Smelling your way around a big city can be a hazardous undertaking. In just a few steps, pleasant aromas of flowers and food can give way to car exhaust, ripe garbage and pee-soaked dog parks.

Even so, a group of brave people is sniffing their way around several global cities in order to create a series of "city smell maps." The researchers, who come from institutions including Cambridge University and the Web company Yahoo, have so far created maps for London and Barcelona.

The results are interesting, and kind of pretty. For example, here's a "smelly map" of London, with "emissions" smells, which the researchers describe as smells of car exhaust, gasoline and dust, marked in red and nature smells, which include flower, grass and soil, marked in green. Unsurprisingly, the auto emissions smells follow the city's major roadways, while the nature smells are concentrated in parks and green spaces.


Daniele Quercia, Rossano Schifanella, Luca Maria Aiello, and Kate McLean

And here is a map of "animal smells," which are unsurprisingly concentrated in London's zoo and in parks. The animal smells include smells of skunk and horse.


Daniele Quercia, Rossano Schifanella, Luca Maria Aiello, and Kate McLean

Compared with London, Barcelona has a different, and arguably better, smell map. Here is the same map for Barcelona, with exhaust smells marked in red and nature smells in green:


Daniele Quercia, Rossano Schifanella, Luca Maria Aiello, and Kate McLean

Smell is hard to record, analyze and depict visually. So to make these maps, the researchers first created what they call a "smell dictionary" with the help of volunteers around the world. They asked dozens of residents in seven cities in Europe and the U.S. -- Amsterdam, Pamplona, Glasgow, Edinburgh, Newport, Paris and New York -- to conduct “smell walks,” in which they walked around, identified distinct odors and took notes on what they were smelling. The researchers took the words that were mentioned by two or more locals and used them to compile a small dictionary. Some of the more common terms are listed below:


Daniele Quercia, Rossano Schifanella, Luca Maria Aiello, and Kate McLean,

The researchers then experimented with using social media to fill in their maps. For London and Barcelona, the researchers searched for the smell words on geo-tagged social media posts, including on Twitter, Instagram and Flickr, and then used them to create maps.

Here's a more technical breakdown of how the smell categories compared for London and Barcelona:


Daniele Quercia, Rossano Schifanella, Luca Maria Aiello, and Kate McLean

The researchers argue that smell has a big impact on our behavior, attitudes and health. Humans can detect up to a trillion smells, and smell is one of our most evocative senses. Yet the smells of cities have been almost entirely overlooked by both urban planners and scientists.

Many people have studied how to improve the visual elements of a city's design, but few have considered its aromas. When we do think about city smells, it's usually in a negative context -- garbage or smog. But that doesn't have to be true, the researchers say. City smells are a lot more diverse than people think, and they can play a positive role in urban life.

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