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Reminder to go touch grass A scientist in Japan demonstrated that the air inside a forest strengthens your immune system for a full month The study is incredibly fascinating: Qing Li, an immunologist at Nippon Medical School in Tokyo, took 12 men on a 3-day forest trip. These were slow walks with no breathing exercises and no set route. Then he tested their blood and found a surge in natural killer cells -- the immune cells your body uses to fight cancer and virus-infected cells. Remarkably, the effect didn't fade when they went home, it was still measurable 30 days later. A control group that walked the same distance through a city showed no change at all. Pine, cedar, and cypress trees release compounds called phytoncides to defend themselves from insects and bacteria. You breathe them in, and Li believes they switch your immune cells into high gear. He estimates one forest walk a month may be enough to keep those cells elevated all year. The best medicine might not even need a prescription -- just a walk among the trees.