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Particle and Fibre Toxicology
Review
Health effects of residential wood smoke particles: the
importance of combustion conditions and physicochemical particle
properties
Anette Kocbach Bølling*1, Joakim Pagels2, Karl Espen Yttri3, Lars Barregard4,
Gerd Sallsten4, Per E Schwarze1 and Christoffer Boman5
Address: 1Division of Environmental Medicine, Norwegian Institute of Public Health, Oslo, Norway, 2Division of Ergonomics & Aerosol
Technology (EAT), Lund University, Lund, Sweden, 3Department of Atmospheric and Climate Research, Norwegian Institute for Air Research,
Kjeller, Norway, 4Department of Occupational and Environmental Medicine, Sahlgrenska University Hospital and Academy, University of
Gothenburg, Gothenburg, Sweden and 5Energy Technology and Thermal Process Chemistry, Umeå University, Umeå, Sweden
Email: Anette Kocbach Bølling* - anette.kocbach@fhi.no; Joakim Pagels - joakim.pagels@design.lth.se;
Karl Espen Yttri - karl.espen.yttri@nilu.no; Lars Barregard - lars.barregard@amm.gu.se; Gerd Sallsten - gerd.sallsten@amm.gu.se;
Per E Schwarze - per.schwarze@fhi.no; Christoffer Boman - christoffer.boman@chem.umu.se
* Corresponding author
Abstract
Background: Residential wood combustion is now recognized as a major particle source in many
developed countries, and the number of studies investigating the negative health effects associated
with wood smoke exposure is currently increasing. The combustion appliances in use today
provide highly variable combustion conditions resulting in large variations in the physicochemical
characteristics of the emitted particles. These differences in physicochemical properties are likely
to influence the biological effects induced by the wood smoke particles.
Outline: The focus of this review is to discuss the present knowledge on physicochemical
properties of wood smoke particles from different combustion conditions in relation to wood
smoke-induced health effects. In addition, the human wood smoke exposure in developed
countries is explored in order to identify the particle characteristics that are relevant for
experimental studies of wood smoke-induced health effects. Finally, recent experimental studies
regarding wood smoke exposure are discussed with respect to the applied combustion conditions
and particle properties.
Conclusion: Overall, the reviewed literature regarding the physicochemical properties of wood
smoke particles provides a relatively clear picture of how these properties vary with the
combustion conditions, whereas particle emissions from specific classes of combustion appliances
are less well characterised. The major gaps in knowledge concern; (i) characterisation of the
atmospheric transformations of wood smoke particles, (ii) characterisation of the physicochemical
properties of wood smoke particles in ambient and indoor environments, and (iii) identification of
the physicochemical properties that influence the biological effects of wood smoke particles.
Published: 6 November 2009
Particle and Fibre Toxicology 2009, 6:29 doi:10.1186/1743-8977-6-29
Received: 3 June 2009
Accepted: 6 November 2009
This article is available from: http://www.particleandfibretoxicology.com/content/6/1/29
© 2009 Bølling et al; licensee BioMed Central Ltd.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0),
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Particle and Fibre Toxicology 2009, 6:29 http://www.particleandfibretoxicology.com/content/6/1/29
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Background
Exposure to ambient particulate matter (PM) has been
associated with a range of negative health effects, includ-
ing increased morbidity and mortality from pulmonary
and cardiovascular diseases [1-3]. Although residential
wood combustion is a major source of particulate air pol-
lution in many countries, relatively few studies have been
performed to investigate the health effects associated with
wood smoke exposure. The two most recent reviews on
the topic concluded that the adverse health effects associ-
ated with wood smoke exposure in developed countries
do not seem to be weaker than for ambient particles from
other sources [4,5]. However, the reviewed literature sug-
gested that the respiratory effects of wood smoke may be
somewhat larger than the cardiovascular effects [5]. The
use of wood or charcoal for heating or cooking during
female adolescence was recently associated with chronic
obstructive pulmonary disease later in life [6], providing
further support for an association between wood smoke
exposure and negative respiratory effects. In addition, a
human inhalation study reported that wood smoke expo-
sure affected both systemic and lung biomarkers, suggest-
ing a potential impact of wood smoke particles also for
cardiovascular diseases [7,8]. Recently, the International
Agency for Research on Cancer (IARC) classified indoor
emissions from household combustion of biomass fuel
(mainly wood) as probably carcinogenic to humans
(group 2A) [9].
The term residential wood smoke comprises emissions
from a variety of biomass combustion appliances, such as
open fireplaces, wood and pellet stoves, masonry heaters,
and boilers for wood, wood chips and pellets [10-12] (see
Additional file 1 for a brief description of the different
types of combustion appliances). The combustion tech-
nology and air supply varies considerably between these
different appliances, but also between old and new mod-
els of each type of appliance. In addition, the fuel type
(e.g. wood logs, wood chips and pellets) and the condi-
tion of the fuel (e.g. moisture content and log size) also
influence the efficiency of the combustion [11,13,14]. The
physicochemical properties of particles emitted from resi-
dential biomass combustion differ considerably with
combustion conditions and between combustion appli-
ances [13,15]. Since epidemiological and experimental
studies provide increasing evidence for the importance of
physicochemical characteristics in the particle-induced
biological effects [16,17], the differences in the physico-
chemical properties of particles originating from varying
combustion conditions may influence their potential to
induce biological effects.
Exposure to ambient PM in general has been associated
with a range of pulmonary effects, such as decreased lung
development and function, exacerbation of asthma,
allergy, chronic obstructive pulmonary disease (COPD),
pulmonary fibrosis and increased risk of lung cancer
(reviewed in [3,18,19]). The cardiovascular diseases asso-
ciated with particle exposure include atherosclerosis,
myocardial infarction and stroke [20,21]. Several mecha-
nisms, including particle-induced oxidative stress, inflam-
mation, cytotoxicity and genotoxicity, have been
proposed to explain the associations between particle
exposure and adverse health effects observed in epidemi-
ological studies. The inflammatory potential of particles
has been linked to chronic pulmonary diseases, but has
also been suggested to contribute to atherosclerosis and
acute cardiac effects [20,22,23]. Particle-induced cytotox-
icity may be involved in tissue damage in the lung and in
other organs, whereas the carcinogenic risk primarily is
linked to genotoxiciy [17,24]. Markers of negative health
effects (i.e. oxidative stress, inflammation, cytotoxicity
and genotoxicity) are commonly monitored in cultured
cells (in vitro), acute and chronic animal models (in vivo)
or voluntary individuals in exposure chambers (in vivo) to
study the effects of particles on human health.
