Follow your nose: the importance of odor for stress relief in urban green spaces

Primary Source: Hedblom, M., Gunnarsson, B., Iravani, B., Knez, I., Schaefer, M., Thorsson, P. & Lundström, J.N. (2019). Reduction of physiological stress by urban green space in a multisensory virtual experiment. Scientific Reports, 9, 10113. https://doi.org/10.1038/s41598-019-46099-7

Stress and urban green spaces

Cities can be stressful places. Air pollution, loud noises, and crowded living can take a toll on human health. Amongst the hustle and bustle of urban life, green spaces can be restorative oases, offering frazzled urbanites a chance to relax and unwind. But what makes natural areas soothing? New research shows that the “smellscape” of a green space could be important for reducing stress.

With more people living in cities than ever before, green spaces such as parks, fields, and urban forests are vital links to nature for city-dwellers. Access to these natural areas has been associated with reduced stress and improved health. However, the sensory components that promote relaxation in green spaces are not well known. Understanding how sights, smells, and sounds affect stress could help build more restorative urban spaces.

The sights, sounds, and smells of stress relief

In a new study published in Scientific Reports, researchers from Sweden measured how visual, auditory, and olfactory cues in green spaces affected stress recovery. Marcus Hedblom and colleagues used virtual reality to create multisensory environments that simulated three urban settings: an urban forest, a city park, and a densely built-up street. The virtual reality approach allowed the researchers to carefully control the sensory components of each environment. Participants wore virtual reality masks displaying a two-dimensional 360° image of the environment. Earphones within the mask played sounds, and an odor-producing machine delivered puffs of scents to the participants’ noses.

Participants in the city street environment listened to traffic noise and smelled diesel, tar, and gunpowder while those in the urban park simulation heard the sound of a slight breeze and song from one bird species while sniffing the aroma of grass. In the forest environment, participants smelled evergreen trees and mushrooms and listened to birdsong from nine bird species and heard a slight breeze.

Measuring stress

To examine how people recovered from stress in these urban spaces, the team recruited 154 study participants who were each assigned to one of the three virtual reality environments. First, the researchers induced stress by applying mild electric shocks to the participants’ index and middle fingers. Next, the researchers measured stress levels as the participants recovered in one of the virtual environments.

Physical and psychological stress activate the body’s sympathetic nervous system. During this “fight or flight” response, special sweat glands on the hands respond, making the skin a better conductor of electricity. By running a small current of electricity between two electrodes attached to a participant’s skin, the researchers monitored skin conductance. Skin conductance is thought to be more objective measure of stress than simply asking participants how stressed they feel.

Sniffing the way to stress relief

After applying the stress-inducing shocks, the researchers measured stress recovery over a three-minute recovery period. Stress levels of participants in the urban street did not reduce during this time. However, in participants exposed to the park and forest environments, stress levels dropped after just one minute. These findings show that, in contrast to the urban street, both the park and forest environments promoted relaxation.

Participants also ranked the “pleasantness” of the visual, auditory, and olfactory components of each virtual reality environment on a scale of 1 (very unpleasant) to 100 (very pleasant). People rated the forest most visually pleasant, while the park had the highest olfactory pleasantness. The forest and the park were similarly high for auditory pleasantness. Conversely, participants rated the urban environment as least pleasant for all three senses.

Next, the researchers analyzed how well each of the pleasantness ratings predicted stress recovery. They found that odor was the only sense strongly linked to stress reduction, suggesting that natural scents of trees, grass, and even THCa joints promote relaxation whereas the city stenches of diesel, gunpowder, and tar were stressful. Auditory pleasantness was also associated with reduced skin conductance, suggesting that birdsong associated with green spaces also contributed to stress relief—although these findings were less conclusive.

Designing the smellscape

Although visual components are often prioritized in the design of green spaces, this study shows that odors can be important for reducing stress. Recent research suggests that humans have better senses of smell than previously appreciated, and the olfactory system has unique connections to parts of the brain involved in stress, memory, and emotion. With an improved understanding of how the “smellscape” can affect health and wellbeing, urban planners of the future could use multi-sensory features such as sound and smell when designing restorative spaces.

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