New research shows that besides lowering blood pressure and pulse rate, and reducing levels of the stress hormone cortisol, spending time in nature boosts your immune system, too.

In a study in Japan, 12 healthy men between the ages of 37 and 55 took a three-day, two-night trip to the forest. On the first day, the men spent two hours in the afternoon walking in the forest. On the second day, they walked in the forest for two hours in the morning and two hours in the afternoon. Blood samples tested on days two and three showed that in 11 of the 12 men there was a 50 percent increase in natural killer cells. These cells are one of the main components of the immune system, and they play a central role in fighting tumors and viruses. Further studies show that these positive immune benefits can last up to a month after a visit to the woods.

Researchers believe that the increase in natural killer cells is in part a response to phytoncides, the essential oils in wood that help protect it from insects and rotting. In Japan, where the studies were first conducted, the practice, dubbed “forest bathing,” has become so popular that the government there is creating official “forest therapy” sites, and companies are adding “forest therapy” to their health plans.