It sounds hokey that evergreen scents—the kind of thing given off by those cardboard trees dangling from the rearview mirrors of taxicabs—could help us live longer. But Li found similar results with NK cells in a petri dish: they increased in the presence of aromatic cypress molecules. So did anti-cancer proteins and proteases called granulysin, granzymes A and B, and perforin, which act by causing tumor cells to self-destruct. Li’s olfaction theory is unconventional, but it contains some of that Zen five-sense wisdom. While American researchers are mostly showing people pictures of nature, the Japanese are pouring it into every orifice.

Li invited me to his lab to have a sniff. The building was practically empty—the medical students were on break—and eerily dark, the result of power shortages in the wake of the Fukushima nuclear disaster.

He held up a small cinnamon-colored glass bottle filled with oil. “This is very toxic!” he said, giggling. “It’s very good but very toxic.” Phytoncides, from the Greek and Latin for “plant” and “killer,” are antimicrobial compounds that ward off pests. At low levels, though, we find them pleasant, and sometimes we don’t consciously detect them at all. Li believes that, while being around big trees in forests offers us the greatest benefit, flora from other landscapes, and possibly even houseplants, release these substances, too.

Before taking a drag, I stuffed my right arm into yet another blood-pressure machine. Then we unscrewed the cap of the forest elixir and I inhaled. The oil gave off a nice pitchy, vaguely turpentine scent. We put the cap back on and read my blood pressure again. It had dropped 12 points.

I looked at Li, who nodded delightedly. “This is a very big effect, bigger than people get with pharmaceuticals,” he said. “In fact, I use a humidifier with cypress oil almost every night in the winter.” You don’t need to harvest your own, Li said. Standard health-store aromatherapy oils work fine.

“What else do you do?” I asked the middle-aged man with the bowl haircut.

Clearly, Li gets asked this a lot. He had a small list. “If you have time for a vacation, don’t go to a city. Go to a natural area. Try to go one weekend a month. Visit a park at least once a week. Gardening is good. On urban walks, try to walk under trees, not across fields. Go to a quiet place. Near water is also good.”

My morning walk back in D.C. was transforming before my eyes.