The two previous wood smoke reviews focused on the
health effects of residential wood smoke particles based
on epidemiological studies [4,5] and experimental studies
[5], whereas the present review focuses on the physico-
chemical properties of the particles, but from a health
based perspective. Naeher et al. (2007) concluded that
wood smoke may affect pulmonary immune defence
mechanisms, with the lung macrophages as a likely target
for wood smoke induced immunotoxicity, based on in
vivo toxicological studies of wood smoke [5]. However,
the combustion conditions used to generate wood smoke
particles and their physicochemical properties were not
discussed, neither was the relevance of these particles with
respect to ambient exposure. In the end of their paper
Naeher et al. (2007) recommended topics for further
research, including; i) 'Better understanding of the simi-
larities and differences of smokes generated by combus-
tion of different categories of biomass in different
conditions (...)' and ii) 'Source and exposure apportion-
ment studies to determine the degree to which residential
wood combustion contributes to both indoor and out-
door particle exposures (...)'. Although further research is
necessary, a notable amount of information is available in
the literature concerning both topics. In the present
review, we summarise current knowledge on physico-
chemical properties of PM from residential wood com-
bustion in developed countries with focus on how these
properties change with varying combustion conditions
and their relevance to human exposure. We also discuss
the combustion conditions and the resulting particle
properties applied in recent experimental studies of the
biological effects of wood smoke, and the relative toxicity
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of different types of wood smoke particles. The review is
organized according to the following outline:
Particle characteristics relevant for health effects
Brief introduction to how the physicochemical prop-
erties of particles may influence their biological effects
Physical and chemical characteristics of wood smoke particles
Summary of the current knowledge on the physico-
chemical properties of wood smoke particles from dif-
ferent combustion conditions, organised into three
different particle classes:
- spherical organic carbon particles
- soot particles/carbon aggregates
- inorganic ash particles
Wood smoke exposure
The exposure studies are reviewed to investigate to
what extent they provide information about the phys-
icochemical properties of the wood smoke particles
Emissions from different wood combustion appliances
As an alternative to the exposure studies, the emission
factors, activity data and emission characteristics of
different types of wood combustion appliances are
combined to obtain information about the type of
wood smoke particles we are exposed to
Transformation of wood smoke emissions in the atmosphere
Discussion of the influence of atmospheric transfor-
mations on the physicochemical properties of wood
smoke particles and its potential influence on their
biological effects
Experimental studies of wood smoke toxicity
Discussion of the combustion conditions and the
resulting particle properties applied in recent experi-
mental studies, divided into three parts:
- human inhalation studies
- in vivo animal studies
- in vitro studies
Summary and conclusions
Particle characteristics relevant for health
effects
The adverse health effects of inhaled particles are highly
dependent on the deposition and retention of particles in
the lung. The deposition probability and deposition site
of particles is governed by their aerodynamic properties,
such as size, density and shape, but also by other physico-
chemical properties such as hygroscopicity (i.e. water
uptake) [25,26]. Experimental studies have identified a
range of physicochemical properties that influence the
toxic and inflammatory potential of PM, and possibly par-
ticle-induced health effects (reviewed in [16,17,27,28]).
Since these data are discussed in detail in several reviews,
they are only described in brief in the following. The most
relevant particle properties and a selection of references
are summarized in Table 1.
Small particles, exhibiting a large surface area per mass,
have been found to induce a more pronounced pro-
inflammatory response than larger particles of the same
material. This has been demonstrated in both in vitro and
in vivo experiments where ultrafine particles are more
potent in inducing inflammatory responses than fine par-
ticles [29-32]. Consequently, surface area has been sug-
gested as a new dose metric for the inflammatory effects
induced by low-solubility low-toxicity particles in vitro
and in vivo [32-34]. However, particle structure, surface
properties and chemistry may override the importance of
particle size and surface area. For example, inflammation
and cytotoxicity after exposure to ultrafine TiO2 has been
found to depend on crystal structure (anatase vs. rutile)
rather than size and surface area [35,36]. Furthermore, the
inflammatory, cytotoxic and genotoxic responses to
quartz particles were reduced by surface coating, indicat-
ing that surface properties were important for the toxicity
of quartz [37-39]. With respect to chemical composition,
the content of metals such as vanadium, zinc, iron, copper
and nickel, as well as the content of organic compounds
such as polycyclic aromatic hydrocarbons (PAHs), seem
to influence the particle-elicited health effects [17,40-42].
Table 1: Physicochemical properties reported to influence the
biological effects of PM in experimental studies
Physicochemical properties References
Particle size [29-32]
Surface area per mass [32-34]
Crystal structure [35-39]
Chemical composition
- metals [41,42]
- organic compounds [40,43-45]
Solubility [50,51]
The table lists the most relevant physicochemical particle properties
and the references used in the text. For a more comprehensive
reference list please refer to one of the reviews [16,17,27,28].
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Quinones, a special group of carbonyl containing PAH
compounds, have recently been pointed out as particu-
larly reactive organic components of PM with potential to
produce reactive oxygen species (ROS) and to induce oxi-
dative stress via their redox capacity [43]. Accordingly, var-
ious oxy-PAHs, including quinones, were found to be
involved in inducing cellular oxidative stress in a murine
monocyte-macrophage cell line during exposure to
organic extracts of wood smoke and diesel exhaust parti-
cles [44,45]. However, the organic fraction of particles
from various sources comprises a large number of com-
pounds besides PAHs, such as aldehydes, ketones, organic
acids and various chlorinated organics [5,46,47], and the
biological effects of many of these compounds, and their
contributions to particle-induced inflammation, are
largely unknown.
Solubility is another property that may influence the tox-
icity of PM. For particles that dissolve upon contact with
aqueous solutions, such as most salt particles, cellular
uptake of dissolved ions may occur through ion channels.
In contrast, insoluble particles are usually taken up by
phagocytosis, which subsequently may initiate a cascade
of intracellular signalling [48]. Organic compounds, on
the other hand, can enter cells directly through the cell
membrane by a partitioning process [49], which in turn
may result in activation of other intracellular signalling
pathways. Insoluble particles exert a prolonged exposure,
while dissolved particulate material is likely to be cleared
more rapidly. In vitro studies indicate that insoluble nickel
compounds are more cytotoxic than soluble nickel salts
[50]. On the other hand, the in vitro cytotoxicity of manu-
factured nanoparticles was greater for partly soluble than
insoluble particles [51]. Thus, for different types of parti-
cles the solubility seems to influence the particle-induced
cytotoxicity to different extents.
