Though the trend has been to listen to environmental claims about the benefits of trees in cities, in science they have a well-established dark side: in urban cities, they produce a lot of ground-level ozone, the very thing environmentalists spent decades lobbying against.

During hot days, trees emit compounds that worsen ozone, such as formaldehyde, which forms from isoprene, a volatile organic compound that trees can give off when temperatures are hot, and 
terpenes, which also interact with sunlight to create a "natural" smog. If you have witnessed the haze of the Great Smoky Mountains, you are breathing in natural pollution.

Storing carbon, controlling storm water and cooling areas off by providing shade are all positives, but when trees dominate a city street, they can raise the ozone level considerably, and cause asthma, bronchitis and other respiratory illnesses that the U.S. Environmental Protection Agency claims will be solved by more regulations on cars and businesses. 

For a new analysis, researchers compared computer models of air pollutant concentrations in the Berlin-Brandenburg metropolitan area in Germany in the summer of 2006, when there was a heat wave, and the summer of 2014, which had more typical seasonal temperatures. The simulation showed that during the summer of 2006, VOCs from urban greenery contributed to about 6 to 20 percent of the ozone formation, and that during the heat wave period, the contribution spiked to up to 60 percent.

The researchers suggest that in addition to tree-planting campaigns, efforts to improve cities' environments should include other measures such as reducing vehicular traffic, a major source of nitrogen oxides that can react with VOCs and form ozone. Or be smarter about which trees you plant, rather than killing their economy. Plant "a million trees", just make sure you think about the trees you use (avoid poplar, for example). Don't do what General Mills did and claim they were helping bees by mailing invasive species all over the country and telling kids to plant them in their backyards.