Smell Maps of Cities

Daniele Quercia and colleagues have published research aimed at mapping the smells of entire cities [2]. They want to analyze social media to detect recent descriptions of smells to create city wide maps of what people are smelling in different places. To do this, they needed to create a dictionary of terms for smells.

They authors are mainly concerned with aesthetics, not with chemical analysis of the air or sources of smells.  They are concerned with “the positive role that ‘smell’ as opposed to ‘air pollution’ can play in the environmental experience” ([2], p.334) They comment that there is little work on this topic, so they hope to “enrich the urban smell toolkit” ([2], p.327)

The study collected residents’ reports of what they smell, and clustered similar terms to form a dictionary of smells (i.e., of concepts about smells). This was also correlated with existing dictionaries of smell terms.

Smell terms from geotagged social media entries were used to create maps of smells across the city. The researchers suggest that there are different spatial scopes for the smells, from broad to very localized. They call these “base notes”, “mid-level notes”, and “high notes”, an to perfume advertisements that is pretty shaky in this case.

The resulting maps seem to capture coarse features of the city (e.g., industrial concentrations, large food market), and are slightly correlated with air quality measures.

The main implications are public awareness of the cityscape, and perhaps an increased attention from urban designers.

Perhaps.

I found this study to be competently done, but not really useful. It borders on what my statistics teachers would call a “Type 3 Error”: they may be asking the wrong question.

Their concept of “smells” seems to be rather questionable.

First of all, as they generally acknowledge, the sense of smell is rather complex. There are large individual differences, not just the demographic variation they mention. Worse, smell is highly affected by both short term and long term experience. People habituate to smells rapidly, and learn over time. Our sense of smell changes as we age, as well as due to illness, exercise, and other activities. For that matter, we wear scents and scented clothing, that form a private smellscape right under our own nose.

The study worked hard to cluster the words people use to describe smell, but this exercise in linguistics brings in  a slew of factors of culture and learning. These cultural and cognitive elements are definitely relevant to their interest in urban experiences, though describing smells isn’t simply perceiving smells. As the paper notes, there are quite a few contextual factors that may go into how a smell is perceived and described.

For example, I suspect that future studies might find that people report more negative words for how the “bad parts of town” smell, compared to areas they prefer, regardless of the objective chemistry of the air.  (This might be called “the New Jersey effect”.)

The terms chosen for a given smell also reflect personal and cultural contexts. For example, the smell of human sweat can be attractive or awful, depending on the people involved and the situation.

This study treats smells as rather permanent and large grain features, though they are ephemeral and subject to micro weather, e.g., wind direction. They do enumerate very localized “high notes”, though these smells can be extremely localized, detectable only within a meter or less, which cannot be represented on their maps.

In other words, the maps are coarse-grained in both time and space. Perhaps this level of analysis is useful for urban design, but it is certainly not self-evident just how much this matters. The reported low correlations with other measures probably reflects this overly coarse granularity of the geotagged terms and the other measures.

This methodology is primarily about outdoor smell. But urban experiences are mainly indoors, and indoor smells are a totally different animal. Sure, the open air farmer’s market smells wonderful for that 15 minutes I am passing it, but when I go inside, I can’t smell it any more, no matter where I am on the city map.

The researchers make an interesting point about wanting to create attractive urban spells, not just mitigate pollution and repellant odors. On the other hand, this work shows little reason to think that this kind of analysis is an accurate measure of harmful pollution, despite what the authors may sometimes claim in the popular media. I’m all in favor of my city smelling nice (at least to some people), but it is important to monitor and reduce dangerous pollution, much of which cannot be seen or smelled.

This method is not, I repeat, not a good way to monitor the air quality of a city.


  1. Matt McGrath, Can city ‘smellfies’ stop air pollution? . BBC News.March 10 2017, http://www.bbc.com/news/science-environment-39179098
  2. Daniele Quercia, Rossano Schifanella, Luca Maria Aiello, and Kate McLean, Smelly Maps: The Digital Life of Urban Smellscapes. International AAAI Conference on Web and Social Media; Ninth International AAAI Conference on Web and Social Media:327-336, 2017. http://www.aaai.org/ocs/index.php/ICWSM/ICWSM15/paper/view/10572/10516

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