Physical and chemical characteristics of wood
smoke particles
The physical and chemical properties of wood smoke par-
ticles emitted during various combustion conditions dif-
fer considerably. Fine particles (equivalent aerodynamic
diameter < 2.5 μm, PM2.5) emitted from residential wood
combustion appliances may be divided into three typical
classes based on chemical composition and morphology;
spherical organic carbon particles, aggregated soot parti-
cles and inorganic ash particles. The physicochemical
properties of these three classes are described in the fol-
lowing sections, and summarised in Figure 1. It should be
pointed out that in real combustion situations, especially
during transient cycles, the particle classes may co-exist
and interact. Since the combustion conditions in an appli-
ance change during a burn cycle, especially during batch-
wise combustion of wood logs, the emissions are likely to
contain several of the defined particle classes.
Spherical organic carbon particles
Burning wood of poor quality (e.g. high moisture con-
tent), overloading the firebox or insufficient air supply,
are examples of conditions that can lead to incomplete
combustion, characterised by low temperature [11]. In a
conventional wood stove without modern combustion
technology, emissions from such poor combustion condi-
tions (low temperature, air deficiency and/or poor mix-
ing) are dominated by spherical organic carbon particles
with diameters that have been measured to be between 50
and 600 nm by electron microscopy [52,53]. Spherical
organic carbon particles have also been observed during
smouldering combustion [54-56], and are therefore likely
to be emitted from open fireplaces. The origin of this
organic material is the thermal degradation products of
the wood constituents (i.e. cellulose, hemi-cellulose and
lignin) that are released at low temperatures (300-500°C)
without being further combusted due to poor mixing con-
ditions.
Freshly generated particles from smouldering combustion
contain large amounts of highly oxygenated water-soluble
organic species, including monosaccharide anhydrides
and methoxyphenols [57-60]. During ageing in the
atmosphere (> 10 min) insoluble 'tar-balls' may be
formed through polymerisation of primary emitted
organic matter [61]. These tar-balls contain low levels of
elemental carbon and lack the internal turbostratic micro-
structure exhibited by the primary particles of carbon
aggregates generated at higher temperatures [61]. Particles
from incomplete combustion are also characterised by a
low content of inorganic constituents such as potassium,
sulphur and chlorine [12,62,63]. Wood smoke emissions
contain a large number of organic compounds, and
detailed chemical speciation of several hundred individ-
ual compounds has been reported [57,58,64]. Using on-
line aerosol mass spectrometry (AMS), Weimer et al.
(2007) showed that organic emissions, particularly those
with signatures similar to levoglucosan, were strongly
enhanced during the start up phase. The mass spectra
recorded during the smouldering phase were, in contrast,
dominated by highly oxygenated species [65]. However,
the changes in organic chemistry for different combustion
conditions and temperatures and for the various phases of
the combustion cycle are not well described in the litera-
ture.
The carbon present in combustion particles can be classi-
fied as either organic or elemental carbon, and may be
determined in thermal/optical carbon analysers. Organic
carbon (OC) comprises hundreds to thousands of organic
compounds, whereas elemental carbon (EC) is defined as
the carbon that is not organic, but EC can also be charac-
terised as refractory carbon [66]. The sum of the organic
and elemental carbon in a sample is defined as the total
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carbon (TC). For low-temperature combustion in conven-
tional stoves, the reported ratios of elemental to total car-
bon (EC/TC) range from 0.01 to 0.11 [62,64,67],
confirming that PM from these combustion conditions
are dominated by organic carbon. It should be kept in
mind though that different measurement techniques give
rise to large differences in the EC/TC ratio [66], hence
great caution should be taken when comparing such data.
The mobility equivalent diameter, which determines the
deposition by diffusion in the human lung (typically
important for mobility diameters below about 500 nm),
equals the physical diameter for spherical particles. The
count mean diameter (CMD) of particles from low tem-
perature biomass combustion has been found to range
from 100-175 nm [68,69]. Similarly, Hueglin et al.
(1997) measured mobility sizes with CMD between 200
and 300 nm during the start up phase of a residential
wood stove, when organic emissions are expected to dom-
inate [70]. Thus, the CMD seems to range from 100 to 300
nm for spherical organic carbon particles. The aerody-
namic equivalent diameter determines particle deposition
The physicochemical characteristics of the three classes of wood combustion particlesFigure 1
The physicochemical characteristics of the three classes of wood combustion particles. The numbers refer to the
references used in the text. * For the aggregated soot particles the listed diameter refers to the primary particle diameter.
50-125 nm
69, 98, 99
50-300 nm
68, 76
100-300 nm
68-70
Mobility diameter
NoYes / No
81-83
No
61
Internal turbostratic
microstructure
Soluble Insoluble Depends on ageing
61
Solubility
(H
2
O)
Schematic
drawing
High-temperature,
complete combustion
120
High-temperature,
incomplete combustion
52
Low-temperature, incomplete
combustion
11, 52-56
Combustion
conditions
Combustion in pellets
stoves, boilers for wood,
wood chips and
pellets
69, 120
Alkali salts (mainly KCl
and K
2
SO
4
with small
amounts of trace
elements (e.g. Zn))
78, 92
50-125 nm
97
Inorganic ash particles
Combustion in
conventional stoves, open
fireplaces, boilers for
wood, wood chips and
pellets
14, 52, 75-79
Elemental carbon with
variable amounts of
organics condensed on the
surface
12, 62, 81
(Most abundant organic
compounds: hydrocarbons
and polycyclic aromatic
hydrocarbons)
84, 85
20-50 nm
52, 73
Soot (elemental
carbon aggregates)
Air starved combustion or
start-up phase of batch wise
combustion in conventional
stoves, open fireplaces
58,62,64,67
Organic carbon
62, 64, 67
(Most abundant organic
compounds: metoxyphenols
and monosaccaride
anhydrides)
57-60
50-600 nm
52, 53
Spherical organic carbon
particles
Main chemical
characteristic
Possible sources
Diameter measured by
electron microscopy*
50-125 nm
69, 98, 99
50-300 nm
68, 76
100-300 nm
68-70
Mobility diameter
NoYes / No
81-83
No
61
Internal turbostratic
microstructure
Soluble Insoluble Depends on ageing
61
Solubility
(H
2
O)
Schematic
drawing
High-temperature,
complete combustion
120
High-temperature,
incomplete combustion
52
Low-temperature, incomplete
combustion
11, 52-56
Combustion
conditions
Combustion in pellets
stoves, boilers for wood,
wood chips and
pellets
69, 120
Alkali salts (mainly KCl
and K
2
SO
4
with small
amounts of trace
elements (e.g. Zn))
78, 92
50-125 nm
97
Inorganic ash particles
Combustion in
conventional stoves, open
fireplaces, boilers for
wood, wood chips and
pellets
14, 52, 75-79
Elemental carbon with
variable amounts of
organics condensed on the
surface
12, 62, 81
(Most abundant organic
compounds: hydrocarbons
and polycyclic aromatic
hydrocarbons)
84, 85
20-50 nm
52, 73
Soot (elemental
carbon aggregates)
Air starved combustion or
start-up phase of batch wise
combustion in conventional
stoves, open fireplaces
58,62,64,67
Organic carbon
62, 64, 67
(Most abundant organic
compounds: metoxyphenols
and monosaccaride
anhydrides)
57-60
50-600 nm
52, 53
Spherical organic carbon
particles
Main chemical
characteristic
Possible sources
Diameter measured by
electron microscopy*
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by sedimentation in the lung (typically important for aer-
odynamic diameters larger than about 200 nm). Since
these spherical organic carbon particles have densities of
around 1-1.5 g/cm3 [71], the CMD based on aerodynamic
equivalent diameter is slightly larger than the CMD based
on mobility diameter.
