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Wood burning in general
In winter, emissions due to heating are an important source of air pollution. Wood-burning emits combustion particles, nitrogen oxides and sulphur dioxide, which are shown to have unwanted health effects. Particle emissions from oil and paraffin heaters are small compared with wood-burning. On cold days, high concentrations of these particles in the outside air can be a problem, especially in cities with high population densities.
Biggest problems in the cities
According to statistics from Statistics Norway, emissions from wood-burning account for more than 60 per cent of the total emissions of particulate matter in Norway. Emissions from wood-burning in Oslo are somewhat lower at around 40 per cent but high population densities make emissions more concentrated in cities and towns than in rural areas. Therefore, wood-burning leads to the biggest problems in the cities. A new method for measuring particle emissions from wood-burning suggests that previous figures are somewhat overestimated. In addition, the concentrations at ground level will be considerably lower since smoke is released over rooftops as opposed to particulate matter from road traffic.
New stoves reduce emissions significantly
On a national basis, almost 80 per cent of wood is burnt in old wood stoves or open fireplaces. Much of the wood-burning takes place with an insufficient oxygen supply, which causes incomplete combustion and high emissions of particles. The old wood-burning stoves release six times as many particles as new clean-burning stoves. Particle emissions from wood-burning can therefore be greatly reduced by replacing old polluting stoves with new clean-burning stoves. However, in order to achieve this reduction in emissions, it is a prerequisite to burn wood correctly in clean-burning stoves.
Substances in the lungs
The particles in wood-burning smoke are mostly fine and ultrafine combustion particles consisting mainly of carbon aggregates in the range of 0.02 to 0.3 μm. Such small particles will be able to reach the smallest branches in the respiratory tract and the lungs, where they can cause damage and respiratory problems. These particles have a large surface where harmful chemical substances and proteins (allergens) can bind. The particles can thus be problematic both in themselves and by transporting substances into the lungs. Polyaromatic hydrocarbons (PAHs) are formed by incomplete combustion, and several of these are carcinogenic. Another carcinogenic substance found in wood-burning smoke is benzene. Several irritants are also present, including aldehydes and organic acids.
Comparable to diesel particulate matter
Few experimental studies have been conducted to investigate whether emissions from wood-burning are harmful to health. Animal testing has found some negative effects of wood-burning smoke. However, neither lung function nor symptoms of respiratory diseases have been evaluated in animal studies. This is the endpoint that is tested in population studies. The particles released from Norwegian wood stoves are comparable to diesel particulate matter in terms of size and chemical composition. There is therefore reason to believe that they may have similar health effects.
Unlike traffic, wood-burning is a pollution source that is only problematic in some countries so there is less focus on its effects. In addition, wood-burning is limited to a few months a year, and studies of long-term effects of wood-burning particles are therefore difficult. In studies investigating the effect of wood-burning particles in outdoor air, only a worsening of acute asthma has been found. When it comes to particles from indoor fires and simple stoves in developing countries, more studies have been done, but these particles or particle levels are not comparable to those found in wood-burning in outdoor air in Norway.
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.
1. Replace an old stove with a modern, clean-burning stove.
2. Use dry wood.
3. Light the fire on top of the wood - not underneath.
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.
You should light the fire on top of the wood because when the wood is lit from above, the heat radiates to warm the wood underneath. The wood will then begin to release gases which will rise, meet the flames and ignite. If you light from below, the heat radiation will cause the wood above the flames to release gases which will rise. If there are no flames on top of the wood, the gases will be released unburnt out of the combustion chamber to the chimney, where they will form particles.
Lighting the fire on top of the wood will halve the particle emissions from the chimney. You also gain maximum heating efficiency from your wood and thus lower your heating costs.
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.
- 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 in the lungs. In addition, the smallest particles have a larger surface per mass unit, providing a larger area for interaction with lung cells.
How do we burn wood?
There are many types of stoves available and people use different types of fuels of varying quality. During good combustion conditions with sufficient oxygen supply and high temperature, most of the organic substances will be burnt in the stove and the particle emissions will be lower.
There are several factors that determine how complete the combustion process is in a stove:
- type of fuel (wood versus pellets)
- moisture content
- combustion technology in the stove (new clean-burning versus old, conventional stove)
Particles emitted during burning
Wood-burning particles can be split into three classes based on their physical and chemical properties and the combustion conditions:
- The particles emitted from poor combustion conditions (“smouldering” combustion) contain relatively large amounts of unburned organic substances from the wood, some of which may be carcinogenic, such as polycyclic aromatic hydrocarbons (PAHs). These particles are quite water soluble and are assumed to be removed relatively quickly from the lungs.
- With improved combustion conditions ('burning with flames'), carbon particles with an insoluble core are formed. Depending on how good the combustion conditions are, the carbon particles have varying amounts of organic compounds on the surface. These particles are likely to remain in the lungs for longer periods of time since they are insoluble.
- With complete combustion, for instance in pellet stoves, all the organic material in the wood is broken down in the combustion chamber. Non-combustible substances (ash) will be emitted from the chimney, these are mainly water-soluble salt particles like potassium sulphate. When these particles are deposited in the lungs, they will dissolve quickly in the lung lining fluid and thus be removed.
Several of the major cities in Norway have programmes to replace old, conventional stoves with new clean-burning ones. The purpose is to reduce emissions of particulate matter and contribute to cleaner air.