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, carcinogens, in the cleanest air we supposedly have in this country. (1)
Trees and plants release more than just oxygen into the atmosphere as a result of photosynthesis. They also release a variety of gases that contribute to air pollution. In fact, the planet's vegetation accounts for about two-thirds of the pollutants known as volatile organic compounds (VOCs) emitted globally, reports the EPA. (2)
There's also another way a seemingly 'green' forest can be a source of pollution. Old plants shed more nitrogen than they 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. In normal forests these needles would be broken down by bacteria, mix back into the soil, and provide nutrients for newly growing plants. The commercial tree plantations don't have young growth. New trees and competing plants were cleared out, and the canopies of older trees make it less likely that new growth will come. What these plantations have, then, is a build-up of nitrogen in the topsoil, which, when it rains gets washed into local streams. (3)
Nitrogen pollution in water starts a whole new chain of events. Because nitrogen is such a vital nutrient, having an excess of it in the water supply causes a rapid increase in the algae population, which then depletes the water of oxygen. This can literally suffocate the fish living in the water. Nitrogen pollution and algal bloom are common problems in waterways near commercial farms, making the forests, in this respect, no better for the environment than big farms. (4)
The point of this is to show that just because something looks both green and pristine doesn't mean it's an unqualified environmental boon.
The Good Side
Trees and other plants make their own food from carbon dioxide in the atmosphere, water, sunlight, and a small amount of soil elements. In the process they release oxygen for us to breathe. (5)
They also:
- Help to settle out, trap and hold particles, dust, ash, pollen and smoke that can damage human lungs.
- Absorb carbon dioxide and other dangerous gases and, in turn, replenish the atmosphere with oxygen.
- Produce enough oxygen on each acre for 18 people every day.
- Absorb enough carbon dioxide on each acre, over a year's time, to equal the amount you produce when you drive your car 26,000 miles. Trees remove gaseous pollutants by absorbing then through the pores in the leaf surface. Particulates are trapped and filtered by leaves, stems, and twigs, and washed to the ground by rainfall. (5)
The loss of trees in our urban areas not only intensifies the urban 'heat-island' effect from loss of shade and evaporation, but we lose a principal absorber of carbon dioxide and trapper of other air pollution as well.
In Sacramento, California, its 6 million trees contribute to an annual net reduction of CO2 by about 335,000 tons. Of that total, 262,300 ton of CO2 remains sequestered in the trees. But, the encouraging piece of this annual reduction is that an additional 83,300 tons—nearly 25% of the reduction—is attributable to tree shade on homes buildings and other structures. (6)
Trees and forests in the US removed 17.4 million tons of air pollution in 2010.(7)
Although this pollution removal equated to an average air quality improvement of less than 1 percent, its effects on human health were significant ($6.8 billion), especially in urban areas.
According to the World Health Organization, the health impacts of trees on air pollution resulted in the avoidance of more than 850 deaths, 670,000 cases of acute respiratory symptoms, 430,000 incidences of asthma exacerbation, and 200,000 school days lost. (7)
References
Jay H. Lehr, “Toxicological Risk Assessment Distortions,” in Rational Readings on Environmental Concerns, Jay H. Lehr, Editor, (New York, Van Nostrand Reinhold, 1992), 682
“Trees and air pollution,” EPA Science Matters, 2010
Danny Clemens, “Excess trees in Japan are harming the environment,” news.discovery.com, June 11, 2015
Esther Inglis-Arkell, “Here's how trees can be sources of pollution (and how they can stop),” io9.com, June 10, 2016
“Trees reduce air pollution,” dnr.state.md.us, accessed June 20, 2015
“Trees- the air pollution solution,” Center for Urban Forest Research, accessed June 20, 2015
Dr. Mercola, “Trees actually lower your risk of dying,” articles.mercola.com, August 16, 2014