The organic compounds from wood combustion are not
only emitted in the particulate phase, but also in the gas
phase. In the hot flue gas leaving the combustion chamber
of boilers and stoves, most of the organic material is
present in the gas phase, but can condense on existing par-
ticles (e.g. soot and/or inorganics) during cooling in the
heat exchanger and chimney [13,57,64]. Atmospheric
processes, for example reactions with OH and O3, can
result in reaction products with lower vapour pressure
that may condense onto existing particles through forma-
tion of secondary organic aerosols [72]. There is still insuf-
ficient knowledge about the relative contributions of
primary emissions and secondary particle formation to
the total particulate organic carbon from biomass com-
bustion. It should be pointed out that the gas to particle
partitioning of organic compounds depends relatively
strongly on concentration. To accurately represent the par-
ticle phase of primary organic aerosols from biomass
combustion, measurements should preferentially be
made at conditions relevant for ambient air.
Soot (Elemental carbon aggregates)
During incomplete combustion with air-starved condi-
tions at higher temperatures (~800-1000°C), PM emis-
sions are more dominated by solid carbon aggregates
(soot). These consist of a large number of primary spheri-
cal carbon particles with diameters that have been meas-
ured to be between 20 and 50 nm by electron microscopy
[52,73]. The formation of soot is very complex and Bock-
horn has given a well adapted soot formation pathway,
via polycyclic aromatic clusters, particle inception, surface
growth and coagulation [74]. Carbon aggregates of soot
may be emitted during incomplete combustion in con-
ventional wood stoves and masonry heaters [52,75,76],
from open fireplaces [14] or during incomplete combus-
tion in boilers for wood, wood chips or pellets [77-79].
In general, soot can contain some percent of hydrogen,
originating from the primary aromatic compounds, and is
subsequently more or less graphitized in the combustion
process. Primary particles of soot have been reported to
exhibit an internal turbostratic microstructure, consisting
of a concentric arrangement of layer planes with a two
dimensional graphitic structure, lacking the ordered stack-
ing of graphite, and thus its three dimensional structure
[80]. Kocbach et al. (2006) observed a turbostratic micro-
structure consisting of concentric carbon layers surround-
ing a single nucleus in primary particles from incomplete
high-temperature wood combustion by high resolution
transmission electron microscopy (HR-TEM) [81]. In the
same study, the graphitic character, defined as the degree
of similarity to the structure of graphitic carbon, was
investigated by selected area electron diffraction (SAED)
and electron energy loss spectroscopy (EELS). Wood
smoke particles from high-temperature combustion were
found to have graphitic character similar to that of traffic-
derived particles, confirming the observations by HR-
TEM. In contrast, Braun and colleagues recently reported
that particles from a range of residential wood stoves did
not have a graphitic character or a less graphitic character
than diesel exhaust particles by application of near-edge
X-ray absorption fine structure spectroscopy (NEXAFS)
[82,83]. Whereas the wood smoke particles in Kocbach et
al. (2006) were collected by aerosol sampling, Braun and
co-authors analysed samples collected either from the
interior walls of various wood stoves or from chimneys.
The differences in applied collection methods could lead
to a selection of different populations of particles. This
might explain the conflicting results concerning the
graphitic structure of wood smoke soot particles. The
present data is insufficient to conclude on a possible dif-
ference in the graphitic character of soot particles from dif-
ferent combustion conditions.
Aggregated soot particles contain higher levels of elemen-
tal carbon and lower levels of organic carbon compared to
carbonaceous particles emitted at lower temperatures, and
the EC/TC ratios for incomplete high-temperature com-
bustion in conventional stoves and masonry heaters have
been reported to range from approximately 0.5 to 0.75
[12,62,81]. Both the concentration and the relative contri-
bution of various particle associated organic compounds
change with combustion temperature. Overall, the total
concentration of non-combusted organic matter in the
emissions decreases with increasing combustion tempera-
tures, and the primary organic pyrolysis products formed
at lower temperatures are "transformed" to purer aromatic
hydrocarbons at higher temperatures. Accordingly, the
content of methoxyphenols decreases with increasing
combustion temperature, whereas the levels of PAHs
increase [84,85]. Thus, soot emitted from different com-
bustion conditions may differ in organic chemistry. The
most abundant PAHs in wood smoke emissions are naph-
thalene, acenaphthene, fluorene, phenanthrene, anthra-
cene, fluoranthene and pyrene [15,47,64], but with regard
to carcinogenicity, benzo(a)pyrene (B(a)P) and fluoran-
thene seem to be the most important compounds in
wood smoke emissions [47,86,87]. Although a rather
extensive amount of work has been performed to charac-
terise the organic fraction of wood smoke
[47,57,58,64,88], little information is so far available con-
cerning compounds that influence the biological effects of
wood smoke particles and on how the organic composi-
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tion varies with the combustion temperature. However,
fractionation of organic extracts, chemical analyses and
measurements of oxidative stress were recently combined
in order to identify the organic compounds involved in
the biological effects of wood smoke particles [44]. In that
study, oxy-PAHs and quinones were found to contribute
to oxidative stress. Interestingly, emissions of oxy-PAHs
have been reported to increase with increasing wood com-
bustion temperature [85].
