Trees might actually make summer air pollution even worse

Talk about treeson.

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.

During heat waves, the researchers found, trees can emit volatile organic compounds (VOCs) that help make air pollution, especially ground level ozone pollution, as much as sixty percent worse.

"We were surprised that the contribution of the plants to air pollution was so high," says lead author Galina Churkina, a senior fellow at the Institute for Advanced Sustainability Studies in Potsdam, Germany.

To be clear, this is still entirely our fault. On their own, the trees' VOCs don't pose a threat. But once in the air, they undergo a chemical reaction with Nitrogen Oxides (NOx) —a byproduct of burning fuel, like the gas that power our cars' engines—that adds more ozone to the mix. NOx causes a host of negative health effects ranging from respiratory issues to heart attacks. At the same time, ground level ozone irritates respiratory systems, aggravates asthma and chronic lung disease, and can even cause permanent lung damage.

RELATED: Photos show how bad air pollution has gotten in China and India:

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Smog in China and India, air pollution
Commuters drive on a road in heavy pollution in Beijing on December 1, 2015. Beijing ordered hundreds of factories to shut and allowed children to skip school as choking smog reached over 25 times safe levels on December 1, casting a cloud over China's participation in Paris climate talks. AFP PHOTO / WANG ZHAO / AFP / WANG ZHAO (Photo credit should read WANG ZHAO/AFP/Getty Images)
BEIJING, CHINA - DECEMBER 01: A woman wearing a mask practices roller blading at Olympic Park during dheavy smog on December 1, 2015 in Beijing, China. The representatives of the governments of more than 190 countries are meeting in Paris this week, including Chinese President Xi Jinping will attend the Paris meeting. (Photo by Feng Li/Getty Images)
A woman uses her scarf to cover her mouth as she crosses a street in Beijing on December 1, 2015. Beijing ordered hundreds of factories to shut and allowed children to skip school as choking smog reached over 25 times safe levels on December 1, casting a cloud over China's participation in Paris climate talks. AFP PHOTO / WANG ZHAO / AFP / WANG ZHAO (Photo credit should read WANG ZHAO/AFP/Getty Images)
BEIJING, CHINA - DECEMBER 01: A man rides riding a tricycle with plastic bottles to be recycled on a day of heavy pollution on December 1, 2015 in Beijing, China. China's capital and many cities in the northern part of the country recorded the worst smog of the year with air quality devices in some areas unable to read such high levels of pollutants. Levels of PM 2.5, considered the most hazardous, crossed 600 units in Beijing, nearly 25 times the acceptable standard set by the World Health Organization. The governments of more than 190 countries are meeting in Paris this week to set targets on reducing carbon emissions in an attempt to forge a new global agreement on climate change. (Photo by Lintao Zhang/Getty Images)
Commuters drive on a road in heavy pollution in Beijing on December 1, 2015. Beijing ordered hundreds of factories to shut and allowed children to skip school as choking smog reached over 25 times safe levels on December 1, casting a cloud over China's participation in Paris climate talks. AFP PHOTO / WANG ZHAO / AFP / WANG ZHAO (Photo credit should read WANG ZHAO/AFP/Getty Images)
BEIJING, CHINA - DECEMBER 01: A subway passing bridge on a day of heavy pollution on December 1, 2015 in Beijing, China. China's capital and many cities in the northern part of the country recorded the worst smog of the year with air quality devices in some areas unable to read such high levels of pollutants. Levels of PM 2.5, considered the most hazardous, crossed 600 units in Beijing, nearly 25 times the acceptable standard set by the World Health Organization. The governments of more than 190 countries are meeting in Paris this week to set targets on reducing carbon emissions in an attempt to forge a new global agreement on climate change. (Photo by Lintao Zhang/Getty Images)
People arrive at the airport on a polluted day in Beijing on December 1, 2015. China has ordered thousands of factories to shut as it grapples with swathes of choking smog that were nearly 24 times safe levels on December 1, casting a shadow over the country's participation in Paris climate talks. A thick grey haze shrouded Beijing, with the concentration of PM 2.5, harmful microscopic particles that penetrate deep into the lungs, climbing as high as 598 micrograms per cubic metre AFP PHOTO / FRED DUFOUR / AFP / FRED DUFOUR (Photo credit should read FRED DUFOUR/AFP/Getty Images)
Visitors wear masks as they walk in Tiananmen Square during heavy pollution in Beijing on December 1, 2015. Beijing ordered hundreds of factories to shut and allowed children to skip school as choking smog reached over 25 times safe levels on December 1, casting a cloud over China's participation in Paris climate talks. AFP PHOTO / WANG ZHAO / AFP / WANG ZHAO (Photo credit should read WANG ZHAO/AFP/Getty Images)
People walk through a overpass in Beijing on December 1, 2015. Beijing ordered hundreds of factories to shut and allowed children to skip school as choking smog reached over 25 times safe levels on December 1, casting a cloud over China's participation in Paris climate talks. AFP PHOTO / WANG ZHAO / AFP / WANG ZHAO (Photo credit should read WANG ZHAO/AFP/Getty Images)
