DOI: 10.3303/CET2292040 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Paper Received: 31 January 2022; Revised: 2 March 2022; Accepted: 22 April 2022 
Please cite this article as: Skreiberg Ø., Seljeskog M., Kausch F., 2022, A Critical Review and Discussion on Emission Factors for Wood Stoves, 
Chemical Engineering Transactions, 92, 235-240  DOI:10.3303/CET2292040 
  

 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 92, 2022 

A publication of 

 

The Italian Association 
of Chemical Engineering 
Online at www.cetjournal.it 

Guest Editors: Rubens Maciel Filho, Eliseo Ranzi, Leonardo Tognotti 

Copyright © 2022, AIDIC Servizi S.r.l. 

ISBN 978-88-95608-90-7; ISSN 2283-9216 

A Critical Review and Discussion on Emission Factors for 

Wood Stoves 

Øyvind Skreiberg*, Morten Seljeskog, Franziska Kausch 

Thermal Energy Department, SINTEF Energy Research, Trondheim, Norway   

oyvind.skreiberg@sintef.no 

Small scale heating appliances such as wood stoves, significantly contribute to domestic heating and energy 

security in many European countries. However, emissions from wood stoves remain a significant concern, even 

though modern wood stoves are continuously improved to reduce emissions due to incomplete combustion. 

Most State-Of-The-Art (SOTA) stoves after the 1990s, achieve significantly lower emissions than stoves 

produced in the period 1940-1990. The main reason being the introduction of emission limits and test standards 

both in Norway and other European countries. SOTA stoves today, including catalyst stoves and the more recent 

downdraft concepts, all apply a strategic and more or less optimized staged air supply. In stoves without catalyst, 

optimized air supply and combustion chamber geometry as well as combustion chamber insulation, are the main 

reasons modern stoves achieve better burnout. When comparing national emission inventories, we find 

unacceptable large variations in emission factors for most reported compounds in all official stove categories. 

There are in some cases plausible reasons for such differences, but for some cases the differences can hardly 

be justified. Both stove categories, old and new, suffers from differences of up to two magnitudes, when 

comparing emission factors used in the national emission inventories in the Nordic countries. Hence, there is a 

real need to correct and align these for inclusion in national emission inventories, which should be reflecting 

real-life emissions as accurately as possible. As stoves are continuously being improved, we also suggest yearly 

updates accounting for such improvements, in the annual national emission inventories reports. 

1. Introduction 

Emissions from wood stoves remain a significant concern, even though modern stoves are continuously 

improved regarding emissions due to incomplete combustion, e.g. particles (PM) of organic origin [Skreiberg 

and Seljeskog, 2018]. Today, all SOTA stoves apply some optimized air supply strategy to comply with the ever 

more stringent emission regulations. Currently there is a large variation in the emission factors used for different 

emission compounds and for different wood stove categories when reported in official national emission 

inventories. These national emission inventories are further reported at international levels, e.g. EU, as a basis 

for further elaborations and estimations of e.g. health and climate impacts related to wood stove emissions. The 

large discrepancies in some emission factors, even within the Nordic countries, justify an updated review and 

discussion around current emission factors, with the final aim of recommending corrected and aligned factors 

and as well as advice for further work, accounting for the impact severity of the reported components. 

Today’s SOTA stoves outperform the first generation staged air stoves introduced in the 1990s [Skreiberg and 

Seljeskog, 2018], as producers continue to optimize them. Further reductions are expected, also due to the 

development of secondary emission reduction measures as catalytic converters, automated air supply and 

distribution control and electrostatic filters. All these efforts have the potential to bring emissions down to very 

low levels, close to those of pellet stoves, e.g. well below 1 g/kg for PM emissions. As stoves reach such low 

emission levels, they become a much stronger complement to other small-scale heating appliances for 

households such as e.g. heat pumps and solar thermal solutions. Combinations of such systems also becomes 

much more defendable for future home heating systems, to achieve a sustainable 100% renewable heating 

within 2050. The aim of this study is to contribute to this future through a critical review. 

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1.1 Wood stove development history 

In the late 1980s development of new concepts was addressed to make stoves more environmentally friendly, 

user-friendly and cost-efficient. In parallel, regulations on emissions from wood stoves were introduced in 

Norway on 1 July 1998. To comply with these emission limits, the stove manufacturers introduced staged air 

combustion where fuel is gasified in the primary combustion zone and additional air is injected in the secondary 

combustion zone for complete combustion. Following the introduction of the staged air technology, continuous 

optimization has led to important reductions of emissions and increased efficiency. This holds true when testing 

the stoves according to different test standards, however, the individual emission levels typically can vary 

significantly depending on the test standard. 