A particle diameter is hard to define for aggregated parti-
cles such as soot, and the mobility equivalent aggregate
diameter for soot from wood combustion has been found
to vary considerably between different studies; from 50 to
300 nm [68,76]. The aerodynamic equivalent diameter of
soot from wood smoke has not been reported in the liter-
ature, but may be considerably smaller than the mobility
equivalent diameter [89]. Condensation of organic com-
pounds onto soot agglomerates may lead to a transition
from highly agglomerated to compact particles. This has
been demonstrated for soot from other sources during
interaction with water or H2SO4 [90,91]. The present
knowledge of the morphology and the mobility and aero-
dynamic diameters of aggregated particles from wood
combustion is, however, limited.
Inorganic ash particles
Combustion of pellets, wood chips and wood logs in boil-
ers or stoves with modern technology provides favourable
combustion conditions with high temperatures (>
900°C), good oxygen supply and sufficient mixing
between combustable gases and air in the combustion
chamber. This results in almost complete combustion and
the emissions are dominated by inorganic ash particles,
such as the alkali salts of potassium/sodium-sulphates,
chlorides and carbonates [78,92]. The content of organic
and elemental carbon can be below 1% of the particle
mass emitted during these favourable combustion condi-
tions [69]. Fine particles emitted during combustion of
some types of wood and bark pellets may also contain
phosphorous, which is probably related to elevated com-
bustion temperatures [93]. It is also believed that potas-
sium phosphates may be present in fine particles during
combustion of more phosphorous rich (non-woody) bio-
mass, as demonstrated during combustion of agricultural
fuels in some recent studies [94-96].
Studies using electron microscopy have revealed that the
fine inorganic ash particles emitted from complete com-
bustion conditions have a sphere-like shape with diame-
ters between 50 and 125 nm [97]. The corresponding
mobility diameters have been measured to be in the same
size range [69,98,99]. Since mobility diameters are close
to the physical diameter for compact particles, the inor-
ganic ash particles from biomass combustion are also
likely to have physical diameters in the same range. Aero-
dynamic diameters may be calculated assuming an effec-
tive density of about 2.0 g/cm3 [68,70]. For example a
particle with an equivalent mobility diameter of 100 nm
and effective density of 2.0 g/cm3 would have an equiva-
lent aerodynamic diameter of 168 nm. Overall, the parti-
cle morphology and size distribution has been relatively
well described for inorganic ash particles.
Inorganic ash particles such as potassium sulphates and
chlorides have rather high hygroscopic growth factors and
are mainly water soluble. This solubility may affect the
biological effects induced by these particles in two man-
ners; (i) hygroscopic particles may grow at the high
humidity in the respiratory tract, which can reduce the
deposition probability and may alter the deposition site
[25,100] and (ii) the solubility may increase the clearance
rate from the lung. In addition, the solubility of PM may
affect the biological effects on a cellular level, for instance
with respect to uptake mechanisms and activation of
intracellular pathways.
In addition to the three classes of particles described
above, coarse inorganic fly-ash particles with diameters
larger than 1 μm, containing refractory species such as cal-
cium, magnesium, silicon, phosphorus and aluminium,
have been detected in emissions from large scale grate
fired biomass boilers [78,101] and wood chip burners
[70]. In grate fired appliances, the air is supplied to the
combustion chamber through a grate beneath the cham-
ber. The coarse fly-ash particles are entrained from the fuel
bed and their emissions may therefore be strongly
dependent on the primary air flow through the grate [98].
Wood smoke exposure
In order to evaluate the negative health effects that may be
associated with exposure to wood smoke particles, it is
necessary to determine the human exposure to these par-
ticles. The number of studies regarding ambient wood
smoke exposure in developed countries increases rapidly.
Source apportionment studies have estimated that wood/
biomass combustion contribute with 10-40% to the fine
particle concentrations (PM2.5) in large cities such as Seat-
tle, Phoenix, Beijing, Prague and Helsinki [102-105]. Res-
idential wood combustion has also been reported to
contribute substantially to increased levels of air pollution
locally, both with respect to increased levels of PM2.5, the
organic particle fraction, particle bound PAH and volatile
organic compounds [106-111]. The contribution of wood
smoke to ambient air pollution is, however, highly
dependent on season, time point and week day [105,112].
In general, people in developed countries spend the
majority of their time indoors. For instance, the partici-
pants in a recent Swedish study reported that they spent
more than 90% of their time indoors and around 60% at
home [113]. Thus, the indoor particle levels have a large
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impact on human exposure. The penetration of wood
smoke from ambient sources to indoor environments has
not been investigated in any detail. However, both the
personal exposure and the indoor concentrations of parti-
cle associated K, Ca, Zn, and possibly Cl, Mn, Cu, Rb, Pb
and black smoke (~soot), were found to be increased in
homes heated with a wood stove or boiler [114]. Personal
exposure and indoor levels showed high correlations for
all elements, and the personal exposure levels were usu-
ally higher than or equal to the indoor levels, but the asso-
ciations between personal exposure and outdoor levels
were generally weak [114]. Residential wood combustion
also increased personal exposure to 1,3-butadiene as well
as indoor levels of 1,3-butadiene and benzene and possi-
bly acetaldehyde [115]. The cancer risk from these com-
pounds due to wood smoke exposure in developed
countries was estimated to be low [115]. In the same
study, the levels of B(a)P and several other PAHs were
found to be significantly higher (3- to 5-fold) in homes
with wood combustion appliances compared to homes
without [87]. While phenanthrene made the largest con-
tribution to the total PAH concentration in indoor and
outdoor air, most of the cancer potency was due to B(a)P
(about 60%) and fluoranthene (about 20%). Moreover,
the median indoor B(a)P concentration in the homes
with wood combustion appliances (0.52 ng/m3) was 5
times higher than the Swedish health-based guideline of
0.1 ng/m3.
