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Quickly, quietly, its leafy tendrils crept across American soil, engulfing hills, fields, trees, even abandoned cars and houses. They called it the vine that ate the South. Now kudzu, the relentlessly invasive weed that has already colonized 7 million acres of the southeastern United States, has revealed another prong to its attack: it also contributes to air pollution, says Jonathan Hickman, a post-doctoral fellow at Columbia University’s Earth Institute.

Kudzu snags nitrogen from the air and pulls it through to its roots, where bacteria convert it into nitrogen oxides, or NOx. Some of the NOx feeds the weed, but some is released into the air. There NOx spurs the formation of ozone and, in turn, smog, which can irritate throats, aggravate asthma, and even cause permanent lung damage.

To measure kudzu's pernicious influence, Hickman used gas-monitoring chambers to collect the NOx released from invaded and uninvaded plots of soil. He found that the kudzu-choked soil burped up about two times as much NOx. In the future, if kudzu were to invade all the undeveloped land across the nine-state belt bounded by Arkansas, Georgia, and Virginia, Hickman calculates that the number of smoggy days in some areas could increase by up to 70 percent. And take note: the alien vine can grow as rapidly as a foot a day.