Are Wood-burning Stoves Good or Bad?
One of a number of topics being discussed in terms of technology that pollutes our atmosphere and affects our health is the question of whether wood-burning stoves are harmful or safe as a way in which to create heating. Much debate has been made, on both sides, suggesting that it is either harmful to our health or not.
A Norwegian researcher has some of the answers, thanks to her research into a variety of different types of wood-burning stoves and the health impacts they each represent.
”The physical and chemical properties of particulate matter from wood-burning have great influence on how these particles may affect our health. Worsening of cardiovascular diseases and respiratory diseases such as asthma and chronic obstructive pulmonary disease are the main concerns,” says Anette Kocbach Bølling from the Norwegian Institute of Public Health, who spent 2011 touring a variety of symposia, sharing her findings on how wood smoke particles generated under different conditions may or may not influence our health.
Different burning conditions give different particles
“Particulate matter from different sources has different physical and chemical properties, and several factors are important when we study their health effects” says Bølling.
- Chemical composition – some metals and organic substances are more harmful to the cells in our bodies than others.
- Solubility – water soluble particles will dissolve easily in the lung lining fluid and be removed from the lungs.
- Size – particle size determines the deposition rate and probability in our lungs. In addition, the smallest particles have a larger surface per mass unit, providing a larger area for interaction with the cells in our lungs.
How do we burn wood?
One of the real important factors in this issue is how well we are able to burn the contents of the stove. There are a number of different types of stoves and probably an even greater number of types of fuel to be used.
According to Bølling, good combustion requires sufficient oxygen supply and a high temperature, and in these cases most of the organic substances will be burnt in the stove. As such, there are several factors which help determine how complete the combustion process is;
- type of fuel (wood versus pellets)
- moisture content
- combustion technology in the stove (new clean-burning versus old, conventional stove)
There are three different classes, according to Bølling, of wood-burning particles, based on their physical and chemical properties and the conditions of combustion.
First you have the particles being emitted during poor combustion conditions, known as “smouldering” combustion. These particles contain relatively high amounts of unburned organic substances from the wood, including carcinogenic particles like polycyclic aromatic hydrocarbons (PAHs). These particles are highly water soluble and scientists believe that they are removed relatively quickly from the lungs.
Secondly you have improved combustion conditions, “burning with flame”, wherein carbon particles with an insoluble core are formed. Once again, it depends on how good the combustion conditions are, but the carbon particles can have a varying amount of organic compounds on the surface which are likely to remain in the lungs as they are insoluble.
Lastly, you have complete combustion stoves, like in pellet stoves, where all the organic material is broken down in the combustion chamber. Water soluble ash will be emitted through the chimney and, when deposited in the lungs they dissolve quickly.
The Health Effects
So it is clear that complete combustion stoves are preferable, as the particles emitted from these are relatively small and are dissolved quickly regardless. However when it comes to the emissions produced by burning of logs in old stoves, we simply don’t have enough knowledge to distinguish between the health effects from smouldering and flaming combustion.
The final say is that emissions are reduced by improved combustion.
The Norwegian Institute of Public Health provided these “tips for health-friendly and efficient wood-burning”;
When wood burns it releases gases. Half of the energy (heat) in the wood is in these gases.
The most important thing is to replace an old stove with a modern, clean-burning stove. With this type of stove, any uncombusted gases and particles from the first chamber can be burnt.
- Use dry wood
- Light the fire on top of the wood – not under, because:
- when the wood is lit from above, the heat radiates to warm the wood underneath. These pieces of wood will begin to emit gases which will rise, meet the flames and ignite
- if you light from below, the heat radiation will cause the wood over the flames to release gases which will rise. Without flames at the top of the wood, the gases will be released unburnt out of the combustion chamber to the chimney, where they will form particles
- the particle emissions from the chimney will be halved
- you gain maximum heating efficiency from your wood and thus lower your heating costs
‘Guidebook – Effective and environmentally friendly firing of firewood’ by Edvard Karlsvik, SINTEF Energy Research, Norway and Heikki Oravainen, VTT, Finland