During a bad hot spell in 2006, plants in Berlin contributed to as much as 60% of the observed ozone pollution

Surprising Pollution From Trees

By —— Bio and Archives--October 14, 2017

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Forests have been called the lungs of the Earth because growing trees remove carbon dioxide from the air and replace it with oxygen. Planting trees is often touted as a strategy to make cities greener, cleaner and healthier.

However, there is some degree of pollution associated with trees.


Research from the metropolis of Berlin shows that green spaces, from forests to public parks can intensify ozone pollution in cities, an effect that is pronounced during heat waves. During a bad hot spell in 2006, plants in Berlin contributed to as much as 60% of the observed ozone pollution, with potential risks for the health of city-dwellers. 1

The problem is caused by a class of chemicals called volatile organic compounds, or VOCs. Plant leaves pump out VOCs naturally but some of these gases can also create chemical reactions in the air that form ground level ozone—a pollutant harmful to people, especially those with preexisting respiratory disease. Summer is a big concern because plants increase the amount of VOCs they emit during hot weather. 2

In Oregon, about 22 million metric tons of emissions are released a year as the states trees die. That’s the equivalent of nearly all annual statewide emissions derived from the transportation and power generation sectors combined reports Catherine Mater. 3

Jay Lehr asks the question, what do you get when you go into the North Woods, a great, beautiful unspoiled area where there is no industry for miles? The answer is you inhale the pine odor. Guess what? Pine odor is made up of polycyclic aromatics, which are carcinogens, in the cleanest air we supposedly have in this country. 4

Janet Pelley reports, “Scientists are beginning to blame local forests and pollutants blown in from overseas for the fact that concentrations of ground level ozone and its accompanying smog have not declined during the past 10 years in the United States. Thus, national smog fighting regulations may actually be doing a better job than they have been given credit for, but measurably reducing ozone levels may require international efforts and rethinking forestry management.” 5

Isoprene is a volatile organic compound that is emitted in large quantities by forest vegetation. Its annual emission is estimated to be 500 million tons worldwide. But although isoprene readily oxidizes to form volatile products, popular wisdom held that it didn’t form products that could contribute to aerosol product formation. (Aerosols are small particles such as soot, dust, and smoke that can influence rainfall and temperature). Intensive measurements taken over the Amazon basin, however, revealed the presence of two novel methyltetrol compounds. These compounds are formed in the atmosphere through reaction of isoprene with hydroxyl radicals. It’s estimated that the photooxidation of isoprene results in an annual production of about 2 million tons of the new compounds. This represents between 5 to 25% of the organic aerosols formed by atmospheric photochemistry from biogenic precursors. 6

Research at the University of California at Berkeley suggests that pollution from oak trees is destroying the pine forests of the Sierra Nevada Mountains. Its been found that the oaks are producing between 40 to 70 percent of the ozone that is damaging and killing Jeffrey and Ponderosa pines that are the dominant species in the forest. 7

There’s also another way a seemingly ‘green’ forest can be a source of pollution. Old plants shed more nitrogen than they can use. Every time the cypress and cedar trees in old forests shed their needles, they drop nitrate (NO3) on the ground. This is causing massive amounts of nitrogen runoff in Japan resulting in harmful algae blooms. 8


  1. Daniel Strain, “City plants can actually decrease air quality during heat waves,”  anthropocenemagazine.org, July 6, 2017
  2. G. Churkina et al., “Effect of VOC emissions from vegetation on air quality in Berlin during a heat wave,” Environmental Science & Technology, May 17, 2017
  3. Catherine Mater, “The surprising emissions from Oregon’s forests,” The Oregonian, September 17, 2017
  4. Jay H. Lehr, Toxicological risk assessment distortions,” in Rational Readings on Environmental Concerns, Jay H. Lehr, Editor, (New York, Van Nostrand Reinhold, 1992), 682
  5. Janet Pelley, “Neglected sources of ozone,” Environmental Science & Technology, 38, 86A, March 1, 2004
  6. Magda Claeys et al., “Formation of secondary organic aerosols through photooxidation of isoprene,” Science, 303, 1173, February 2004
  7. Geoffrey Lean, “Revealed: the dirty way in which trees are killing each other,” independent.co.uk, November 24, 2002
  8. Danny Clemens, “Excess trees in Japan are harming the environment,” news.discovery.com, June 11, 2015


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Jack Dini -- Bio and Archives | Comments

Jack Dini is author of Challenging Environmental Mythology.  He has also written for American Council on Science and Health, Environment & Climate News, and Hawaii Reporter.

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