The physicochemical characteristics of ambient wood
smoke particles are highly dependent on factors that vary
between locations, such as the relative numbers of differ-
ent types of residential combustion appliances, and on
factors that vary with both time and location, such as the
combustion activity (e.g. use frequency and burn rate),
the wood species and wood quality. The contribution
from residential wood combustion to ambient, indoor
and personal wood smoke exposure is commonly esti-
mated by application of various markers for wood smoke,
such as the content of organic and elemental carbon, spe-
cific organic compounds (levoglucosan, 1,3-butadiene,
benzene, or PAHs) or metals (K, Ca, Zn) [87,105,114-
117]. However, these markers provide limited informa-
tion regarding the exposure to the different classes of
wood smoke particles, as they are usually only represent-
ative for one of the three classes of residential wood com-
bustion particles. Thus, a broader range of wood smoke
markers with specificity for each of the three classes of
wood smoke particles should be applied in future expo-
sure studies. This would provide a better characterisation
of wood smoke exposure in epidemiological studies, and
also a better basis for choosing relevant particles in exper-
imental/toxicological studies. Further characterisation of
the personal and indoor wood smoke exposure is also
necessary, since we generally spend more than 60% of our
time indoors at home.
Emissions from different wood combustion
appliances
The physicochemical properties of ambient wood smoke
particles depend on the wood smoke emissions to ambi-
ent air. Data collected for individual classes of combus-
tion appliances may be applied to obtain information
about the physicochemical characteristics of residential
wood smoke emissions in a specific area, as illustrated in
the flowchart in Figure 2. For each class of wood smoke
appliances it is possible to determine emission factors,
activity data and emission characteristics. By combining
the activity data with the emission factors the classes of
combustion appliances that account for the majority of
the emissions are determined, and in combination with
the emission characteristics the classes of PM that domi-
nate the residential wood smoke emissions can be sug-
gested. This approach provides a rough estimate for the
main characteristics i.e. the particle class(es) dominating
the emissions, but application of exposure studies provide
more relevant chemical characterisation and also includes
the atmospheric modifications that have occurred in the
time span between emission and exposure.
Emission characteristics
In this section, the class of PM (i.e. organic carbon/soot/
inorganic ash) that dominates the emissions from the dif-
ferent types of combustion appliances is suggested based
on the available data for EC/TC ratios and morphology of
the emitted PM (Table 2). If the data is limited a sugges-
tion is made based on knowledge about the combustion
conditions in that type of appliances.
Open fireplaces
Wood combustion in open fireplaces is a mixture of flam-
ing and smouldering combustion. These emissions are
therefore likely to be dominated by spherical organic car-
bon particles and soot. Scanning electron microscopy of
samples from a range of fireplace emissions suggested that
carbon aggregates (soot) was the dominating particle class
[14]. However, the reported EC/TC ratios, ranging from
0.04 - 0.46 [14,58,62,64,67], indicate that organic carbon
is the major component of PM emissions from open fire-
places. A possible explanation for the discrepancy
between the EC/TC ratios and the morphology observed
by electron microscopy may be condensation of organic
carbon onto soot particles. Overall, the reported data on
emission characteristics from open fireplaces suggest that
the contribution to ambient air from this class of wood
combustion appliances is a mixture of soot and organic
carbon.
Conventional wood stoves
As discussed previously, particle emissions from incom-
plete low-temperature combustion conditions are domi-
nated by spherical organic carbon particles and low levels
of elemental carbon (EC/TC ratios 0.01-0.11), while soot
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and high EC/TC ratios (0.50-0.75) characterise emissions
from incomplete combustion at higher temperatures
[12,62,64,67,81]. Kocbach et al. (2005) observed soot,
but not spherical organic carbon particles, in ambient
samples collected in two areas dominated by smoke from
conventional stoves. The samples comprised emissions
from different combustion conditions and several wood
species, suggesting that the contribution from conven-
tional stoves to ambient air was mainly soot [52]. In con-
trast, another study indicated that spherical carbon
particles observed in ambient air samples originated from
household wood combustion [61]. Although soot seems
to constitute a large part of the emissions from conven-
tional wood stoves, organic carbon, either condensed
onto soot or as individual spherical carbon particles, also
appears to be an important contributor to the particle
emissions from this class of combustion appliances. Gas-
eous organics emitted during poor combustion condi-
tions are also likely to contribute to the particulate OC
levels due to formation of secondary organic aerosols
[118].
Flowchart illustrating how information about the physicochemical properties of ambient wood smoke particles may be obtained from data collected for individual classes of combustion appliancesFigure 2
Flowchart illustrating how information about the physicochemical properties of ambient wood smoke parti-
cles may be obtained from data collected for individual classes of combustion appliances. See text for explanation.
DIFFERENT CLASSES OF WOOD COMBUSTION APPLIANCES
For each class it is possible to obtain:
EMISSION CHARACTERISTICS
The physicochemical properties of the PM emitted
from a class of appliances, classified according to
the three defined particle classes:
organic carbon / soot / inorganic ash
EMISSION FACTORS
Defined as the amount of PM emitted per
unit of wood combusted (energy unit or weight unit )
ACTIVITY DATA
The amount of wood combusted in a class of
appliances in a specific area/country
(including firing habits)
Determine the classes of
combustion appliances that
account for the major emissions
Suggest the class(es) of PM that
dominate the residential wood
smoke emissions
Table 3
Table 2
Table 2: Emission characteristics for the different classes of wood combustion appliances
Type of combustion appliance Particle class(es) dominating the emissions References
Open fireplaces organic carbon/soot [14,58,62,64,67]
Conventional wood stoves organic carbon/soot [12,62,64,67,81]
Masonry heaters organic carbon/soot [11,76,119]
Conventional boilers for wood logs organic carbon/soot *
Modern wood stoves inorganic ash/organic carbon/soot *
Modern boilers for wood logs inorganic ash/organic carbon/soot *
Pellet stoves and boilers inorganic ash [69,120]
Based on the available data on the physicochemical properties of particles emitted from different types of combustion appliances we have suggested
the class(es) of particles that are likely to dominate the emissions. The references used to support the text are included in the table.
* Limited data available, see text for details
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Conventional wood log boilers and masonry heaters
Conventional wood log boilers and masonry heaters can
be defined as appliances without new technology, such as
down-draft combustion, sucking fan and electric combus-
tion control. Conventional wood log boilers may be
installed with a water heat accumulation tank, which
improves the user comfort and combustion efficiency
considerably. In Sweden, less than 30% of the households
with wood log boilers have a water heat accumulation
tank. Analyses of carbon content or morphology of the
particulate emissions from wood log boilers have not
been reported in the literature. However, since the com-
bustion conditions (e.g. temperature, residence time and
mixing) in such systems can vary significantly, the emis-
sions can be expected to vary with respect to the fractions
of organic carbon, elemental carbon (soot) and inorganic
ash constituents. In general, the PM is dominated by car-
bonaceous material and for conventional masonry heat-
ers, the EC/TC ratios have been reported to range from
approximately 0.10 to 0.35 in both field and laboratory
studies [11,76,119]. Thus, the emissions from conven-
tional wood log boilers and masonry heaters are likely to
be dominated by soot and organic carbon.
