An Alternative Way To Tour A City — By Your Nose!

Note: This article was written last year!



Just last night, I came home from work and I was making dinner using the microwave. I was preparing a ‘microwavable meal’ of pasta–sounds great right–the meal did the job to say the least (which needs no further comment). While I was stirring the meal in between cooking cycles in the microwave, I started to smell an unfamiliar smell which I could not place in my kitchen. Although, when you are hungry–the stomach tends to ‘out rank’ all senses in ‘reason’ or ‘logic.’ I noticed that the door was open and went outside to see if by any chance the smell was coming in through the door...



Before I finish the story that I started let me cut to the chase.  I was reminded of an article that I read concerning our (humans) relationship with smell and design layout in a large city. What was suggested as a ‘smell walk’ in the article, I was actually mimicking in my own home without realizing this behavior. We do this all the time without really placing emphasis on this talent. This method could be extended to touring the city according to the news. Let me explain in the post below.

What Is A ‘Smell Walk’?

Until I read an article in the ‘New York Times ’ recently, I was unaware that a ‘smell walk’ existed. I would reserve such a description to the tour of a factory (industry) – food, perfume, chemical –exclusively not to the topic of touring a city, much less observing the history behind a city based largely on the sense of smell. The article was titled ‘Don’t Turn Up Your Nose At The City In Summer".  Dr. Victoria Henshaw describes our relationship with odor as follows:

We tend to think of a heightened sense of smell as an animal skill — something our pets are better at than we are — but our olfactory systems, too, are clever at filtering and categorizing the smells we detect. We sort odors into the familiar, which represent no threat, like the scent of our own homes, while the unfamiliar are brought to our conscious attention, whether as potentially pleasurable or possibly insalubrious. 

The downside of this filtering process is that we fail to appreciate the sheer volume of smell information we process on a daily basis. Those who lose their sense of smell attest to the scents they desperately miss, as well as to the fear that comes with being unable to detect smoke, gas or rotten food.

My fascination with smell as stated earlier was limited in scope up to a few months ago. In a previous blog post, I highlighted the research conducted that determined that humans can differentiate between around 1 trillion different scents. Of course, the larger degree of overlap in scents increases the difficulty in differentiation on the part of the test subject (human). To observe a city through the sense of smell was surprising and foreign to me. Usually, olfactory information is suppressed or less prioritized in the grand scheme of day to day interactions with the environment. According to Dr. Henshaw this sensory information contributes more to the design of a city than inhabitants (residents) know which is highlighted in the article from ‘The Times’ here:

In New York, smell was a key factor in the city’s original planning. Devising the grid system in the early 19th century, the city commissioners aimed to maximize the benefits of westerly winds to dissipate the supposedly deadly miasmas thought to spread disease. Odor standards were enforced by local smelling committees. In 1891, in the 15th ward (Greenwich Village), members identified pollution from an oil refinery by following their noses. 

The surprising thing is that they could detect anything over the smell of human waste. In 1890, The New York Times reported that the city’s sewers dumped an estimated 100,000 tons of fecal matter a year into the harbor. Industry added to the problem: An inspection in 1910 found that in Yorkville, home to German immigrants at the time, brewery effluent flowed in the pipes, while around Canal Street, the smell of banana oil — used as a solvent by painters — dominated. 

Odors can often provide a guide to the city’s industrial past, even decades after smoke has stopped pouring from the stacks. In London’s Olympic Village, for example, the main stadium was built on a former industrial zone — and when it rains, locals report detecting the smell of soap seeping from the site of an old factory. 

Smell also provides a sociological map of the city. Poorer people tend to have less control over their smell environments. Residents of Hunts Point, in the Bronx, suffered for decades from the acrid odors of the waste-treatment plants there. Only after a public and private nuisance suit was brought by residents and activists in 2007 did the city settle the case and clean up its act.

