Trees were supposed to be the urban jungle's salvation. After all, trees provide sweet, beautiful shade which helps cool the metropolis, a place prone to overheating thanks to a proliferation of surfaces like asphalt and concrete. And by pumping oxygen into the air—the same oxygen humans need to breathe— while also filtering out harmful air pollution, trees were supposed to help the eighty percent of Americans who live in urban areas breathe easy. But a new study out today in the journal Environmental Science & Technology has found that planting trees while doing nothing about underlying air pollution is a bit like putting spinach on your double bacon donut burger: you're still going to die of a heart attack.