Researchers are finding evidence that spending time in the woods can lower your blood pressure, control stress and even prevent cancer. Japan's Forest Agency has designated 100 'Forest Therapy' sites where people can get a dose of shinrin-yoku or "forest bathing". One theory is that phytoncides, antimicrobial compounds that ward off pests, are produced by trees. People walking in the wood breath in these beneficial chemicals. Nature may increase a persons natural killer immune cells.