1.2 Wood stove test standards 

Since the 1990s, introduction and development of test standards for required type approvals have taken place 

individually in many countries around the world, resulting in a variety of different standards and emission 

regulations. Many of the standards do not even produce comparable results, as they are physically different in 

their approach. In Norway, NS 3058-1/2:1994 and NS 3059:1994 was, and still is, the prevailing test standard 

since 1998, applying the principle of a full flow dilution tunnel in combination with a gravimetric method to 

determine the mass of emitted PM. In Europe, the prevailing standard was the EN 13240:2001, to determine 

CO emissions and efficiency, in combination with DINplus:2004, for OGC, NOx and PM. PM is sampled 

gravimetrically directly in the chimney on a heated filter (70°C). Ecodesign (2015) and its associated EN 16510 

(2018) standard takes over from the previously mentioned test standards from 2022. However, still three 

different PM measurement methods are included in the standard, sampling on a hot filter, in a full flow dilution 

tunnel or with an electrostatic precipitator. Comparing stoves between different standards might be very difficult, 

as both testing procedures, measurement methods and wood species vary. I.e., the test standards are primarily 

useful for comparing stoves, and not for assessing real-life emissions. Some test standards try to account for 

real-life operation, through testing at different operating conditions, e.g. loads, however, they fail in accounting 

for real-life behaviour, as when ordinary people operate their wood stoves in their homes. Creating a test 

standard that accounts for real-life behaviour is very difficult, maybe impossible, so the key question will then 

be to what extent wood stoves should be tested to ensure low real-life emissions, in a cost-efficient way. 

1.3 Environment, climate and health related emission compounds 

Wood log combustion in wood stoves results in emissions of many emission compounds, gaseous and 

particulate. Their levels vary, depending for some compounds mainly on the fuel composition, and for others 

mainly on the combustion conditions. For the first category, avoiding using the fuels containing high levels of 

minor and trace elements, especially N and some ash elements, is an option, while for the second category, 

combustion process optimisation is key, enabling reducing emission levels of unburnt resulting from mainly the 

C and H content of the fuel, down to very low levels. Climate impacting emissions includes CH4, N2O and black 

carbon as climate forcers while aerosol forming compounds (organic carbon, SOx, NOx) contribute to reversed 

climate impact. Particles of different origins, especially the smallest particles, contribute to adverse health effects 

as do CO, Polycyclic Aromatic Hydrocarbons (PAH), dioxins and heavy metals. Minor species (N, S, Cl) and 

ash elements contribute to environmental impacts, e.g. acid rain. When type approving a wood stove, the 

emission levels of many of these harmful emissions are not measured, and hence determining such emission 

levels depends on specific projects where such measurements have been included, and the representativeness 

of the values can be questioned if there are testing deviations compared to type approval testing. 

2. Emission factors 

Emission factors are typically developed to represent a technology category, and are used e.g. in national 

emission inventories. Commonly, the emission factor is kept for a number of years, while relative technology 

stock changes between technology categories (as replacing old stoves with new stoves) leads to overall 

emission reduction. For wood stoves, the typical technology categories are old stoves, new stoves including 

fireplace inserts, and open fireplaces. As technology improves through continuous development, a differentiation 

within a technology category with respect to year makes sense, but this is only to a low degree done in practice. 

Such a differentiation will also showcase the actual technology improvements, which is a central driver for 

replacing older and (much) less performant technology. 

2.1 Wood stoves in the national emission inventories - The case of Norway 

Wood stoves in the Norwegian national emission inventory (2021) are represented by emission factors for TSP, 

PM10, PM2.5, CO, CH4, NMVOC, PAH-4 (LRTAP), SO2, NOx, NH3, N2O, Cd, Pb, Hg, As, Cr, Cu and dioxins, 

while additional emission factors, e.g. for the climate forcer black carbon, have been derived but are not yet 

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accounted for in the Norwegian national greenhouse gas inventory. Emission factors have been updated 

through the years based on new knowledge, and the last major revision was based on a work published in 2017 

(Seljeskog et al., 2017). Even after many years of research activities, still significant uncertainties exist with 

respect to the accuracy and representativeness of emission factors. Main uncertainties in emission factors are 

caused by different test standards or procedures used, different measurements methods, different fuels as well 

as operator influence. In the end, the real life emissions when people are using the stoves might be quite 

different for some emission compounds due to less skilled operators and the influence of several building related 

and atmospheric factors and also a wider spread in the fuel quality (e.g. moisture content) and the stove 

condition (e.g. tightness). Figure 1 shows the current and earlier emission factors for wood stoves in Norway. 
 

