Small chimneys – big emissions

Illustration: Lars-Erik Håkansson

The Danish Government and the European Commission have separately presented proposals for emission standards for new boilers and stoves. But to achieve noticeable near-term air pollution reductions it is essential to combine such standards with measures for existing installations.

Residential wood burning in the EU is a significant source of several air pollutants: fine particulate matter (PM2.5), black carbon (soot), dioxins, polycyclic aromatic hydrocarbons (PAHs) and volatile organic compounds (VOCs). It accounts for about one-third of the total emissions of soot and PM2.5. These emissions contribute significantly to premature mortality and morbidity in the EU as well as to Arctic warming and thereby climate change. The share of emissions from residential wood burning is expected to increase as other key emission sources gradually become more efficiently regulated and because increasing costs of conventional home heating will continue to stimulate cheap wood burning.

Emission levels per unit of energy from residential wood burning are so high that emission levels from other heat sources are overshadowed (figure). In addition, detailed Danish and Swiss measurements from chimneys show that emissions of PM2.5 may increase up to 30 times if a stove isn’t operated properly and up to 250 times if it is misused, which underlines the fact that emission levels can be much higher under real-life conditions.

Figure: PM2.5 emission levels including condensates (Norwegian standard NS 3058-2) from boilers and stoves compared to other heat sources. In comparison, the emission level of a truck without filter (EURO V) is included.
Note: The emission level for boilers may be expressed as mg/Nm3 and 10 per cent oxygen. This emission does not include condensates. Usually, 1 mg/Nm3 ≈ 5.5 g/GJ including condensates.  Newer estimates suggest that emission factors from boilers may be reduced up to 40 per cent. Emission from stoves may be expressed as mg/Nm3 and 13 per cent oxygen. This emission does not include condensates. Usually, 1 mg/Nm3 ≈ 7.7 g/GJ including condensates.

The key challenges related to residential wood burning are the high pollution levels combined with a long lifetime of each unit (usually more than 30 years) and the intensive use of wood burning because it is a cheap or sometimes even free way of heating in many parts of Europe.

The Danish Centre for Environmental Research has conducted very detailed studies on pollution from residential wood burning, and found that it is responsible for about 80 per cent of Danish PAH emissions, 70 per cent of PM2.5, 60 per cent of black carbon, 50 per cent of dioxin and contributes significantly to emissions of VOCs and CO (ozone precursors). In comparison, all Danish power plants emit about 2 per cent of the total PM2.5 emissions but produce more than 60 per cent of the energy.

Detailed measurements in residential areas in Denmark show that wood burning can increase local PM2.5 and PAH concentrations to levels similar to those in rush hour traffic in central Copenhagen.

The Danish Centre for Energy, Environment and Health estimates that pollution from Danish residential wood burning causes about 250 premature deaths yearly in Denmark and about 650 premature deaths in the EU (due to pollution spread to the rest of Europe). This mortality is related to chronic exposure to increased PM concentrations and not to acute exposure to wood smoke. On top of this, tens of thousands of cases of serious diseases (heart, airways and cancer) can be related to the pollution. This estimate does not take into account secondary particles from VOC emissions. The yearly socio-economic health costs related to pollution from Danish wood burning are estimated to be around €500 million per year.

If residential wood burning remains practically unregulated, the sector will by 2025 be responsible for more than 90 per cent of the total Danish PM2.5 emissions.

In the light of this situation, the Danish government put forward a proposal to regulate stoves and boilers that has just been out for public consultation. The key point is to introduce emission standards for new stoves and boilers. The limit values are aimed at both sale and resale and must be fulfilled to sell stoves and boilers in Denmark and will only come into force if allowed as national standards by the EU. The Commission has also published a working document with possible EU-wide emission limit values for new stoves in the form of the Ecodesign Directive.

