If you're feeling stressed or down at holiday time, take a moment to breathe in the scent of rain. Can't step outside into a forest? No worries, you can find these scents all around you in your home. There's citrus in the kitchen and, if you decorate with pine or fir tree boughs, the forest scents may be in your living room or hanging on your front door. I have an old pine chest of drawers that reminds me of childhood road trips in the mountains of Vermont and New Hampshire.

Now close your eyes and imagine yourself taking a quiet stroll through a forest. You may be able to conjure the smell of rain wafting up your nostrils. All animals, including insects and mammals, are attracted to that scent, and of course, we humans love it too. This is because plants release hundreds of chemicals, dissolved in oils, onto their leaves, twigs, branches, bark, petals, and stems, and when the air gets moist, the chemicals in the oils dissolve in the water droplets, which waft into the air and swirl around in the wind. If you happen to be in their path and inhale deeply, they signal your brain to relax and calm.

Scientists have studied the chemical content of the plants in the Shinrin-yoku parks in Japan and Korea and in the saguaro cactus forests of the Sonoran Desert in Arizona, and even though the desert and forest scents differ, hundreds of similar chemicals were found in these far-flung places. These are “biogenic volatile organic compounds,” which means they come from plants (biogenic), evaporate easily (volatile), and are mainly made up of carbon molecules stuck together (organic). In Korea and Japan, these fragrant oils come mostly from a specific kind of cypress tree—the Hinoki cypress. In the desert, they emanate from creosote bushes and many other desert plants.

One scientist at the University of Arizona, Gary Nabhan—who in 1990 received the MacArthur Fellowship – the so-called “Genius Grant,” studies the smell of rain in the Sonoran Desert. He likens the combination of hundreds of chemical oils released before, during, and after a rain to an “orchestra” of fragrances. This blend also bears similarities to the taste of fine wines or the scent of perfumes—with high and low notes, both shorter and........

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