 

Figure 1: Current (from 2017) and earlier emission factors for wood stoves in Norway 

Before 2001 a limited set of emission factors was implemented in the national emission inventory and no 

differentiation was made between open fireplace, old stove and new stove. As new stoves, based on the staged 

air combustion principle, had clearly lower emission levels than old stoves, a distinction between open fireplace, 

old stove and new stove was made from 2001. The new stoves had much lower emissions of PM10, CO and 

PAH compared to old stoves. Interestingly, the open fireplaces had significantly lower values for the same 

emission compounds compared to old stoves. There is no logical reasoning behind this, but merely a lack of 

own data, and then choosing emission factors from literature that are not derived in a comparable manner. This 

can be seen also for NOx, where the emission factor for open fireplaces is much higher than for old and new 

stoves, due to different wood species used. For CH4, NMVOC and NH3, products of incomplete combustion, no 

differentiation between the stove categories was made, which was hardly a correct assumption. For SO2, N2O 

and Cd, also no differentiation between the stove categories was made, which is reasonable, as these emissions 

are mainly influenced by the fuel composition. In 2013, based on the BLACKOut project (2013), the emission 

factors for PM10 for old and new stoves were revised based on experiments with two assumed representative 

stoves, and the PM10 value for open fireplaces was now set as the TSP value. Based on TSP measurements, 

the PM10 emission factors were derived as a percentage (98%) of TSP, and for PM2.5, 95% of TSP. Such an 

assumption is commonly applied, however, with much uncertainty connected to it as the particle size distribution 

depends on a number of factors. The new emission factors for PM were significantly lower for old stoves and 

higher for new stoves than the previous emission factors. In fact, the emission factor for new stoves well 

exceeded the allowable emission level when type approving stoves according to NS 3058/59. The reasoning 

behind it was that the new emission factors derived from measurements in the SINTEF lab, were more realistic 

compared to emission factors derived from type approval testing, where type approval testing experts are able 

to get significantly lower emission levels than a non-expert would. In 2013 also emission factors for NH3, dioxins 

and heavy metals were introduced. No distinction between the stove categories was made, which is hardly 

correct for NH3 and dioxins since their emission levels depend on also the combustion conditions. For the heavy 

metals, the emission factors were set identical to emission factors for quite different fuels, which is hardly correct. 

Open Old New Open Old New Open Old New

fireplace stove stove fireplace stove stove fireplace stove stove

<2001 2001 2013 2017

PM10 g/kg 10 17.3 40 6.2 17 22.2 13.1 17 23.13 8.3

CO g/kg 100 126.3 150 50.5 126.3 150 50.5 126.3 102.025 85.73

SO2 g/kg 0.37 0.2 0.2 0.2 0.2 0.2 0.2 0.348 0.348 0.348

NOx g/kg 0.7 1.3 0.97 0.97 1.3 0.97 0.97 1.3 0.97 0.97

N2O g/kg 0.07 0.032 0.032 0.032 0.032 0.032 0.032 0.032 0.032 0.032

CH4 g/kg 5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3 16.1445 3.883

NMVOC g/kg 6.9 7 7 7 7 7 7 7 22.284 15.218

Cd mg/kg 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.16 0.16 0.16

PAH-total mg/kg 17.4 52 0.0226 17.4 52 0.0226

PAH-6 (OSPAR) mg/kg 6.1 8.1 0.045 6.1 8.1 0.045

PAH-4 (LRTAP) mg/kg 3 2.7 0.025 3 2.7 0.025 3 2.7 0.025

TSP g/kg 17.3 22.7 13.4 17.3 24.145 8.44

PM2.5 g/kg 16.4 21.6 12.7 16.4 20.855 7.85

NH3 g/kg 0.066 0.066 0.066 0.066 0.066 0.066

Pb mg/kg 0.05 0.05 0.05 0.487 0.487 0.487

Hg mg/kg 0.010244 0.010244 0.010244 0.0025 0.0025 0.0025

As mg/kg 0.159 0.159 0.159 0.00036 0.00036 0.00036

Cr mg/kg 0.152 0.152 0.152 0.152 0.152 0.152

Cu mg/kg 0.354 0.354 0.354 0.354 0.354 0.354

Dioxins ng/kg 5.9 5.9 5.9 5.9 9.9375 3.758

BC g/kg 0.96 0.86 1.47915 1.044 0.653

OC g/kg 16.74 10.47 13.2405 4.497

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In addition, emission factors for BC and Organic Carbon (OC) were established for old and new stoves, with 

slightly lower BC value and significantly lower OC value for new stoves, which is reasonable. In 2017, based on 

