Mathew White, an environmental psychologist, is on a mission to give Mother Nature the respect he thinks she deserves when it comes to human health. For decades, scientists and health-care professionals have recognized that exposure to green spaces, such as public parks or forests, is linked with lower risks of all sorts of illnesses common in the world. Experimental work has demonstrated various physiological responses that occur when people spend time in natural environments: blood pressure drops, heart rate decreases, immune function improves, and the nervous system directs the body to rest and digest.
As humans increasingly populate urbanized areas, they are spending less and less time in natural environments. But before doctors can start advising their patients to head to the nearest park, there is an important outstanding question, says White: How much time in nature do you need to generate these apparent benefits? Most of the research that has linked health outcomes with exposure to the natural world didn't use frequency or duration of park visits, but rather the amount of green space within a certain distance of a person's home, White says. But "it's not so much where you live; it's whether you use it or not."
So he collected data to estimate what dose (剂量) of nature was needed to show benefits to a person's health. White's group found the answer he was after: Spending at least two hours in nature per week was strongly correlated with self-reports of being in good health or having high wellbeing. "I was very surprised, to be honest," says White, who had been expecting a much longer time. "We had no idea that such a clear threshold of time per week would emerge from the data."
He was further surprised to learn that it didn't seem to matter how many trips to a park people took, so long as they got in their two hours per week. It could be a long visit one day, a couple of hour-long trips, three visits of 40 minutes, or four half-hour excursions. He and his colleagues speculate that, if nature's apparent health benefits are a result of being able to de-stress, then whatever pattern of green space exposure fits one's schedule is probably the best way to achieve that goal.
Health-care recommendations for people to spend time in nature are probably years away, but the movement has begun. Several organizations around the world are working to promote awareness of nature's contribution to health. Some researchers have used the term "a dose of nature" to evaluate the amount of exposure needed to gain benefits. "That was kind of the deliberate medicalization of the language around nature and health," says White.
