Does the smell of pine make you smarter?

Short-term exposure to essential oils from two tree species — the Douglas fir and the Hinoki cypress — produced no significant effects on mood, psychological stress, or cognitive performance in a well-designed laboratory study. Researchers noted that most participants could not reliably identify the scent they were smelling, according to research published in the Journal of Environmental Psychology.

Research consistently shows that being outdoors — especially in forests — improves mood, reduces stress, and sharpens mental performance. But as cities grow and green spaces shrink, scientists are increasingly interested in whether the specific elements of nature that drive these benefits can be recreated indoors.

One underexplored candidate is smell. Forests are rich with airborne molecules released by trees, including substances called terpenes, which have been linked to reduced stress and improved immune function in prior studies. However, those earlier studies exploring cognitive performance were very small and produced inconsistent results.

The researchers behind this new study wanted to test the effects of tree scents more rigorously and in a larger sample, while also exploring whether the familiarity of a scent matters.

Led by Djo Juliette Fischer of the University Clinic Hamburg-Eppendorf, together with Simone Kühn of the Max Planck Institute for Human Development, the team conducted two linked studies in Germany. In the first study, 68 participants (mean age 28.2 years, 53% female) were randomly assigned to either a Douglas fir group or a Hinoki cypress group.

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Each participant completed the same tests on two separate days — once with the tree oil diffusing into the room, and once with plain water as a placebo — in a randomised order. Because the Douglas fir is a scent common to German forests while the Hinoki cypress is native to Japan, the design allowed researchers to test whether odour familiarity mattered.

When the first study produced a marginal hint of an effect on one vigilance measure in the Douglas fir group, a second study added 34 more Douglas fir participants to boost statistical power. Across both studies, participants completed seven cognitive tasks spanning working memory, attention control, task-switching, inhibition, vigilance, and executive control, alongside mood and stress questionnaires.

Neither tree scent produced any significant effects on any outcome. The initial hint of a vigilance benefit from the Douglas fir vanished once the larger combined sample was analysed. Crucially, additional statistical testing did not merely fail to find an effect — it provided strong evidence that no meaningful effect existed.

One potentially telling finding came from a small subgroup. Only around 15% of the participants in the second Douglas fir study correctly identified the scent as coming from a tree or forest. When looking at the 14 participants across both studies who correctly identified the origin of the scent, the researchers found tentative hints of reduced fatigue and sharper inhibition performance. This suggests that consciously recognising a smell as nature-related may be necessary for it to trigger psychological benefits.

As the researchers note, “most participants could not reliably identify the odour, suggesting limited conscious awareness and/or semantic associations.” Indeed, most participants in the second study who noticed any smell at all described it as citrusy, floral, or like a cleaning product.

Several aspects of the study design may have limited the chances of detecting an effect. For example, the researchers did not explicitly tell the participants the study involved scents, introducing the diffuser simply as a humidifier. Additionally, the 75-minute cognitive battery likely induced mental fatigue that could have masked subtle benefits. Previous studies showing stress-relief effects typically had participants sit and inhale the oil for a short period without any mental demands.

The researchers also note that individual tree essential oils cannot replicate the full chemical richness of real forest air, and the plain laboratory setting offered none of the visual or contextual cues that might help people associate a scent with nature.

The study, “Scent of trees: 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.