Of course, I admit that I am not a city planner, instead a chemist. In the course of a chemistry education, the average student (and graduate student) is exposed to a wide range of volatile chemicals and as a result becomes accustomed to differentiating his/her way through the vast ‘smellscape’ of the chemistry research laboratory. Although, when the same student steps out onto the street, a disconnect is present to the urban environmental smells surrounding them. This is an area of science communication in which there is tremendous room for improvement. Courses such as ‘the science of cooking’ have been concocted to combat this disconnect between the laboratory and the outside world (a much larger laboratory). There exists a vast array of smells (some concocted and some uninteded) that invade our noses on a daily basis that we are unaware of and do not appreciate as a result of other dominant senses. After reading this I decided to look further into Dr. Henshaw’s research of our relationship with smell and city planning. Dr. Henshaw is a Professor of Urban Design and Planning at the University of Sheffield and has studied the role of our relationship with the sense of smell in a variety of environments. Work like that of Dr. Henshaw provides us with a new vernacular from which to view problems associated with the environment and sustainability. She has a blog on which there is a large number of articles highlighting her adventures with smells.  In addition, she has written a book that should be in print soon. On her blog post describing her book, she expands further on her interest with smell and urban planning as highlighted here in this excerpt:

In Urban Smellscapes I draw from detailed research with participants in Doncaster, Manchester, Sheffield and London (UK) and the wide range of smellwalks I have conducted around the world including those in Edinburgh (UK), Seattle and New York (US), Grasse (France), Montreal (Canada) and Barcelona (Spain). I explore relationships between our individual senses of smell and personal characteristics, alongside factors relating to the world surrounding us such as the buildings and streets we walk through, the activities that take place within them and the people and objects we encounter. By drawing from people’s descriptions and perceptions of the smells they encounter, I identify a range of different tools and models for deconstructing and designing smell environments, and in doing so I hope that city leaders, architects and urban designers are better equipped to take a more positive approach to smell when designing places and spaces in the city.

I love to learn about new ideas and research. Since I read about this research last week, I have been thinking about various ways in which I have ignored the smells surrounding me on a daily basis. Throughout the day, I tend to ignore smells, but as the research above indicates tourists and city dwellers are missing out on an untapped dimension of pleasure continuously. Without even realizing it, when we step into our homes each night–each of us perform experiments with our noses that send a signal to our brain that the building (our home) is indeed matched with our sight (our visual observation). This simple task may seem mundane since each of us perform an experiment continuously unconsciously. Think about how often you carry out an experiment with your nose to validate your visual experimental observation–very frequently. The combination of smells and sight provide us with comfort, whether it is at home, work, a favorite restaurant, a family physician, etc.. This leads me back to the story of cooking my pasta at the beginning of the blog post. Let me explain below.

An Unfamiliar Smell In A Familiar Place

Navigating the ‘smellscape’ in our home is task that is continuously done. As I started the post, I was cooking a meal in the microwave and smelled an unfamiliar smell that I had never experienced while cooking this particular meal before. My mind started to wander all over the place. I started asking myself – did I burn something? Is that the smell of burned plastic? Is there something else burning that was left over in the microwave? I was smelling while hungry–or anticipating my meal. After the meal was cooling, I was ready to eat and I looked over at the ‘trash can’ only to find that there was a used up bag of ‘microwaveable popcorn’. In the midst of being hungry, I could not even determine the correct origin of the unfamiliar smell. In addition, I was recalling a history of cooking events that had taken place in our kitchen over the last 24 hours only to realize that my wife had cooked some popcorn earlier that day. I did not connect the fact that she had been on vacation (off from work) and stayed home and produced an unfamiliar (or unknown to me). I was amazed at the events that unfolded in this scenario.



Here I was worried about a smell that I linked to the current meal that I was making–which is normal. I was not enjoying the variety of smells that were in our kitchen as a result of meals that had been made. The history of odor was dominated by my immediate task which was to make dinner. This observation ties into the above discussion of smell walks–in that–I was only concerned about the smell in relation to the present. I was not appreciating the variety of history that was contained in our ‘trash can’ since the last disposal. I realize that I have rambled on a bit. This blog is rather loosely tied together. The point is that we are inundated with a variety of odors on a daily basis and choose not to filter them out for the pleasure of observation–unless there is an immediate need to differentiate between them. Restaurants along with retail stores make use of our odor differentiation to sell us products everyday consciously and unconsciously.