work (Seljeskog et al., 2017) carried out in CenBio (The Norwegian Bioenergy Innovation Centre), a number of 

new emission factors were derived for old and new stoves, and they are still used today. The reasoning behind 

it was to derive representative emission factors based on a wider set of stoves and experiments, while taking 

into account a user distribution between part load and nominal load operation. For fuel composition dependent 

emission factors (NOx, N2O, NH3, SO2 and heavy metals), they were derived based on fuel analysis and a wood 

species distribution. For Cr and Cu the old emission factors were kept, due to lack of analysis data. An 

assumption was also made regarding the dioxin emission levels for old and new stoves at part load and nominal 

load operation, with highest level for old stoves at part load operation and lowest level for new stoves at nominal 

load operation. However, the weighted average dioxin emission factor was maintained. Overall, it can be said 

that the changing emission factors through the years are a result of the use of more representative data and 

more logical assumptions, however, still up to large uncertainties exist for some emission factors, and there has 

not been enough focus on assessing the improvement for new stoves over the years. This improvement has 

shown to be very significant and many of the emission factors for the new stove category today are clearly not 

representative for SOTA stoves within this stove category. On average a continuous improvement is occurring, 

which is not accounted for when keeping an average emission factor for the new stove category unchanged 

over the years. Hence, there is a real need to split the new stove category into sub-categories based on stove 

introduction year. This has been done in the Danish national emission inventory (2021), as will be shown and 

discussed later. Introducing an "advanced/ ecolabelled" stove category has been done by EU in their EMEP/EEA 

air pollutant emission inventory guidebook (2019) and in the Danish national emission inventory as well. This is 

also a step in the right direction, but the emission factors used are all referring to relatively old works.  

2.2 Comparison with other Nordic countries 

Figure 2a compares the currently used emission factors in the Nordic countries Norway, Denmark, Finland and 

Sweden for the old stove category (national emission inventory reports, 2021). Emission factor ratios between 

the other Nordic countries and Norway have been calculated to visualise the differences. Large deviations in 

emission factors can be seen for many of the emissions, both for the factors mainly influenced by the fuel used 

and for the factors mainly influenced by the combustion conditions. Especially worrisome are the large 

deviations found for PM. The deviations can partly be explained by different stove testing procedures used, as 

the PM emission factors are derived using NS in Norway and Denmark, and EN in Sweden and Finland. The 

PM emission factors are lower in Denmark, relying more on type approval testing data, i.e. data achieved by 

type approval testing experts. In Finland and Sweden much lower PM emission factors are used, due to testing 

according to the EN standard, which does not account for PM formed from condensables. The different test 

standards used also results in much lower emission factors for emissions typically influenced mainly by the 

combustion conditions, e.g. CO and CH4, and also for dioxins. However, for NMVOC the emission factor in 

Finland is much higher than for all the other countries. For PAH all the other Nordic countries have higher 

emission factors than Norway. For the climate forcer black carbon, Denmark and Finland have much lower 

emission factors than Norway and Sweden, with Sweden having the highest emission factor. For emissions 

influenced mainly by the N-content, and to some extent by the combustion conditions, as NOx, N2O and NH3, 

large deviations exist due to partly different fuel-N contents, but mainly due to different assumptions regarding 

the distribution of N between NOx and NH3, the latter typically increasing in importance at poor combustion 

conditions. Overall, unacceptable deviations exist for a number of the emission factors for old stoves between 

the different Nordic countries, which highly influence the outcome of the national emission inventory for this 

stove category. 

 

Figure 2: Relative comparison of emission factors - Old (a) and new (b) stove category 

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Figure 2b compares correspondingly the currently used emission factors in the Nordic countries Norway, 

Denmark, Finland and Sweden for the new stove category. Also for this stove category large deviations in 

emission factors can be seen for many of the emissions, both for the factors mainly influenced by the fuel used 

and for the factors mainly influenced by the combustion conditions. Again, especially worrisome are the large 

deviations found for particles, now with much lower emission factors also in Denmark compared to Norway. This 

is also the case for CO, but no longer for CH4. For dioxins, the ratio between the Danish emission factor and 

the other countries' emission factors has increased further. For NMVOC the emission factor in Finland is now 

on par with those in Denmark and Sweden and is much lower than in Norway. For PAH all the other Nordic 

countries have much higher emission factors than Norway. For the climate forcer black carbon, now Finland 

and Sweden have lower emission factors than Norway and Denmark, with Denmark having the highest emission 

factor, but quite close to the Norwegian emission factor. For NOx, N2O and NH3, large deviations exist due to 

partly different fuel-N contents, but mainly due to different assumptions regarding the distribution of N between 

NOx and NH3. Especially Denmark assumes a high NH3 emission factor also for the new stove category, which 

is hard to defend. Overall, unacceptable deviations exist for a number of the emission factors for also new stoves 

between the different Nordic countries, which highly influence the outcome of the national emission inventory 

also for this stove category. 