Modern stoves, masonry heaters and boilers for wood logs
In "modern" residential combustion appliances for wood
logs, the applied combustion technology leads to
improved combustion conditions with good burn out and
low emissions of PM [120]. The emissions from modern
appliances for combustion of wood logs are dominated
by inorganic ash during ideal operation, and organic car-
bon and soot may constitute less than 10% of the emitted
particle mass [120]. However, during the start-up phase
and during low burn-rates, the combustion performance
can be deteriorated causing increased emissions of both
organics and soot. Moreover, the emissions from modern
appliances for wood logs may increase ten-fold if they are
not operated appropriately [118] and then the emissions
are most likely dominated by soot and organic carbon
rather than inorganic ash. The data reported concerning
detailed chemical composition of the PM for modern
wood boilers and stoves are still very scarce.
Pellets stoves and boilers
Wood pellet boilers and stoves can in general be consid-
ered as "modern" technology with high combustion effi-
ciency, and situations with poor combustion conditions
are assumed to be very rare in these systems due to for
instance the homogeneous character of the fuel, continu-
ous fuel feeding and fan driven air supply [118]. Based on
laboratory studies, the emitted PM from these appliances
is therefore assumed to be dominated by inorganic ash
and to contain very low levels of elemental and organic
carbon (TC ~5 - 12% and EC/TC ~0.65 - 0.80) [120]. The
total carbon level may be as low as < 1% [69]. However,
the efficiency of these appliances may be deteriorated if
they are installed or operated in an inappropriate manner,
this could possibly cause emission of soot or organic car-
bon.
Overall, the emissions of gases and PM from the various
types of combustion appliances have been rather well
described, although the data available for modern wood
stoves and boilers for wood logs have some limitations. A
more detailed characterization of the variations in particle
properties between the different types of appliances is
however still missing. Efforts should be made to resolve
this issue, since a more complete characterisation of the
variation in particle properties would provide a better
basis for an evaluation of the impact of wood smoke
exposure on human health.
Emission factors and estimates
The emission factors of PM for different classes of residen-
tial biomass combustion appliances have recently been
discussed in several reports [118,120-122], and are sum-
marised in Table 3. Conventional stoves and boilers for
wood logs account for the highest emission factors, fol-
lowed by open fireplaces and modern stoves and boilers
and finally pellet stoves and boilers. Generally, the ranges
of emission factors reported for the various classes of
appliances are very large. This variation is partly due to
application of different measuring techniques; both sam-
pling of particles in the chimney at gas temperatures of
120-160°C and sampling of particles in a dilution tunnel
at lower temperatures (< 35°C) are commonly applied.
Application of a dilution tunnel allows for condensation
of organic compounds onto the particles, and the result-
ing emission factors can be up to 10 times higher than the
factors based on collection of particles in the undiluted
chimney gas [118,120]. However, if even higher dilution
ratios are applied (above 20:1) the emission estimates for
organic carbon may decrease with increasing dilution
ratios [123]. The fraction of primary formed organics (i.e.
products of incomplete combustion) which partitions to
the particle phase is strongly dependent on both concen-
tration and temperature [118,123-125]. Thus, to accu-
rately quantify the primary organic particle phase fraction
in atmospheric wood smoke pollution, dilution condi-
tions close to ambient should be applied. It should also be
kept in mind that wood combustion appliances are often
pre-heated when their emission factors are determined.
This is in contrast to real-life wood combustion, where the
burning of wood starts in a cold stove. Since organic com-
pounds are likely to dominate the emissions from a cold
stove, this procedure may contribute to an underestima-
tion of the real-life emissions of organic carbon.
The operation conditions, e.g. ideal, typical or poor oper-
ation, also have great impact on the measured emission
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factors for appliances fired with wood logs, and the emis-
sion factors for wood stoves and boilers have been esti-
mated to increase with a factor of 10 during typical
operation as compared to ideal operation [118]. In addi-
tion, the large variations in combustion technology
within each class of combustion appliances also contrib-
ute to increased variation in the reported emission factors.
The data in Table 3 suggest that the emission factors for
residential wood combustion appliances are highly uncer-
tain, and their uncertainty was recently estimated to be ±
54 - 88% for Finland (95% confidence interval) [121]. As
illustrated in Figure 2, emission estimates for the different
class of combustion appliances may be obtained by com-
bining the activity data with the corresponding emission
factors. In comparison to the large uncertainty determined
for the emission factors, the uncertainty related to the
activity in the domestic wood combustion sector was
found to be considerably lower (± 10%), while the uncer-
tainty regarding the activity in different types of combus-
tion appliances were between ± 15% and ± 25% [121].
Recently, the number of biomass combustion appliances,
the activity, and the calculated estimated emissions based
on emission factors were summarized for several Euro-
pean countries (Denmark, Finland, Norway, Sweden, Ger-
many and Switzerland) [120-122]. To our knowledge,
Table 3: Emission factors for different types of residential combustion appliances
Type of combustion appliance Reported emission factors
Approximate range
(mg/MJ)
Reported data
(mg/MJ)
Open fireplaces 160 - 910 800 a
160 - 447 b,1
860 - 910 b,2
Conventional wood stoves 50 - 2100 700 a
94 - 650 b,1
50 - 1932 b,2
100 c
150 - 2100 d
Other conventional stoves, including masonry heaters and sauna stoves 30 - 140 140 a
30 - 100 c
Conventional boilers for wood logs
without accumulator tank 50 - 2000 700 a
300 - 2000 b,1 and 2
1300 c
300-900 d
with accumulator tank 50 - 250 80 a
50 - 300 b,1 and 2
95 d
Modern wood stoves 34 - 330 34 c
330 d
Modern boilers for wood chips or logs 5 - 450 5-450b,1
20 - 25 c
30-100 d
Pellet stoves and boilers 10 - 50 30 a
10 - 50 b,1 and 2
20 c
30 d
Emission factors are reported as mg particles emitted per MJ of fuel burnt (MJ = Mega Joule)
a mean emission factors based on available literature, as reported in [121].
b range of emission factors based on data from members of the International Energy Agency, as reported in [118]. 1 = measurement of particles at
temperatures > 100°C, 2 = measurement of particles in dilution tunnel at temperatures < 100°C.
c range of emission factors [120].
d data from [122].
