Oak trees in Missouri's Ozarks may be tainting air

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Posted on Sun, Nov. 24, 2002

By SARA SHIPLEY St. Louis Post-Dispatch

ST. LOUIS - Oak trees in Missouri's Ozark forests might be contributing to inexplicably high levels of a cancer-causing air pollutant in St. Louis, according to a federal environmental agency.

Officials with the U.S. Environmental Protection Agency suspect that naturally occurring emissions from the trees could help explain why St. Louis has among the highest levels nationwide of formaldehyde, a toxic gas believed to cause cancer.

The woods haven't looked so bad since President Ronald Reagan declared in the early 1980s that trees were worse polluters than smokestack industries. More recently, trees have been blamed for up to 65 percent of ozone-forming chemicals in cities such as smog-choked Houston.

The EPA doesn't believe trees are the sole reason for St. Louis' formaldehyde problem - cars and industries are known sources. Nevertheless, officials installed an air monitor last week in a forest in St. Francois County, 60 miles southwest of St. Louis, that could help solve the puzzle.

Oak trees give off isoprene, a gas that reacts with sunlight and water to create formaldehyde. Researchers want to see whether the chemicals emitted by the oak-rich Ozark forests could affect air quality in St. Louis.

"We tend to not think that nature can be a source of pollution. But nature is putting out things that, if they were there in large enough amounts, we wouldn't want to breathe," said Jay Turner, an associate professor of chemical engineering at Washington University who is helping the EPA with its study.

The science of "biogenic emissions" - naturally occurring air pollution - is still relatively young, and environmental activists say there's no reason to jump to conclusions.

"On balance, in any community, trees are an asset," said John Wilson, executive director of the Galveston-Houston Association for Smog Prevention. "Cutting down trees won't clean your air."

The mystery started when federal officials discovered high formaldehyde levels during a two-year study of toxic air chemicals in the city. The St. Louis Community Air Project, funded by a $715,000 EPA grant, is one of 40 similar programs nationwide.

Of about 250 chemicals monitored in St. Louis, four showed up at risky levels: formaldehyde, benzene, arsenic and acetaldehyde. Formaldehyde caused the most concern because of its reputed ability to cause cancer, which EPA scientists are currently re-evaluating. Formaldehyde is familiar to many people as a preservative used in science labs.

The formaldehyde levels detected at an air monitor stationed at Grant School surprised the EPA. The levels were higher than those taken in Baltimore, Phoenix, Chicago, Detroit or El Paso, Texas.

Existing estimates of air pollution from local businesses and cars didn't explain the readings. Could the excess be coming from some unreported industry source?

Or could it be the trees?

Atmospheric scientists have known for more than 20 years that plants and trees emit chemicals related to air pollution. Oak trees are among the worst culprits, giving off the highly reactive isoprene, which can lead to the formation of both formaldehyde and ozone. Pine trees emit turpine, a similar gas.

Missouri's dense oak forests are unique, said Thomas Pierce, a meteorologist who tracks natural emissions for the EPA's Biogenic Emission Inventory System. "There are some counties down there that are more than 50 percent oak trees. That's one of the highest densities of oaks anywhere in the country," he said.

Pierce worked with Washington University's Turner to produce a 1998 study of isoprene emissions from the Ozark forests. Their work, along with other EPA studies, found that trees in heavily forested counties in the region might emit up to 300 tons per day of isoprene.

Trees are believed to emit the chemical as part of a sun protection system, Pierce said. About half of the isoprene eventually turns into formaldehyde, but it's unknown how far the chemical travels, he said.

In its natural setting, isoprene is not a danger, Pierce said. The concern is when wind carries naturally emitted chemicals into urban areas or near other pollution sources, he said.

Pierce would not estimate how much formaldehyde comes from trees versus other sources. "My working hypothesis is, not very much, but I think it deserves a look," he said.

The monitor that was installed Wednesday near Bonne Terre is one of three new monitors that will test for formaldehyde and related chemicals. Two other monitors will operate in the St. Louis area, one at Washington University, the other in the northern part of the city.

Members of the Air Project, which includes residents, civic leaders, business representative and others, welcomed the EPA's offer to further investigate formaldehyde sources.

"It doesn't mean we're going to cut down all the trees," said Emily Andrews, who coordinates the Air Project for the St. Louis Association of Community Organizations. "If we can figure out what percentage those forests are contributing to the problem, then maybe we can figure out what to do about it."

Wilson, of the Houston organization, cautioned against putting too much emphasis on trees as pollution sources. He said trees' contribution to ozone formation in Houston originally was overestimated and should be 20 percent at most. Industry, meanwhile, was underestimating its own emissions by a factor of 10, he said.

Jim Hirtz, an environmental protection specialist in the EPA's Kansas City office working on the project, said he couldn't determine how much formaldehyde trees may contribute.

"We have to be open," he said. "There could be other sources out there."

-- Anonymous, November 24, 2002


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