Marketing and advertising consultants use the sense of smell to guide the consumer into a store (or try to). Historically, the use of chemicals (fragrances, odor trapping, odor masking, etc.) in products and advertising is a well-developed area of marketing. Although–with the technological development of ‘diffusers’ along with other distribution devices coupled with chemical synthetic methods, chemistry has helped the economy through sense of smell in a variety of ways. I was not surprised to read that certain stores spray odors (synthetic chemicals) that match the products that are sold inside. As technology is developed further into odor identification and synthesis, expect more exciting and undeniable hidden influences to draw you into a given establishment. The use of odor to attract customers was embedded in the design layout of a store or restaurant in past time. Now, with the precision of chemical synthesis to make remarkable odors that are very familiar as well as enticing to our olfactory sense, our ability to withhold the temptation to a given food or fragrance product is decreasing. Retailers are enlisting the help of psychologists, sociologists, and other scientists to hone in on a product that will sell with little resistance (on the customer's part). These tactics used by marketers and advertisers should not be cast in a negative light. Instead, the consumer should appreciate the diversity and precision that science lends in producing products that are appealing to the nose of consumers and invoke a desire to purchase or eat. There is a lot of science involved in the product development. As a consumer, I am always fascinated by the creative effort that is behind such products.



The new lesson learned from reading and exploring Dr. Henshaw’s research is that I will from now on try to focus more attention to my nose than I did previously in the past. By the way, if you would like to learn 5 tips to having a better ‘smell walk’ from the expert herself, check out Dr. Henshaw’s blog post outlining suggestions. I challenge the reader to do the same. Think about all of the neglected smells that you encounter on a daily basis. Further, challenge yourself to view the layout of a big city (such as New York City, Los Angeles, Chicago, etc.) in terms of the smell distribution. Compare the layout of the city with the history. Does the smell match the past? What differences have you observed? Summer time is a time to get out an enjoy the beautiful weather outside. In addition, with this new information learned, an added component awaits your adventure. Enjoy the wide range of sensory information provided by a city’s odors alongside the traditional ‘sight seeing’ in your adventures.

How Many Smells Can Humans Differentiate Between?

Note: This post was originally written last year.  Enjoy the article!!




Last Thursday, I rode a new route by bicycle home from work which entailed riding longer >22 miles (and in the heat) instead of using my normal dominant mode of transportation — the metrolink commuter train. The previous weekend, my wife and I discovered a new bicycle path that passes (in a round about way) our work. At the time, I was excited and said “I am going to ride this route home at least once this week” — today was that day–Thursday. On my ride I could not help but think about two articles that I read recently–one from a newspaper and the other from an academic journal detailing a study of odor differentiation. In short, turns out that humans have around 2.5 million sweat glands in distributed strategically throughout the surface of the body (to stabilize the body temperature) and all of these were working today in concert on my ride home–I was sweating a large amount. At the same time, I was giving off “chemical signals” without even knowing during which I was telling a story inadvertently to the surrounding species (fellow riders, runners, walkers, cars w/windows down, non human species included).



Why is the above any concern to the reader of this blog post? Maybe the discovery is not and simply boring and is reason enough to stop reading now (fair enough). If the reader is interested how these ‘chemical signals’ are correlated with the ability to distinguish our partner’s various states of odor (stressed, happy, nervous, sexual, etc.), then keep on reading. Furthermore, if you would like to know why this should not be of any surprise due to recent scientific findings regarding the ability of the human to distinguish between various smells (including complex overlapping of smells), then this post is for the reader to use as a springboard to read more into the scientific and non-scientific literature and form ones own opinion on the matter. In addition, you can perform your own experiment on yourself to test the findings. Or, simply just go back in time in memory and think about various odors that have been observed on a partner at a given time.

2.5 million sweat glands and chemical signaling?

As I mentioned above, I was sweating profusely on the ride home on my bicycle this evening which made me think of an article that I read in a newspaper. According to the ‘Los Angeles Times’ article titled "Sweat: A Cooling System That’s An Ancient Language Too," the human body has more than 2.5 million sweat glands across our surface. These glands provide an avenue for releasing large amounts of water (some people up to 3 gallons per day) which is surprising in itself–unless you are an athlete–wrestler, boxer, martial artist, etc.. I was not too surprised at this number–in fact I would not be surprised if this is a low value of the true number of total sweat glands. What caught my eye is contained in this excerpt from the article:

Sweat itself is 99% water, with traces of salts and metabolic wastes. When secreted onto the skin’s surface, sweat evaporates, taking heat from our bodies as it vaporizes and cooling the blood that flows beneath our skin. This evaporative cooling system is likely the reason that human bodies are nearly hairless. And it turns out we can thank our efficient sweat glands for the trait that makes us uniquely human: our big brains.

