In a well-designed laboratory study, short-term exposure to essential oils from two tree species, Douglas fir and cypress, had no significant effects on mood, psychological stress, or cognitive performance. In a study published in , researchers noted that most participants could not reliably identify the scent they were smelling. Journal of Environmental Psychology.
Research consistently shows that being outdoors, especially in the forest, improves mood, reduces stress, and improves mental performance. But as cities grow and green space shrinks, scientists are increasingly interested in whether certain elements of nature that provide these benefits can be recreated indoors.
One candidate that has not yet been considered is smell. Forests are rich in airborne molecules released by trees, including substances called terpenes, which have been linked in previous studies to reduced stress and improved immune function. However, early studies examining cognitive ability were very small and yielded inconsistent results.
The researchers behind this new study wanted to test the effects of wood scents more rigorously and on a larger sample, while also exploring whether scent familiarity matters.
The team, led by Djo Juliette Fischer from the University Clinic Hamburg-Eppendorf, together with Simone Kühn from the Max Planck Institute for Human Development, carried out two related studies in Germany. In the first study, 68 participants (mean age 28.2 years, 53% female) were randomly assigned to either the Douglas fir group or the cypress group.
Each participant completed the same test on two separate days. The first time they diffused tree oil into the room, and the second time they used plain water as a placebo. They were done in random order. This design allowed the researchers to test whether odor familiarity is important, since Douglas fir is a common scent in German forests, while cypress is native to Japan.
When the first study yielded a small hint of an effect of the Douglas-fir group on one of the vigilance measures, the second study added an additional 34 Douglas-fir participants to increase statistical power. In both studies, participants completed seven cognitive tasks spanning working memory, attention control, task switching, inhibition, vigilance, and executive control, along with mood and stress questionnaires.
The scent of neither tree had a significant effect on the results. Initial hints of a vigilance effect by Douglas fir disappeared when larger combined samples were analyzed. Importantly, additional statistical tests not only found no effect, but also provided strong evidence that no meaningful effect exists.
One potentially important finding came from a small subgroup. In the second Douglas fir study, only about 15% of participants correctly identified the scent as coming from a tree or forest. Examining the 14 participants who accurately identified the origin of the scent in both studies, the researchers found tentative hints of reduced fatigue and sharper inhibitory performance. This suggests that conscious recognition of odors as associated with nature may be necessary for them to cause psychological effects.
As the researchers note, “most participants were unable to reliably identify the odors, suggesting limited conscious awareness and semantic associations.” In fact, in the second study, most participants who noticed any odor described it as citrusy, floral, or detergent-like.
Several aspects of the study design may have limited the opportunity to detect an effect. For example, the researchers introduced the diffuser to participants simply as a humidifier, without making it clear that the study was about scent. Additionally, the 75-minute cognitive battery may have caused mental fatigue, which may have hidden subtle benefits. Previous studies that have shown stress-reducing effects have typically had participants sit and inhale the oil for a short period of time without causing mental strain.
The researchers also note that essential oils from individual trees cannot fully recreate the chemical richness of real forest air, and that the simple laboratory environment did not provide any visual or contextual cues that could help people associate scents with nature.
The study, “Wood Scent: Investigating the Short-Term Effects of Two Tree Essential Oils on Mood, Psychological Stress, and Cognition,” was authored by Djo Juliette Fischer and Simone Kühn.
