Table ll. Carcinogenic compounds observed in smoke from residential
wood combustion sources.(a,b). Click for the popup.

Health Concerns

Health concerns for emissions from residential woodburning appliances relate to the particulates and chemicals formed due to incomplete combustion. Wood burns with a diffusion flame, that is, oxygen must diffuse from outside the flame into the combustion zone in contrast to a gas flame where the fuel and air are mixed before combustion. As oxygen is consumed, the region inside the flame becomes oxygen starved. Both free carbon atoms and hydrocarbon fragments are formed in this oxygen depleted zone. If these species escape the high temperature combustion region before they are oxidized, they will form soot and hydrocarbons commonly called creosote and tars consisting of a variety of aliphatic, olefinic, and polynuclear aromatic hydrocarbon compounds. It is clear that a myriad of initial pyrolysis products formed during partial combustion are mixed in a sea of chemical reactivity including pyrolysis, oxidation, and reduction. These reactions and the resulting compounds are influenced by the chemical and physical state of the reacting species which can contain metallic compounds and soot that can serve as catalysts, as well as ions, electrons, free radicals, and free atoms, which form molecules that combine and decompose. This tremendously complex chemical manufacturing process is initiated whenever any material containing carbon and hydrogen is burned. A chemical as simple as methane, for example, when burned incompletely, can form some of the same complex organic molecules as when coal or tobacco are burned. The chemicals formed during incomplete combustion are determined by the temperature and oxygen supply in addition to the chemical complexity of the starting material. Thus, many of the same organic compounds formed in the burning of tobacco, coal, leaves, and grass clippings, are also produced when wood burns. The chemical products formed in the combustion of wood have been studied primarily in terms of the major products of combustion such aa water and carbon dioxide and minor components such as CO, HCI, SO,, NO,, etc. Two recent studies, however, measured over a hundred different chemicals and compound groups in emissions from burning wood and wood-burning stoves.2l·22 The results from these studies showed smoke from wood and wood-burning stoves and fireplaces contained

• 17 priority pollutants which combined, account for 4.8% of the particulate mass (Table I);
• up to 14 carcinogenic compounds making up 0.5% of the particulate material (Table II);
• 6 cilia toxic and mucus coagulating agents (Table III); and
• 4 co-carcinogenic, initiating, or cancer promoting agents (Table IV).

Other toxic gases such as CO, noxious and respiratory irritants such as aldehydes, phenols, etc. were also found.(21,22) Even dioxins have been identified in fireplace soot.(23-29) Although other compounds have not yet been reported, it is reasonable to assume that many of the compounds identified in other types of smoke are also present in wood smoke because of the similarity in their combustion chemistries. This appears particularly reasonable since 85% of the condensable emissions were not characterized by DeAngelis, et al. in their GC·MS study(21)

The contribution of emission from RWC sources to both acute and chronic health effects of air pollution are of concern. Acute air pollution episodes where substantial number of people have died are well documented.(33-38) These episodes occur during periods of low inversion heights and low wind speeds such that pollutants in the stagnant air mass accumulate to hazardous levels particularly for individuals with respiratory or other medical problems.

Table III. Emission factors for cilia toxic and mucus coagulating agents observed in smoke and flue gas from residential wood combustion sources(a,b) (g/Kg).

a DeAngelia, el al.(21) (grams emissions per kilogram wood).
b Hoffmann and Wyndcr,(30) page 339.

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