The Danish and EU proposals for PM limit values for stoves are shown in the table and compared to the best stoves on the market today and emissions from other heat sources in Denmark as well as the European Environmental Bureau’s proposal for limit values(see figure). The Danish limit value for 2016 is twice as high as the best stove was in 2010 and the EU limit value for 2015 is almost ten times higher. The proposed limit values are high compared to emission levels for other heat sources and trucks as well. However, a key problem is that the regulation does not focus on replacement of existing stoves with new models or other heat sources. Since stoves and boilers often have a long lifetime (more than 30 years) the Danish Environment Protection Agency estimates that the proposed regulation will only reduce particle pollution by 2 per cent.

Table: Suggested Danish, EU and EEB emission limit values for stoves. All limit values include condensates (Norwegian standard NS 3058-2).

Stoves 2013 2016  
Danish suggestion (in g PM2.5/GJ)
(original units: g PM / kg wood)

(5 g PM/kg wood)

(4 g PM/kg wood)
  2015 2017 2019
Commission suggestion (in g PM2.5/GJ ) (original units: mg/Nm3, 13% O2) 1,150
(150 mg/Nm3)
(75 mg/Nm3)
(40 mg/Nm3)
  2015 2018 2020
EEB suggestion
(in g PM2.5 /GJ )
200 100 50

By replacing old wood stoves with Swan labelled1  stoves and replacing old wood boilers with new boilers the pollution from private wood burning can be halved. This would reduce the total Danish and EU emission of PM2.5 by 35 and 16 per cent, respectively. And if all wood stoves and wood boilers were replaced with wood pellet equivalents the pollution from residential wood burning could be shrunk to less than a tenth of present emissions. Total Danish and EU emission of PM2.5 would be reduced by about 65 and 30 per cent, respectively, and other key pollutants would also be reduced significantly.

Several flue gas cleaning technologies have been tested in Denmark without success. Consequently, the most cost-efficient PM reduction measures for this sector are to replace existing wood stoves and wood boilers with the best ones on the market, or even better, with pellet stoves and pellet boilers or other heating sources.

Economic incentives (taxes or charges) combined with strict emission limit values are efficient instruments to promote replacement (shorten the lifetime) or phasing out of heavily polluting stoves and boilers, thereby promoting the best stoves and boilers and encouraging better home insulation, heat pumps or, in cities, gas and district heating. The Danish Ecological Council has designed a tax proposal concerning stoves in Denmark. The tax is based on the type of installation, varies with emission levels and is differentiated for urban and rural areas. For an old wood stove within an urban area the tax would be 1,000 euros a year, but for an equivalent stove in a rural area the tax would only be half as much. A modern eco-labelled wood stove in an urban home would cost 500 euros a year, while an equivalent stove in a country home would be free from tax. Pellet stoves would be exempted from tax everywhere.

The proposal means fairer taxation of heat sources by increasing the fee on the most polluting types. It has been estimated that this tax model would reduce the pollution from wood burning by more than 50 per cent through the replacement of units, increase Danish tax revenues by €200-300 million per year and result in annual health gains worth €200-300 million in Denmark and almost €1 billion in Europe due to Danish PM reductions. Furthermore, incentives to insulate houses would be enhanced, as well as the promotion, sales and development of better stoves.

In order to further promote the best new stoves and boilers an EU labelling system could be introduced. Another option is to introduce low-emission zones prohibiting or restricting domestic wood burning in residential areas where district heating or gas are – or can be made – available.

Kåre Press-Kristensen

Kåre Press-Kristensen is senior advisor on air quality at The Danish Ecological Council (

 1The swan is an ecolabel established by the Nordic Council of Ministers.


Danish wood burning

Stoves emit about 70% of the PM2.5 from residential wood burning and boilers emit about 30%. About 25% of the wood is used in wood pellet boilers but these only cause about 1% of the emissions. About 20% of the Danish stoves are old (before 1990) and 80% are newer. About 15% of the Danish stoves have the Swan label. About 20-30% of the Danish boilers are old (before 1980) and 70-80% are newer.


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