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Every autumn, the windows close and probably stay closed until spring. That might seem like a healthy thing to do, but by keeping nature "out," we're actually leaving out a lot of what makes us healthy in the first place. Our skin and gut are built to interact with the microbes that come with it, not to go without them for six months.
You might already know that our name comes from a Japanese word: shinrin-yoku, which means "forest bathing." Not just because it sounds nice on a label, but because it's the closest description for what YOKUU does: bring that same contact with nature back into your home.
Where does shinrin-yoku come from?
Japan's Ministry of Agriculture, Forestry and Fisheries coined the term in 1982, as part of a public health response to urbanization and rising stress-related illness. It describes spending time immersed in a forest atmosphere, without a specific goal. Early Japanese research from that period measured lower cortisol, lower blood pressure, reduced sympathetic nervous system activity, and increased natural killer cell activity in people who practiced it.
The biodiversity hypothesis
Finnish researchers later studied why nature has this effect on the body, not just the mood.
Allergologist Tari Haahtela's biodiversity hypothesis offers an answer: human skin and gut are built to interact with a wide range of environmental bacteria. Contact with microbially rich natural environments reduces the risk of immune-mediated diseases like atopy, allergies, and asthma. Modern homes and cities simply don't offer that range of exposure anymore.
The idea isn't theoretical. It grew out of the Karelia study, which compared allergy and asthma rates on either side of the Finnish-Russian border, in populations with a shared genetic background but very different environments. Same genes, different air, different rates of disease.
The daycare study
The clearest test of this came in 2020. Researchers led by Marja Roslund at the University of Helsinki wanted to see what would happen if you gave a city environment its biodiversity back.
They replaced gravel in some Finnish daycare yards with real forest floor: undergrowth like dwarf heather and blueberries, plus planter boxes for crops. Other daycare yards were left untouched as a comparison. Children in the forest-floor daycares played there five times a week for 28 days, and researchers tracked their skin and gut bacteria before and after.
The results were clear. Children at the intervention daycares developed more diverse bacterial communities on their skin, their gut bacterial diversity shifted favorably, and markers of immune regulation improved. The daycares that kept their gravel yards showed no comparable change over the same period.
Direct contact, same result
You don't actually need a whole daycare yard, it turns out. A separate Finnish study tested direct, short-term contact only.
In one experiment, participants had direct hand contact with nature-based material. In another, they simply wore fabric packets containing Sphagnum moss. Both raised skin bacterial diversity within a single short exposure, no month-long intervention required.
Bringing it indoors
The most recent step goes further still. In 2025, researchers built a microbial extract from Finnish plant and soil material, containing over 600 distinct bacterial species and their metabolic products, designed to bring natural biodiversity indoors.
They tested it properly: a randomized, double-blind, placebo-controlled trial, 142 adults with atopic dermatitis. The group using the microbial extract needed less medication to manage their symptoms than the group on placebo.
Why YOKUU exists
YOKUU is built on the same underlying principle these studies point to. You don't have to move to a forest to get some of what a forest does. You just have to stop keeping your home microbially empty behind that closed window. That's what 1 billion cleaning helpers are for.
Bring the outside in.
Quick answers
What does shinrin-yoku mean?
It's Japanese for "forest bathing," coined in 1982 by Japan's Ministry of Agriculture, Forestry and Fisheries to describe spending time immersed in a forest atmosphere.
Is there real science behind forest bathing?
Yes. Early Japanese research measured lower cortisol and blood pressure. Finnish research since then has traced the effect to skin and gut microbiome diversity.
What is the biodiversity hypothesis?
The idea, from Finnish allergologist Tari Haahtela, that contact with microbially rich natural environments protects against immune-mediated disease, and that modern indoor life removes that contact.
Can you get the benefits of forest bathing indoors?
A 2025 clinical trial found that a microbial extract made from Finnish forest soil and plant material reduced medication needs in adults with eczema, suggesting the exposure itself, not just being outside, is what matters.
Where does YOKUU's name come from?
YOKUU takes its name from shinrin-yoku, the Japanese ritual of forest bathing.
Sources: Roslund et al. (2020), Science Advances. Hui et al. (2019), skin microbiota study. Haahtela, T., Karelia Allergy Study and biodiversity hypothesis. 2025 medRxiv preprint, microbial extract trial in adults with atopic dermatitis (n=142).