I started to wonder if the percentage of water varies based on diet and exercise regimen. When I go out for a ride–like the one that day–and I have not been on my bicycle in a short time–I tend to sweat a large amount of salts and other chemicals. The direct observation of not being able to see is the indicator along with wiping the sweat away and the next drop blinding me more than the previous drop. Therefore, I need to look into this number a little more and will report back in a future blog. The importance of the observation was more tied to the next excerpt below:

Humans have two types of sweat glands: eccrine glands, which are distributed evenly throughout the body, and apocrine glands, which are densely packed in the underarm, genital and nipple regions. Apocrine secretions are milkier than eccrine secretions and are friendlier to bacterial growth.

“Sweat doesn’t have much smell itself, but when apocrine secretions and microflora meet, it gives a unique smell,” says Denise Chen, professor of neurology at Baylor College of Medicine in Houston who studies the nuances of our unique body odors.

Interested in how married people seem to understand each other without words, Chen and colleagues recruited couples, then used armpit pads to collect sweat from each individual in different emotional states: fearful, happy, sexual or neutral. Next, individuals blindly judged the sweat samples of their partner as well as the opposite-sex strangers in the group. The study found that individuals were significantly more accurate in distinguishing their partner’s emotional sweat from his or her neutral sweat than they were in distinguishing the emotional sweat of a stranger. And their accuracy was directly related to how long they had been in the relationship.

The article suggests that humans give off chemical messages as a result of sweating throughout the day. Should this be surprising? Not really, research conducted a few years back found that sweat did indeed contain more than just water–including metabolites and other chemicals. If one takes a step back and just thinks about him/herself and their own various body scents during a given occasion (sex, stress, various emotional states, etc.) none of this should be surprising. How many people just avoid thinking about the topic all together and just hit the stick of deodorant for a ‘freshen up’ application layer? I know that I do occasionally during various moods to hide the chemical language that I am giving off.



At first glance what surprised me most was that my wife could distinguish (according to the study in the article) various moods by my odor. Of course, she has other indicators through other senses (visual and audio) that completes her assessment of her experiment. I started to think back to various time points in our relationships when I remembered distinct smells and tried to remember if I could have correlated that odor with a mood. As a disclaimer–I would not suggest starting to collect your spouse’s or partner’s clothing after various moods and writing observations down–as this act might raise a ‘red flag.’ I simply did a thought experiment with no actual verification on my own wife–just to clarify.



The last sentence of the above excerpt (second one) makes logical sense in that humans form memories of observation (of all the senses) at various times in our relationship. Further, the more we reinforce a given scent with a mood–(length of time in a relationship)–the easier it should be able to identify the scent. If you have ever gone wine tasting, your ability to distinguish scents is extremely powerful–even when scents are masked. With this last sentence in mind, I was led to think of a recent article that I read in the Journal ‘Science’ a couple of months ago. The research was concerning the span of odors that make up the range of the human olfactory system. How many different odors can humans distinguish between? And does this number have any correlation with the above statements regarding differentiation between spouse’s moods based on odor exlusively?

How Many Smells Can Humans Differentiate Between?

The human olfactory system is differs greatly from the other senses (visual and audio) in the range of differentiation of various smells. Recently, the realization has been researched into more depth by scientists in the United States and Europe in a collaborative effort as highlighted in a recent article in the Journal ‘Science’. The title of the article is "Humans Can discriminate More Than 1 Trillion Olfactory Stimuli". To really absorb the methodology of the experimental research, I would suggest pouring over the paper itself. In the article, the authors distinguish–quite coherently–the difference in between the various senses in terms of the range of the human sensory system (visual, audio, and odor). Here is an excerpt from the beginning, defining the problem:

