Wood Stove Pollution

For years no news was good news, but now information is beginning to come in on wood stove pollution.

| January/February 1981

If you've spent any time contemplating the curl of smoke from a wood stove flue, you've probably wondered just what was in that cloud and whether it contained pollutants that might someday be recognized as harmful. You may even have heard that some communities—such as Vail, Colorado—have been forced to restrict wood burning to avert health hazards from haze, fly ash, and carbon monoxide.

But, until just a few months ago, there was very little solid technical information about wood stove pollution—the specific kinds and quantities of emissions produced by residential wood-fueled appliances. Well, the facts are now beginning to come in, and though it will be a few years before scientists have a thorough grasp on the scope of the problem, the initial results are, for the most part, depressing.

A study completed in June 1980 (it was carried out by the Monsanto Corporation, at the request of the Environmental Protection Agency) has shown that wood stoves, although they produce different sorts of heavy emissions from those associated with automobiles or power plants, do pose significant pollution problems. It seems that wood burners, especially the airtight models increasingly favored by consumers, are major producers of what scientists call polycyclic organic matter (POM). In fact, the results of the preliminary study suggest that residential wood stoves contribute more POM to the atmosphere than all other sources of combustion combined!

The term "polycyclic" refers to a molecule that contains at least two closed atomic rings, but the most important aspect of POM—as far as we consumers are concerned—is that many of the compounds classed under the heading are known to be mutagens or carcinogens. Two of the most toxic examples are benzo(a)pyrene and dimethylbenzanthracene ... but there were actually 25 POM compounds (or subdivisions of them) identified by the Monsanto study, plus miscellaneous aldehydes and other organics.

Efficient = Dirty

In an attempt to categorize the severity of the problem under differing combustion conditions, the Monsanto team—working at the Auburn University laboratories—tested three different wood burning appliances using logs of green and seasoned oak and pine, each at a variety of draft settings. The results demonstrated conclusively that the production of POM is a function of combustion temperature ... which—in most wood stoves—is closely related to oxygen supply.

Therefore the fireplace insert that the group tested—which burned its fuel supply relatively rapidly, yielding efficiencies that peaked at roughly 25%—produced about 1/10 as much POM as did either a baffled or unbaffled airtight. (In fact, the two airtight stoves performed similarly, yielding efficiencies of between 45 and 50%.) Quite clearly, limiting the rate of combustion in a woodstove by reducing the air supply increases POM emissions dramatically. In other words, the more efficient the unit becomes, the more POM it will produce.

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