Aplethora of studies over the past few decades has indicated that spending time in natural outdoor environments is associated with benefits such as reduced stress and improved mental health. But questions remain about whether such benefits are primarily attributable to features of green environments themselves, or are instead gained indirectly through the way people use these spaces. Compared with the time they spend in busy cities or indoor environments, for example, people in natural environments may be more likely to exercise or socialize with friends or neighbors—activities that are in turn associated with benefits to human health.

It’s a topic that has long interested Mark Nieuwenhuijsen, an urban planning and environment researcher at Barcelona Institute for Global Health. In 2012, he helped launch the long-running project, PHENOTYPE (Positive Health Effects of the Natural Outdoor environment in TYPical populations in different regions in Europe), to...

In one recently published study, the PHENOTYPE team analyzed data collected on time spent in green spaces, plus information about physical activity, socialization, and stress relief. Previous research had suggested that these mechanisms play a role in human health, says Nieuwenhuijsen, “but there was little actual evidence.”

The researchers collected questionnaire responses from nearly 4,000 people in four cities: Barcelona, Spain; Doetinchem, the Netherlands; Kaunas, Lithuania; and Stoke-on-Trent, UK. They also analyzed maps of the cities derived from satellite data to assess the amount of green space near each respondent’s address.

Sometimes you have more physical activity with green space, but other times it does not make a difference.

—Mark Nieuwenhuijsen, Barcelona Institute for Global Health

Overall, the researchers found that people who reported spending more time outdoors in nature also reported spending more time engaging in physical activity and socializing with neighbors. Those same people also tended to have higher mental wellbeing, as measured by self-reported anxiety levels and mood. In line with previous research, “we see a consistent picture showing that green space is good for health,” says Nieuwenhuijsen.

However, the link between time spent in nature and physical activity was less pronounced than the researchers had expected. For every additional hour spent in natural outdoor environments per week, physical activity increased by only around 78 seconds, suggesting that the health benefits of natural environments might be coming primarily through other mechanisms. “In contrast to what we thought before, the evidence appears to be more consistent and stronger for stress reduction, restoration and mental health, and social contacts than for physical activity,” says Nieuwenhuijsen. 

He adds that the relationship between natural environments, health, and physical activity may vary depending on the features of the environments themselves. “Other studies have shown that sometimes you have more physical activity with green space, but other times it does not make a difference,” notes Nieuwenhuijsen. “It may depend on the type and quality of green space.” 

Certainly, the four cities studied in his team’s recent project varied widely in the amount and accessibility of green spaces. For example, in Stoke-on-Trent and Doetinchem, almost 90 percent of respondents had their own garden, while in Barcelona, fewer than 11 percent did.

Other differences between the four cities were also “sometimes extremely large,” says Sjerp de Vries, an environmental psychologist at Wageningen University & Research in the Netherlands who was not involved with the study. He notes that in Doetinchem, for example, just 4 percent of respondents considered themselves in “fair to poor” health, while 62 percent of people in Kaunas said the same. “This makes me wonder about possible cultural differences, especially in the interpretation of the survey questions and the answering options provided,” he says.

As with any observational study, the research doesn’t allow the scientists to determine cause-and-effect links between natural environments, activity, and health. “You do not know what came first,” says Wilma Zijlema, a postdoc in Nieuwenhuijsen’s lab and a PHENOTYPE researcher who was not involved with this particular study. Does spending more time in nature make people healthier, or do healthier people spend more time in nature? 

One way to help tease out the answer could be to conduct longitudinal studies and examine people’s health, and potential mechanisms that accompany time spent in green space, over many years. But this “offers only a partial solution, as it is often challenging to determine the optimal time lag between two rounds of data collection,” says Angel Dzhambov, an epidemiologist at the Medical University of Plovdiv in Bulgaria who was not involved with the study.

In the meantime, the PHENOTYPE project researchers are exploring other ways to gather data on how people make use of green spaces. In another recent study, the team tracked the movement, location, and self-reported mood of 400 smartphone-using volunteers in the same four cities for one week. “We collected objective data of people’s physical activity and location,” Zijlema says, adding that this data collection method, which is less susceptible to bias, could provide a “great benefit” over self-reported data. 

Although analyzing these mechanisms is tricky, it’s crucial to get a better understanding for public health and green space planning, says Dzhambov. “Obtaining an estimate of the relative contribution of each of these pathways to the overall beneficial effect is a cornerstone of planning heath-enhancing green systems and getting the most out of them.”

Emily Makowski is an intern at The Scientist. Email her at emakowski@the-scientist.com.

This story is part of a series by The Scientist on how natural environments affect human health. 

Click through to find out more about some of the other proposed mechanisms linking spending time in nature to physical and mental health benefits.

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