smell according to differences in the blooming season of various plant species.

Conifers are also very important contributors to the atmospheric scent because they release terpenes. The air in a pine forest has a characteristic smell because of these compounds. Terpenes are extremely volatile and large areas of coniferous forests can release large amounts of them into the atmosphere. Terpenes are also the main constituents of all plant essential oils

Plant growth and flowering seasons start earlier in southern (in the northern hemisphere) geographic regions, so it seems not unlikely, that winds blowing from the south can carry floral scents up north to the areas where warm season hasn't come yet or is only starting. This would again depend on the movement of air masses, atmospheric chemistry of the odorous compounds, and their initial amount released into the air. But since land areas occupied by blooming plants and conifers can be huge, so can the amounts of released aromatic compounds.

Temperature:
Temperature affects molecular movement and thus defines how intense smells are and how fast they are transported. Only at a certain temperature level we might begin to sense certain atmospheric odours. In the warm season there are more different scents present in the air and they are more intense, therefore the smell of atmospheric air is more easily noticed.

On the other hand, people often comment on the clean and fresh scent of winter's air. This might be because in cold weather certain shades of the atmospheric smell are muted and therefore other shades or one shade, in particular, comes forward. It's like in the case of warm and cold drinks (e.g. tea, coffee, juice) they smell a bit different when hot vs. cold because different notes of the same smell are brought back or forward. Another aspect that cold air has is a cooling feeling inside your nostrils, this feeling is not a smell, it's a sense of touch, but it contributes to the whole experience.

Wind:
Wind is perhaps the most important of all natural factors in this context because it's what makes odorous molecules travel, mix together and change the output of atmospheric scent in a particular location. Depending on its direction, wind might bring sulfides from the seaside, ozone from a nearby thunderstorm, warm air with plant scents from the south, a cool, fresh air scent from the north, etc. Irregularly moving air masses can mix odorous molecules from different sources and create unique atmospheric "perfumes".

Other natural factors: Those are just few most obvious and perhaps most influential natural factors. There are most likely more and you could help me to point those out.

Anthropogenic factors: The main anthropogenic factors influencing the smell of atmospheric air are various types of human-caused combustion. The smell of burning wood might linger in the air in more rural areas where people still use solid fuel to heat their homes in winter. That is a pleasant odour for most people, but it could be classified as semi-anthropogenic because wild forest fires can cause the same smell, also, humans were spending time near campfires for thousands of years, so from the perspective of evolutionary psychology, the smell had time to "sink in" deep into the human psyche. Other types of fuel produce mostly unpleasant smells when burned.

Even though anthropogenic factors also affect the smell of atmospheric air, they do it rather insignificantly in comparison to natural factors. They are significant only when one is relatively close to the source emitting odorous combustion products. Anthropogenic aerosols constitute 10 percent of all the atmospheric aerosols, the rest is of natural origin. Similar ratio is likely to be the case when it comes to odorous compounds that constitute the smell of atmospheric air. The topic of odorous compounds of anthropogenic origin present in the atmospheric air and influencing our mood might be developed into a separate idea.

[1]https://www.compoundchem.com/2014/07/18/the-chemical-compounds-behind-the-scent-of-the-sea/

[2]https://www.sciencefocus.com/planet-earth/why-does-the-sea-smell-like-the-sea/

[3]https://en.wikipedia.org/wiki/Dimethyl_sulfide#Natural_occurrence

[4]https://pubmed.ncbi.nlm.nih.gov/21431377/

[5]https://earthscience.stackexchange.com/questions/2585/how-can-odor-travel-be-tracked#2598

[6]https://sci-hub.st/10.1038/201993a0

[7]https://www.britannica.com/topic/essential-oil/Chemical-composition

[8]https://www.businessinsider.com/why-cold-air-has-a-scent-2014-1?op=1

[9]https://earthobservatory.nasa.gov/features/Aerosols

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General comments

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Darko Savic
Darko Savic8 months ago
I can relate. For me, the smell elicits feelings from childhood - possibly a memorable event associated with the same smell. I get "transported" back to a time and place where I remember the smell from. It also brings back the memories of people associated with that period/place. Sometimes it takes me way back, other times it goes to the previous spring, autumn, etc.
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Povilas S
Povilas S8 months ago
Darko Savic Are you talking about smells in general or the smell of atmospheric air in particular? What you said is true for most people, talking about smells in general, - smell perception is tightly linked with memory and emotions. But what I've noticed from experience is that people rarely give importance to the smell of the air (outdoors), even though it might evoke strong emotions (for me personally). And since atmospheric air is something we are exposed to every day, but its smell goes largely unnoticed, it seems likely that it might have a significant, although unconscious influence on our daily mood.
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Darko Savic
Darko Savic8 months ago
Povilas S I'm talking about the smell of outdoor air without any obvious influencers such as smoke, etc. So yes, atmospheric air
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