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.
But other factors also influence the rate of POM production in a wood stove. The moisture content of the fuel, for instance, has the direct effect of reducing combustion area temperature since the liquid must be evaporated before the wood can burn. While the tests showed little difference in the pollution produced by burning green—as opposed to seasoned—oak, burning green pine caused roughly a three-fold increase in POM emissions over that of either of the two oak samples or of the seasoned pine. This result suggests that the resinous content of some softwoods has a tendency to increase POM emissions.
Though the POM results were the most significant discoveries made by the study, the research also showed that rates of carbon monoxide production are influenced by many of the same factors as are POM emissions. Again, airtight stoves seem to be far greater polluters than are their less efficient brethren. (We should also note, however, that though wood burners are not heavy producers of nitrous oxides, the higher temperatures that occurred in the fireplace insert did yield more of that particular pollutant than was produced by the more efficient stoves.)
What's to Be Done?
We're not suggesting that you abandon your wood stove, nor is the EPA currently contemplating any specific restrictions on the production or use of such appliances. (In comparison, after a study in Sweden similar to our EPA's project yielded nearly identical results, wood burning was—as of November 1980—strictly limited in that country!) But we are suggesting that you operate your wood burner in ways that tend to reduce emissions!
The object of a clean stove-operating procedure is to maintain the highest combustion temperatures possible. You can help your stove accomplish this by using seasoned wood (and, specifically, by not burning green pine!) and keeping the draft at least partway open. (Ideally, this means sacrificing—at least for the time being—a smoldering all-night burn in return for cleaner air. The choice is yours.)
You can also reduce POM and CO emissions by burning smaller fires in your stove. Such an approach avoids the creation of oxygen-starved areas in the firebox ... spots in which heavy emissions tend to be produced. Furthermore, even though there are no conclusive results to date, there seems to be reason to believe that lined (with ceramic, soapstone, etc.) fireboxes maintain hotter combustion than do "bare" chambers.
Salvation in Technology?
A number of different companies and individuals have recognized the dangers of wood stove emissions, and are developing new products in an attempt to solve the problem. For example, the Corning Glass Works has introduced a catalytic converter—similar to the automotive devices, but with much larger passages to accommodate lower "exhaust" velocities and larger particles —that may help strip flue gases of pollutants while increasing overall stove efficiency. Corning has no retrofit devices available at this time, but Franklin Cast Products (a major importer of Taiwan-made stoves) does have two wood burners—which, at this writing, sell for about $1,000 apiece—equipped to use the Corning device as a wood stove catalytic converter.
Other research groups feel that the solution will involve moving away from current airtight stove designs. Jotul of Norway—while unwilling to divulge specific information about work being carried out under its $2,500,000 annual research budget—is rumored to be working on a wood gas (pyrolysis) stove. Still another possibility is to use high-temperature furnaces (such as Hampton Technology's Jetstream), that employ heat exchangers and storage systems. And, while MOTHER EARTH NEWS is keeping a watchful eye on the corporate solutions to our pollution problems, we're also keenly attentive to work being done by individuals (such as the development of Larry Dobson's sawdust stove, featured in MOTHER EARTH NEWS' Guide to Home Energy).
We're confident that there are solutions to the problem, and—in fact—MOTHER EARTH NEWS' research staff is busy working on a number of retrofit approaches to lessening emissions. What's more, we're doing our best to encourage others to apply their knowledge to the effort. And when the solutions do become available, you'll read about them here!