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Indoor greenery enhances well-being, but new research shows that too much can overwhelm people. The findings could guide the design of healthier indoor environments.

If time outdoors in nature were a pill you could take, the label would read "take as needed." A dose of nature, whether it's a short walk or a weeklong camping trip, reliably improves human well-being. But a new study by Stanford engineers, in the journal Sustainable Cities and Society, found that the relationship between indoor nature and well-being is more nuanced.

The researchers developed a new method to quantify varying doses of greenery and wood materials, and tested their effect on measures of well-being. While a simulated office with a handful of potted plants and a window with a view of trees enhanced participants' perceived restoration and sense of belonging, they became stressed when the greenery grew more crowded.

"I wasn't expecting this at all," said the study's lead author, Eva Bianchi, a recent Stanford civil engineering Ph.D. graduate. "Around 60% total greenery and wood had the highest stress increase, which was contradictory to prior work."

Measuring indoor nature doses

Past research has found that indoor nature, including potted plants and natural materials, reduces measures of stress while increasing feelings of restoration and belonging. But these studies either test people's response to nature versus no nature or use subjective levels of greenery, such as small, medium, and large amounts of plants.

"There's such a huge body of literature, but we can't actually synthesize it, because no one's using the same methods," said senior author Sarah Billington, the UPS Foundation Professor at Stanford and chair of the Department of Civil and Environmental Engineering.

The Stanford researchers wanted to see how people responded to varying "doses" of indoor nature. To help answer this question, Bianchi developed software called the Nature View Potential tool, which is presented in an published in the Journal of Building Engineering. It calculates how much nature you see, on average, inside a given space. Imagine standing in a room with rays extending from your eyes. "The tool is calculating how many of those rays that are coming from your eyes are seeing natural elements," said Bianchi.

Using the nature viewing tool along with 3D modeling software, the researchers designed 11 digital conference rooms with varying levels of wood, greenery, and nature views out the window. They enrolled 412 participants via a crowdsourcing platform and assigned them randomly to the rooms. The participants were told, essentially, "This is your new workplace," said Bianchi, "and we're going to do a bunch of professional development tasks."

Next, the participants were unknowingly introduced to stressors so researchers could see if the natural features eased tension. Before starting their tasks, participants filled out a questionnaire that gauged several measures of well-being. Then, they completed tasks that included difficult anagrams and counting down from 1,022 to zero in increments of 13. After, to add more pressure, "I told them they had scored below average and that they weren't going to get paid," said Bianchi, and they filled out the questionnaire again (all participants were paid in the end).

Getting greenery levels just right

A greenery dose of about 20% had the strongest response in restoration and sense of belonging for participants, the researchers found. That's like standing in a room and seeing plants 20% of the time as you look all around. "Any nature helps, but if you really want to see an improvement, you have to get up to around that value," said Billington.

But in the room with the most nature, 60%, the looming plants, living wall, and wooden ceiling seemed to elevate stress. The researchers suspect that too much nature may lead to people feeling overwhelmed in indoor spaces. Some of the participants were conscious of this, writing "too many plants" and "probably a nightmare to try to get anything done" in comments on the space.

Additionally, wooden furniture and features alone had no effect on the well-being measures. The researchers think this could be a limitation of the rendering. Previous research by Billington using and found well-being benefits associated with wood materials; perhaps the digital wood in this study was not realistic enough.

The findings underscore a dose-sensitive response to indoor nature—there can, in fact, be too much. Plus, the study highlighted that connection to nature was the mediating factor between indoor nature and well-being. If a study participant reported an increased feeling of connection to nature, they reaped more well-being benefits from the greenery.

But when participants didn't feel that connection, the natural features had little effect on them. "You don't just put a bunch of nature inside the space," said Bianchi. "You make sure that the nature that you're putting inside the space will successfully make the occupants feel connected to nature."

In the future, the researchers hope designers can use the Nature View Potential tool to maximize the benefits of different types of indoor spaces, from housing to schools to workplaces.

Billington and her students are also looking closer at what nature means to individuals and how that might vary. The ultimate goal is to find ways for people to reap the restorative qualities of nature even amid increasing urbanization. "How do you get the feeling of nature indoors?" she said. "That's why it's so important to have some kind of quantification … so we can compare across studies and make progress faster."