But how long do the feel-good effects of nature last? Are they simply wiped out by the first traffic jam or cell-phone ditty?

One of Miyazaki’s collaborators, Qing Li, an immunologist in the department of hygiene and public health at Nippon Medical School in Tokyo, had the same question. The chairman of the Society of Forest Medicine, a small but growing international group of academics, Li is interested in nature’s effect on the human immune system. A person’s natural killer immune cells (NK cells for short) can, like cortisol and hemoglobin, be reliably measured in a lab. A type of white blood cell, NK cells are handy to have around, since they send self-destruct messages to tumors and virus-infected cells. It’s been known for a long time that factors like stress, aging, and pesticides can reduce your NK count, at least temporarily. So, Li wondered, if nature reduces stress, could it also increase your NK cells and thereby help you fight infections and cancer?

In 2005 and 2006, Li brought a group of middle-aged Tokyo businessmen into the woods. For three days, they hiked in the morning and again in the afternoon. By the end, blood tests showed that their NK cells had increased 40 percent. A month later, their NK count was still 15 percent higher than when they started. By contrast, during urban walking trips, NK levels didn’t change.

Since most of us can’t spend three days a week walking in the woods, Li was curious to know if a one-day trip to a suburban park would have a similar effect. It did, boosting the levels of both NK cells and anticancer proteins for at least seven days afterward.

What was going on? Li suspected that trees were important. Specifically, he wondered if NK cells are affected by “aromatic volatile substances,” otherwise known as scents, sometimes called phytoncides. These are the pinenes, limonenes, and other aerosols emitted by evergreens and many other trees. Scientists have identified 50 to 100 of these phytoncides in the Japanese countryside and virtually none in city air that’s not directly above a park.

This wasn’t a totally left-field idea. Studies have attributed healthful properties to soil compounds like actinomycetes, which the human nose can detect at concentrations of 10 parts per trillion. And since the mid-1990s, researchers have been studying pinene for its antimicrobial properties and limonene, which is given off by citrus and other trees, as a possible tumor suppressor in cancer patients.

To test the phytoncide theory, Li sequestered 12 subjects in hotel rooms. In some rooms, he rigged a humidifier to vaporize stem oil from common Hinoki cypress trees; other rooms got nothing. The results? The cypress dwellers had a 20 percent increase in NK cells during their three-night stay and reported feeling less fatigued. The control group saw almost no changes.

“It’s like a miracle drug,” said Li.