Pedestrians walk past a billboard scene of green trees and grass on a heavily polluted day in Beijing on December 1, 2015. China has ordered thousands of factories to shut as it grapples with swathes of choking smog that were nearly 24 times safe levels on December 1, casting a shadow over the country's participation in Paris climate talks. A thick grey haze shrouded Beijing, with the concentration of PM 2.5, harmful microscopic particles that penetrate deep into the lungs, climbing as high as 598 micrograms per cubic metre. AFP PHOTO / GREG BAKER / AFP / GREG BAKER (Photo credit should read GREG BAKER/AFP/Getty Images)
BEIJING, CHINA - DECEMBER 01: Chinese Tai Chi practitioners exercise at a local park on a high pollution day on December 1, 2015 in Beijing, China. China's capital and many cities in the northern part of the country recorded the worst smog of the year with air quality devices in some areas unable to read such high levels of pollutants. Levels of PM 2.5, considered the most hazardous, crossed 600 units in Beijing, nearly 25 times the acceptable standard set by the World Health Organization. The governments of more than 190 countries are meeting in Paris this week to set targets on reducing carbon emissions in an attempt to forge a new global agreement on climate change. (Photo by Kevin Frayer/Getty Images)
BEIJING, CHINA - NOVEMBER 30: A Chinese man wears a protective face mask as he passes by the CCTV building on a day of heavy pollution on November 30, 2015 in Beijing, China. China's capital and many cities in the northern part of the country recorded the worst smog of the year with air quality devices in some areas unable to read such high levels of pollutants. Levels of PM 2.5, considered the most hazardous, crossed 600 units in Beijing, nearly 25 times the acceptable standard set by the World Health Organization. The governments of more than 190 countries are meeting in Paris this week to set targets on reducing carbon emissions in an attempt to forge a new global agreement on climate change.(Photo by Kevin Frayer/Getty Images)
BEIJING, CHINA - DECEMBER 01: Tourists visit the Olympic Park during dheavy smog on December 1, 2015 in Beijing, China. The representatives of the governments of more than 190 countries are meeting in Paris this week, including Chinese President Xi Jinping will attend the Paris meeting. (Photo by Li Feng/Getty Images)
This combination image of two photographs taken on December 3, 2015 (top) and two days earlier on December 1 (bottom) shows a skyscraper under clear skies and in heavy pollution, as seen in the central business district in Beijing. The skies cleared in Beijing on December 2 and 3, after being swathed in choking smog that was nearly 24 times safe levels earlier in the week. AFP PHOTO / GREG BAKER / AFP / GREG BAKER (Photo credit should read GREG BAKER/AFP/Getty Images)
This photo combo shows a general view of people visiting Tiananmen Square during a heavily polluted day on December 1, 2015 (LOWER) and two days later when the smog cleared on December 3 (TOP) in Beijing. The skies cleared in Beijing on December 2 and 3, after being swathed in choking smog that was nearly 24 times safe levels earlier in the week. AFP PHOTO / WANG ZHAO / AFP / WANG ZHAO (Photo credit should read WANG ZHAO/AFP/Getty Images)
A woman wearing a mask exits an underpass in Beijing on November 30, 2015. Beijing choked under the worst smog of the year on November 30, with dangerous particulates nearly 20 times healthy levels, as China's president joined other leaders in Paris for key climate change talks. AFP PHOTO / WANG ZHAO / AFP / WANG ZHAO (Photo credit should read WANG ZHAO/AFP/Getty Images)
BEIJING, CHINA - NOVEMBER 30: A couple wear protective masks as they have their picture taken outside the Forbidden City on a day of heavy pollution on November 30, 2015 in Beijing, China. China's capital and many cities in the northern part of the country recorded the worst smog of the year with air quality devices in some areas unable to read such high levels of pollutants. Levels of PM 2.5, considered the most hazardous, crossed 600 units in Beijing, nearly 25 times the acceptable standard set by the World Health Organization. The governments of more than 190 countries are meeting in Paris this week to set targets on reducing carbon emissions in an attempt to forge a new global agreement on climate change. (Photo by Kevin Frayer/Getty Images)
SHANGHAI, CHINA - NOVEMBER 30: (CHINA OUT) Buildings at the Lujiazui Financial District are shrouded in smog on November 30, 2015 in Shanghai, China. Shanghai's real-time Air pollution index (API) read high 189 on Monday morning. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)
Pedestrians walk over a footbridge shrouded in haze as vehicles drive on a highway in Beijing, China, on Monday, Nov. 30, 2015. Air pollution reached 'hazardous' levels in Beijing on Sunday, prompting the city to upgrade to the second-highest alert for the first time in 13 months on the same day that the Chinese government said it has met pollution-reduction targets for the year. Photographer: Qilai Shen/Bloomberg via Getty Images
