Toxic threat from wood burning

The burning of wood and coal for domestic heating is a major source of air pollutant emissions, contributing to more than half of primary PM_2.5 emissions in the EU. A new study compares the emissions from various types of heating options.

Although wood burning is perceived by many as being natural and therefore environmentally benign, it is in fact a significant source of several harmful air pollutants, including fine particulate matter (PM_2.5), black carbon (BC), dioxins, and polycyclic aromatic hydrocarbons (PAHs).

Small-scale combustion of fuels for domestic heating is reported in the European emission inventories under a sector called “residential, commercial & institutional”. According to emission data for 2019 from the European Environment Agency (EEA), fuel combustion in this sector in the EU28 was the major emission source of primary PM_2.5, contributing 53 per cent of total emissions, as well as PM₁₀ (40%), black carbon (39%), dioxins (42%) and the carcinogenic PAH compound benzo(a)pyrene (87%).

These emissions contribute significantly to premature mortality and morbidity, and black carbon also contributes to Arctic warming and global climate change. Moreover, the share of emissions in the EU from residential solid fuel burning has increased over the last 20 years and is expected to continue increasing, as other important emission sources gradually become more efficiently regulated.

A new study by the European Environmental Bureau (EEB) and Green Transition Denmark has compared the emissions of air and climate pollutants from wood burning to those from several other heating sources, such as oil, gas, electric, heat pumps, and district heating. The expressed purpose of the study, which is referred to as a pre-study, is to raise awareness about pollution from wood burning.

Expressed as grams of pollutant per unit of energy, the emission levels of PM_2.5, BC, PAH, volatile organic compounds (NM-VOC), and methane (CH₄) from residential wood burning are many times higher than those from other heat sources (see Table). It should be noted that emissions of PM_2.5 from wood burning may increase significantly if the stove/boiler is not operated properly and therefore can be much higher in real-life conditions.

Moreover, wood burning in stoves can give rise to very high levels of indoor air pollution – a recent study in the UK showed a threefold increase in harmful PM levels in homes using wood stoves.

The climate impacts of different heating options are also compared, summarising the net global warming potential (GWP) over 20 and 100 years respectively, using the IPCC GWP factors for each of the climate pollutants CO₂, CH₄, N₂O and BC. This calculation shows two options for the CO₂ emissions from wood burning, where these are either accounted for or not.

If the CO₂ emissions from wood combustion are included, it was found that wood-burning results in the highest global warming impact, and if these emissions are ignored (i.e. assuming that trees from sustainable forestry grow as fast as they are burned) wood-burning would be a better option for the climate than fossil fuels (coal, oil, gas) for heating. Even better would be to use the wood to replace fossil fuels burned by district heating or power plants, and then use the resulting district heating or electricity (e.g. in heat pumps) to heat homes. The best climate option would be to use green electricity or solar heating.

The health impacts of air pollution depend on both the toxicity of the individual pollutants and on the location of the emission. To illustrate the air pollution health costs of different heating options, the study used average Danish health cost data for four key air pollutants (PM_2.5, NOx, SO₂ and NH₃), and assumed that the emissions take place in Denmark. It was found that wood-burning is the most expensive heat source, resulting in the highest health damage, both domestically and internationally.

In conclusion, residential solid fuel burning is the most polluting and costly option for domestic heating, and it contributes significantly to global warming. Although new wood-burning appliances are more efficient and emit less particles than the old ones, they still pollute much more than other available heating options. Replacing wood burning with better insulation and alternative clean heat will provide big health benefits.

The study ends with three recommendations:

• Carry out a more detailed study on air and climate pollution from domestic heating;
• Set much stricter emission limit values in the EU EcoDesign directive for wood-fired stoves and boilers;
• Start a Europe-wide campaign for better insulation and cleaner domestic heating, including i.e. local/national actions to ban or limit domestic solid fuel combustion.

Christer Ågren

Source: “Emissions from domestic heating”. By Kåre Press-Kristensen. Published by the European Environmental Bureau (EEB) and Green Transition Denmark (September 2021). Available at: https://eeb.org

Table: Emissions of air pollutants from different heat sources, expressed as grams of pollutant per gigajoule house heating.