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.