BEIJING, CHINA - NOVEMBER 30: A Chinese woman wears a protective mask as she walks outside the Forbidden City on a day of heavy pollution on November 30, 2015 in Beijing, China. China's capital and many cities in the northern part of the country recorded the worst smog of the year with air quality devices in some areas unable to read such high levels of pollutants. Levels of PM 2.5, considered the most hazardous, crossed 600 units in Beijing, nearly 25 times the acceptable standard set by the World Health Organization. The governments of more than 190 countries are meeting in Paris this week to set targets on reducing carbon emissions in an attempt to forge a new global agreement on climate change. (Photo by Kevin Frayer/Getty Images)
A woman wearing a mask walks past in Beijing on November 30, 2015. Beijing choked under the worst smog of the year on November 30, with dangerous particulates nearly 20 times healthy levels, as China's president joined other leaders in Paris for key climate change talks. AFP PHOTO / WANG ZHAO / AFP / WANG ZHAO (Photo credit should read WANG ZHAO/AFP/Getty Images)
A man wears a mask on a polluted day in Beijing on November 30, 2015. Beijing choked under the worst smog of the year on November 30, with dangerous particulates nearly 20 times healthy levels, as China's president joined other leaders in Paris for key climate change talks. AFP PHOTO / FRED DUFOUR / AFP / FRED DUFOUR (Photo credit should read FRED DUFOUR/AFP/Getty Images)
To go with Climate-warming-UN-COP21-India,FOCUS by Trudy Harris In this November 15, 2015 photo, Indian joggers exercise on a smoggy morning near the India Gate monument in New Delhi. India's capital, with 18 million residents, has the world's most polluted air with six times the amount of small particulate matter (pm2.5) than what is considered safe, according to the World Health Organization (WHO). The air's hazardous amount of pm2.5 can reach deep into the lungs and enter the blood, causing serious long term health effect, with the WHO warning India has the world's highest death rate from chronic respiratory diseases. India, home to 13 of the world's top 20 polluted cities, is also the third largest emitter of greenhouse gases behind the United States and China. In Delhi, the air pollution is due to vehicle traffic including cargo trucks running on low-grade diesel, individual fires that residents burn in winter, crop being burnt by farmers in neighboring states, and construction site dust. Burning coal in power plants is also major contributor that is expected to increase hugely in the coming decades to match electricity needs of the ever-growing city and its booming satellite towns. (Photo credit should read ROBERTO SCHMIDT/AFP/Getty Images)
An Indian schoolgirl covers her nose and mouth as she walks with others along a busy road on a smoggy day in New Delhi on November 30, 2015. Some 150 leaders including US President Barack Obama, China's Xi Jinping, India's Narendra Modi and Russian President Vladimir Putin will attend the start of the Paris conference on climate change, which starts on November 30, tasked with reaching the first truly universal climate pact. AFP PHOTO / Money SHARMA / AFP / MONEY SHARMA (Photo credit should read MONEY SHARMA/AFP/Getty Images)
Heavy traffic is seen during a smoggy day in New Delhi on November 30, 2015. Some 150 leaders including US President Barack Obama, China's Xi Jinping, India's Narendra Modi and Russian President Vladimir Putin will attend the start of the Paris conference on climate change, which starts on November 30, tasked with reaching the first truly universal climate pact. AFP PHOTO / Money SHARMA / AFP / MONEY SHARMA (Photo credit should read MONEY SHARMA/AFP/Getty Images)
BEIJING, CHINA - NOVEMBER 30: A residence community is blanketed by smog on November 30, 2015 in Beijing, China. (Photo by Xiao Lu Chu/Getty Images)
A man wears a mask on a polluted day in Beijing on November 30, 2015. Beijing choked under the worst smog of the year on November 30, with dangerous particulates nearly 20 times healthy levels, as China's president joined other leaders in Paris for key climate change talks. AFP PHOTO / FRED DUFOUR / AFP / FRED DUFOUR (Photo credit should read FRED DUFOUR/AFP/Getty Images)
SHANGHAI, CHINA - NOVEMBER 30: (CHINA OUT) People take photos at The Bund in smog on November 30, 2015 in Shanghai, China. Shanghai's real-time Air pollution index (API) read high 189 on Monday morning. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)
Heavy traffic is seen during a smoggy day in New Delhi on November 30, 2015. Some 150 leaders including US President Barack Obama, China's Xi Jinping, India's Narendra Modi and Russian President Vladimir Putin will attend the start of the Paris conference on climate change, which starts on November 30, tasked with reaching the first truly universal climate pact. AFP PHOTO / Money SHARMA / AFP / MONEY SHARMA (Photo credit should read MONEY SHARMA/AFP/Getty Images)
SHANGHAI, CHINA - NOVEMBER 30: (CHINA OUT) Buildings at the Lujiazui Financial District are shrouded in smog on November 30, 2015 in Shanghai, China. Shanghai's real-time Air pollution index (API) read high 189 on Monday morning. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)
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"We knew that if you have more plants, if you have more emissions of VOCs, then you might have a substantial increase in pollution—in ozone—in a city that has high levels of NOx," says Churkina. "We thought it would be interesting to quantify to what degree plants contribute to ozone production. And a heat wave was a logical first step, because there were studies showing that during a heat wave air pollution goes up."

