A few months ago I was at a real estate conference here in New York, and near the end of the day there was a presentation by a non-profit that plants trees in low-income neighborhoods. A big part of the pitch was the idea that the additional trees would not only be an aesthetic improvement and give local kids an opportunity to garden, but they would improve the air quality and potentially cut down on asthma rates.

New York has very, very high rates of asthma. It's a leading cause of emergency room visits in the poorest neighborhoods, and about 8.4 percent of children and 9.3 percent of adults are affected, according to the health department. 

And air pollution in general contributes to about 6 percent of deaths in the city each year, according to the Department of Environmental Protection. During the summer, I regularly get alerts on my phone warning me about dangerously high ozone levels.

Ever since hearing about the tree-planting programs I've been wondering how much of an impact they can really have on these numbers. I haven't been able to find a lot of good research showing that there's a positive impact, at least when it comes to asthma rates. But today I came across this article in the Atlantic's City Lab, explaining how well-intentioned tree-planting can actually have the opposite of the intended effect.

Trees emit volatile organic compounds, which combine with mono-nitrogen oxides to form ozone when exposed to sunlight, and scientists recently found that certain trees are bigger culprits than others. Black gum trees emit about 15 times more VOCs than birch trees, for example. 

Ozone production from VOCs is only expected to become a bigger problem as climate change proceeds, so researchers are working to ensure that programs that plant trees to better local environments don't end up creating an even bigger problem. 

The City Lab article does a great job of explaining the research and what cities should do, and has some cool graphics as well. Definitely worth a look!