Breathe Poland. What’s in the smoke from burning wood?

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In the case of many pollutants, their amount in the smoke may very strongly depend on the type and quality of fuel (especially its humidity). And also from the device in which we burn it. The emission of pollutants from a tiled stove, from a primitive boiler (the so-called “goat”) or from a simple, traditional fireplace will be different, and different – usually much lower – in the case of a modern fireplace (especially equipped with an electrostatic precipitator), wood gasification boiler or a boiler for wood pellets.

However, practice shows that real emissions – for example from fireplaces – can be very high. And even in a country as wealthy and caring for its environment and citizens as Austria. The skills and experience of the person who smokes in the stove or fireplace also have a great influence on the amount of pollutant emissions.

It should be emphasized that all the time we are talking only about burning pure wood, not chipboards or varnished or impregnated wood. Smoking this type of “fuel” significantly increases the emission of various toxic substances, including volatile organic compounds, formaldehyde, PAHs and dioxins (compared to the situation when we smoke with pure wood). It is also illegal to burn hazardous waste and not legal fuel.

Exactly the same can be said about the simultaneous burning of plastic waste, such as plastic bottles, with wood.

Comparison: wood versus coal

Many harmful substances are present in both the smoke that comes from burning wood and the smoke that comes from burning coal. Even, of course, suspended dust. The dust emitted when burning biomass is usually finer than the dust produced by burning coal. In other words, the content of PM2.5 in PM10 is very high. This is important because the smaller the dust particles, the more dangerous it is to our health.

In both cases (coal and wood), we also usually deal with polycyclic aromatic hydrocarbons (PAHs) and with dioxins.

Compared to burning coal, burning wood under unfavorable conditions (primitive boiler, low-quality fuel) can lead to similar or even higher PAH and dust emissions. If both the fuel and the heating device (boiler, stove) are of similar quality / class, then when burning wood, we can usually expect lower PAH emissions than when burning coal. [3]

But once again: for both coal and wood, the chemical composition of the exhaust gases and the amount of pollutants emitted depend on the combustion conditions and the quality of the fuel. Usually, however, the composition of smoke from burning coal and that from burning wood differ so much that we will not confuse them even by looking only at our nose, by the smell. Even more so, air pollution specialists equipped with appropriate instruments and methods of analysis have no problem with distinguishing them. [4]

Wood (more generally, biomass) typically contains much less sulfur than carbon. Sulfur dioxide emissions are therefore much lower when burning biomass than when burning hard coal.

Compared to hard coal, emissions of nitrogen oxides are also lower (due to the lower combustion temperature of biomass).

Contrary to hard coal and lignite, biomass contains at most traces of mercury. When we burn with biomass, the problem of the emission of this toxic element is eliminated.

But we also have a lot of harmful substances that occur when wood is burned and are not (or present in much smaller amounts) when burning coal. (You can find them in the table on pages 57-61 of the publication of the International Agency for Research on Cancer, “Household Use of Solid Fuels and High-temperature Frying”).

Wood smoke and tobacco smoke

The smoke from burning wood can also be compared to cigarette smoke. The similarities here are even greater than in the case of hard coal combustion, which should not surprise us too much.

Burning one kilogram of wood can produce as much dust as burning several hundred cigarettes. But in the case of one of the most harmful compounds from the PAH group – benzoapirene – the equivalent amount of cigarettes is much greater: several – tens of thousands! It follows that the same amount (mass) of wood smoke usually contains much greater amounts of benzoapirene (and many other PAHs) than tobacco smoke!

On the other hand, even breathing air heavily polluted by fumes from the chimneys of wood-heated houses for several hours a day, we still do not draw into our lungs as much smoke (dust) as heavy smokers are exposed to. Fortunately.

Thanks

I would like to thank Piotr Kleczkowski and Michał Krzyżanowski for their help in writing this text.

Footnotes

[1] When burning any biomass, including wood, of course, carbon dioxide (CO2) and water (H2O) are formed, which is present in the exhaust gas as a gas – water vapor or as small droplets of liquid water. However, we are talking about substances that are directly harmful to health in relatively low concentrations. That is, at the concentrations in which these substances are usually found in the outside air (as “smog components”) or inside buildings. Therefore, in the text I do not mention CO2 or H2O. However, it is worth remembering that the increased concentration of carbon dioxide in the air is not indifferent to our well-being and efficient functioning. CO2 is also the most important greenhouse gas.

[2] PAHs with five or more rings are usually found primarily in particulate matter and not as individual particles in the gas phase.

[3] We compare here the so-called emission factors, i.e. the amount of a given harmful substance emitted when burning a kilogram of a given fuel. The amount of emitted substance per unit of energy obtained from a given fuel is also often used, the unit then being e.g. gram / megajoule, g / MJ.

[4] When wood is burned, levoglucosan is formed – a chemical compound that is often used as a marker (indicator) of biomass combustion.

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