Churkina and her colleagues simulated the emissions of volatile organic compounds from different kinds of plants and modeled how those compounds react with the full chemistry of the atmosphere, including nitrogen oxides. Berlin made the perfect study site—the city has air monitor data from its 2003 heatwave.

While VOCs often have a negative connotation because of industrial VOCs (which have been found to harm health) plants producing VOCs isn't unusual or inherently bad. The distinctive smell of fresh pine needles, for example, is caused by a group of VOCs known as the monoterpenes, while lilacs' distinctive fragrance is a mix of (E)-beta-ocimeneE and the eponymously named Lilac aldehyde and lilac alcohol. Plants create VOCs to attract insects (to pollinate their flowers), to repel insects (so they remain unmolested) and in times of stress. It's those times of stress that create the issues outlined in the study: During a heatwave, plants increase their production of VOCs because very hot weather is botanically stressful.

This doesn't mean that we should stop planting trees, since ultimately, it's the underlying human-produced pollution that's the problem. Plus, the study only suggests that trees boost our pollutants during heatwaves, not all the time. And even the increased temperatures of the heatwave that Churkina studied can be blamed on, well, us. A 2015 study found that the 2003 European heatwave was caused mainly by climate change.

But the study makes it clear that it isn't enough for us to plant trees—we have to lower pollution as well.

Churkina hopes that the results can help cities make the most of their efforts at going green—by planting trees that emit fewer VOCs in very hot weather, for example. Municipalities might also take care to give their urban foliage plenty of water, since the heat stress that increases VOC production is caused by thirst.

"I want to stress that planting trees in the city is a good thing," says Churkina. "But if you really want to have the benefits of trees in the city, you really have to go from the other side and reduce the pollutants."

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