To determine how many stimuli can be discriminated, one must know the range and resolution of the sensory system. Color stimuli vary in wavelength and intensity. Tones vary in frequency and loudness. We can therefore determine the resolution of these modalities along those axes and then calculate the number of discriminable tones and colors from the range and resolution. Humans can detect light with a wavelength between 390 and 700 nm and tones in the frequency range between 20 and 20,000 Hz. Working within this range, researchers carried out psychophysical experiments with color or tone discrimination tasks in order to estimate the average resolution of the visual and auditory systems. From these experiments, they estimated that humans can distinguish between 2.3 million and 7.5 million colors (1, 2) and ~340,000 tones (3). In the olfactory system, it is more difficult to estimate the range and resolution because the dimensions and physical boundaries of the olfactory stimulus space are not known. Further, olfactory stimuli are typically mixtures of odor molecules that differ in their components. Therefore, the strategies used for other sensory modalities cannot be applied to the human olfactory system. In the absence of a straightforward empirical approach, theoretical considerations have been used to estimate the number of discriminable olfactory stimuli. An influential study from 1927 posited four elementary odor sensations with sufficient resolution along those four dimensions to allow humans to rate each elementary sensation on a nine-point scale (4). The number of discriminable olfactory sensations was therefore estimated to be 94 or 6561 (4). This number was later rounded up to 10,000 and is widely cited in lay and scientific publications (5–7). Although this number was initially calculated to reflect how many olfactory stimuli humans can discriminate, it has also sometimes been used as the number of different odor molecules that exist, or the number of odor molecules that humans can detect. We carried out mixture discrimination testing to determine a lower limit of the number of olfactory stimuli that humans can discriminate.

From the research described in the paper, the number that was determined to accurately discriminate between odor mixtures of molecules was cast with a lower bound of 1 trillion. As discussed in the accompanying interview with the authors on the ‘podcast’ from the website ‘science,’ this number is most likely going to be a ‘low-ball’ estimate. The authors believe that in time the number will soar into the tens of trillions with emerging research in the future regarding odor discrimination. A reader of this blog might be asking “how did the researchers approach testing for odor discrimination in subjects. Here is an excerpt describing the thought behind the actual experimentation (which I leave to the reader–to access the Journal Article and devour at one’s own speed) below taken from the article:

Natural olfactory stimuli are almost always mixtures of large numbers of diverse components at different ratios. The characteristic scent of a rose, for example, is produced by a mixture of 275 components (8), although typically, only a small percentage of components contribute to the perceived smell. We reduced the complexity by investigating only mixtures of 10, 20, or 30 components drawn from a collection of 128 odorous molecules (table S1). These 128 molecules were previously intensity-matched by Sobel and co-workers, which enabled us to produce mixtures in which each component contributes equally to the overall smell of the mixture (9). The 128 molecules cover much of the perceptual and physicochemical diversity of odorous molecules (10–12) because the collection contains most of a collection of 86 odorous molecules that were selected to be well distributed in both perceptual and physicochemical stimulus space (9).

The researchers tested a variety of mixtures to see the ability of the volunteers to distinguish between mixtures with a varying amount of “overlapping” of odor molecules. By this, the researchers varied the percentage of different odor molecules in each mixture. From this research, the reader then should not be surprised at the above results reported in the study highlighted in the ‘Los Angeles Times’ newspaper. One might be asking the obvious question “Who cares?” The importance of the research introduced above lies in the ability of scientists to push the boundaries of hypothesizing and framing the problem. In this study, a mixture of two odors would compose a single dimension. On can imagine then a multidimensional mixture of 20 different odors. That is, the resolution is cast into multiple dimensions when compared to single dimensions when studying other senses. That is, when one is studying an audio problem, the dimensional analysis is defined in terms of ‘tones’–which is cast into a single dimension. Similarly, in a visual study, the single dimension is defined in terms of wavelength of light. Before this study, either of these senses were cast into a single dimensional space.



Science has progressed to expand our thinking of odor discrimination and continues to grow. The research highlighted in both examples above is fascinating and shows the rapid improvements in study design and experimentation along with the interpretation of the given results. Further, when one is out in nature from now on, there will be extra dimensions to explore with your sensor (your nose). I was enlightened by learning about the above research and the new found knowledge definitely changed the experience of my ride home last Thursday. In addition to odor discrimination, there is a large amount of untapped knowledge regarding chemical sensors/signaling which is encountered in nature on a day to day basis. This should be motivation enough to continue to push the boundaries of designing/building better sensors as technologies progress to try to match a small fraction of the ability of the human sensory system. There is no doubt that scientists have a LONG way to go before competing with the human sensory system. At the same time, current (progress) results are exciting and should be celebrated both in and out of the laboratory.