2.3 Comparison with EU 

When comparing with the EMEP/EEA air pollutant emission inventory guidebook (2019) which as Denmark also 

operates with an ecolabelled stove category, see Figure 3, it becomes clear that a number of the Danish 

emission factors, both for ordinary and ecolabelled new stoves are based on EMEP. However, for NOx and 

NMVOC differences can be seen for the ecolabelled stoves, with e.g. reduced NOx emission factor for Denmark 

and increased for EMEP. It is also worth noting that for CO, PAH-4 and dioxins, there are no reduction in the 

emission factor for ecolabelled stoves, while in EMEP these emission factors are significantly reduced for 

ecolabelled stoves. Interestingly, EMEP emission factors for particles are much closer to the Norwegian ones 

for new stoves.  

 

 

Figure 3: Comparison with EU, including ecolabelled stoves. EMEP/EEA do not give values for CH4 and N2O 

3. A critical discussion 

3.1 Emission factors 

As has been presented in the previous section, up to large uncertainties and for some emission compounds 

large variations exist in emission factors used in national emission inventories in the Nordic countries as well as 

when comparing with EMEP emission factors. Emission factors are in some cases taken directly from type 

approval testing and in other cases from experiments following the type approval test standard but with tests 

carried out by personnel not being experts on type approval testing, and hence higher emission factors results. 

In this case the emission factors can be anticipated to be closer to real-life. For most of the emission compounds 

there are no type approval standard that requires measuring these, and the emission factors are in the best 

case measured as an add-on when measuring according to the type approval test standard, in the second-best 

case the result of a research project where the goal has been to map emissions more extensively but not 

necessarily strictly following a type approval test standard. In the second-worst case the emission factor is 

borrowed from a database or literature without knowing enough about its origin and representativeness, and in 

239



the worst case the emission factor can even be adopted from using a different fuel which might also have been 

used in a different combustion technology. All these cases are today represented to different degrees in national 

emission inventories. Hence, there is a real need to derive more representative emission factors for wood stoves 

and to align these for inclusion in national emission inventories. Of course, there might be to some extent 

differences between different countries with respect to how wood stoves are operated and especially regarding 

the preferred wood species, but for similar technology and the same wood specie used, a representative 

average emission factor should be rather similar: i.e. large differences, as can be seen between the different 

Nordic countries for some emission compounds, cannot be justified.  

3.2 The sustainability of wood stoves 

The sustainability of wood stoves is to a large extent coupled to their emission levels, since the social and 

economic aspects of using wood stoves are easy to defend. Clearly, the sustainability of new and clean burning 

wood stoves is much stronger than for open fireplaces and for old wood stoves without staged air combustion. 

However, as the last decades have shown, emission levels of unburnt compounds have much decreased also 

because of further development and refinement of these modern wood stoves. This needs to be considered in 

the national emission inventories that are updated on an annual basis. To increase the sustainability of the wood 

stove stock, old stoves need to be replaced by new ones and open fireplaces need to be replaced by fireplace 

inserts. Continuous technology development, automation of air supply, improved user-friendliness, more aware 

and responsible wood stoves users and secondary flue gas cleaning as well as sufficient maintenance and 

sweeping of the appliance and chimney will add on to improve the sustainability of the future wood stoves. 

4. Conclusions 

Wood stoves give a very important contribution to domestic heating in many European countries and provides 

energy security. However, high emissions of especially unburnt compounds as fine particles from old and poor 

technology or wrong use results, still today, in a negative focus on emissions from wood stoves, even if the 

modern and clean-burning wood stoves outperform the old and poor technology. As the comparison of emission 

factors used in national emission inventories in the Nordic countries shows, large uncertainties and large 

variations in emission factors still exist today, and there is a real need to put more effort into deriving more 

representative emission factors as well as aligning emission factors between different countries. As the wood 

stoves are continuously improved, emission factors in national emission inventories should take this 

improvement into account, either by updating an average emission factor used by a modern stove category, or 

better, by splitting such a category to showcase the continuous technology improvement resulting in 

continuously reduced emission levels.  

Acknowledgments 

The authors acknowledge the financial support by the knowledge building project SusWoodStoves, financed by 

the Research Council of Norway and industry partners. 

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	A Critical Review and Discussion on Emission Factors for Wood Stoves