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It’s been established that people who spend more time in parks and other natural settings tend to report higher levels of health and happiness, but new research shows there’s actually a magic number for it.

According to a study published this week in the journal Nature Scientific Reports, spending 120 minutes a week strolling a tree-lined street or sitting by a lake can greatly enhance a person’s overall sense of well-being. Less time didn’t yield any significant benefit, the research showed.

Those who got in two to three hours in nature were about 20 percent more likely to report high overall satisfaction with their lives than those who spent no time outdoors at all. The benefits to physical health were even greater, with those who met the outdoors benchmark being 60% more likely to report being in good health than their cooped-in counterparts.

The figures were adjusted for a number of characteristics known to influence health and happiness, including socioeconomic factors, neighborhood characteristics and general demographics.

People who already spend a lot of time outdoors aren’t likely to find these results surprising: There’s already a substantial body of work linking green spaces to lower risks of heart disease, diabetes, obesity, asthma, mental health problems and overall mortality; and to greater rates of health, happiness, and cognitive development in children.

But most of these studies simply measured the physical characteristics of neighborhood environments. They didn’t actually ask people how often they experience the natural world to create a gauge of nature exposure at the individual level.

That’s exactly what the current research does, using a nationally representative sample of 20,000 people living in England. The authors note their approach is similar to what governments have used in the past to develop physical activity guidelines for adults and children. They envision the creation of similar guidelines around exposure to nature.

Overall, they found, two hours or more of nature exposure had a significant impact: Its positive effect on an individual’s health and well-being was comparable to getting recommended amounts of exercise or of living in a high socioeconomic status area versus a low-status one.

They stress, however, that the effect is not necessarily a causal one. Though researchers controlled for a wide range of variables known to affect health and happiness, the study’s design didn’t allow them to completely rule out other factors that could result in higher health and happiness for nature lovers.

It may be the case, for instance, that people who are more inclined to be physically active and have a positive outlook on life are more likely to seek recreation opportunities outdoors. It may also be the case that being outside in nature, which typically involves a lot of moving around, may serve as a proxy for physical activity overall.

However, the authors note that other studies have demonstrated the benefits of being outside even in the absence of physical activity. Research in Japan, for instance, found that simply sitting passively in a natural environment can confer benefits to physical and mental health. Other research has shown that exercising outdoors provides a boost to mental health above and beyond what you’d get from doing the same exercise inside.