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emission estimates for the different classes of wood com-
bustion appliances have not been calculated for other
European countries, the US, Canada, Australia or New
Zealand. As mentioned above, considerable uncertainty is
associated with these numbers, they do however provide
an estimate of the residential wood combustion emis-
sions. All studies reported that wood logs is the most com-
monly applied fuel in biomass-based residential heating,
and that the majority of the wood was combusted in con-
ventional stoves and manually fed boilers [120-122].
Since these combustion appliances have high emission
factors (Table 3), they also account for the majority of the
emissions of PM from residential biomass combustion,
generally more than 80% [120-122]. Due to low activity
or low emission factors, fireplaces and modern wood log
appliances account for less than 15% of the residential
biomass emissions in the Nordic countries [121,122].
This is in contrast to the US, where open fireplaces are
considered to be one of the major contributors to residen-
tial wood smoke emissions [12].
Over the last 10-20 years, the development of new com-
bustion technologies for densified wood fuels, such as
pellets, has been considerable in several countries, like
Sweden, Austria and Germany [10]. Although log wood is
still the dominating fuel type in most European countries,
wood pellets have gained increasing relevance and this
trend is expected to continue [120]. Due to their low emis-
sion factors and the relatively low number of appliances,
the relative contribution from pellets burning to the total
biomass combustion emissions is generally below 10%
[120-122]. The share of modern biomass combustion
appliances, for both wood log and pellets, is likely to grow
steadily, particularly due to replacement of older stoves/
boilers and due to conversion from oil and electricity. The
relative contribution from these appliances to the total
residential wood smoke emissions is, however, likely to
remain low due to their low emission factors.
Emissions of the different classes of wood smoke particles
As discussed in the previous section, conventional wood
stoves and boilers for wood logs account for the majority
of the domestic biomass emissions to ambient air in
Europe. Since these emissions consist of variable fractions
of soot and organic carbon depending on combustion
appliances, operation and fuel quality, these two classes
of particles are likely to dominate the emissions in Euro-
pean countries. The organic compounds may be con-
densed onto soot and/or inorganic particles or be present
as individual spherical organic particles in emissions from
very poor combustion conditions.
With respect to relevance for experimental studies, parti-
cles generated solely during smouldering combustion, not
containing soot, seem to be more representative for bush
and structural fires, and hence for fire fighter exposure,
than for residential wood smoke exposure. In addition,
smouldering combustion and spherical organic carbon
particles are also relevant for the domestic exposure in
developing countries, since open fires that provide poor
combustion conditions are commonly burned indoors in
these countries. Inorganic ash particles are primarily emit-
ted from pellets stoves and boilers and from modern
wood log boilers under optimal firing. Due to the low
emission factors of these appliances, inorganic ash parti-
cles make a small contribution to ambient wood smoke
concentrations presently, but their contribution may
increase in the future.
Combining the activity data with the emission factors and
emission characteristics for the different types of combus-
tion appliances provides some information about the
classes of PM that dominate the residential wood smoke
emissions in specific areas/countries. A major limitation
of this approach is the high uncertainty associated with
the acquired information. In addition, activity data are
unavailable for many regions and the emission character-
istics are insufficient for some types of appliances. This
approach does, however, have promising aspects as it has
the potential to provide information about the general
type of emissions to a specific area/country without per-
forming time consuming and expensive field measure-
ments.
Transformation of wood smoke emissions in the
atmosphere
The physiochemical properties of ambient particles may
change through interaction with atmospheric photo-oxi-
dants (e.g. OH, O3, NO3. NO2), acids (e.g. HNO3,
H2SO4), water and UV radiation [126]. Possible atmos-
pheric transformations include altered size, morphology
and chemical composition [90,91,127-129]. Few studies
have investigated the atmospheric transformations of
wood smoke particles, but the metoxyphenols present in
wood smoke particles have been suggested to enhance the
photochemical degradation of PAHs [128]. In addition,
more volatile compounds have been reported to condense
onto particles, and heavy compounds to be photo-
degraded into lighter ones [129,130]. Photo-oxidation of
wood stove emissions at atmospherically relevant or
slightly elevated concentrations in a Teflon chamber has
been found to increase the organic aerosol mass by a fac-
tor of 1.5-2.8 [130]. The condensed material was highly
oxidised, distinctly different from the primary organic
particle mass. Less than 20% of the formed secondary aer-
osol mass could be explained by known pre-cursors, indi-
cating involvement of large classes of organic compounds
[130]. These effects are qualitatively similar to those pre-
viously reported for diesel exhaust [125]. It is obvious that
more research is needed on this topic, for example on how
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the combustion conditions influence the formation of
secondary organic aerosols [131].
Atmospheric alterations could affect the biological activity
of PM, and cause either increased or decreased potency
with respect to mutagenicity, inflammatory potential or
toxicity [132-136]. The performed studies also suggest
that the effect of ageing on the biological activity could be
related to the particle source.
The present data on atmospheric alterations of particulate
matter suggest that it is necessary to take into account the
atmospheric alterations of the emitted particles in order to
elucidate the potential health effects of wood smoke. Fur-
ther studies are necessary both with respect to changes in
physicochemical particle properties but also with respect
to the influence of these changes on the biological effects.
Experimental studies of wood smoke toxicity
The physicochemical properties of wood smoke particles
applied in experimental studies, have not been discussed
in recent reviews of health effects of wood smoke
[4,5,137]. As discussed previously, particles generated
under varying combustion conditions differ with respect
to physicochemical properties, and this may influence
their potential to induce biological effects. Therefore, the
applied combustion conditions and, if possible, the phys-
icochemical properties of the wood smoke used in recent
experimental studies are discussed in this section, and
summarised in Table 4.
Human inhalation studies
A limited number of human inhalation studies have
investigated the negative effects of wood smoke exposure.
Barregard and colleagues used a conventional stove to
generate wood smoke [7,8,138,139]. The mass concentra-
tion (PM1) was approximately 250 μg/m3 with levels of
B(a)P around 20 ng/m3 and total PAH levels around 800-
1000 ng/m3 (sum of 14 measured PAHs). The major inor-
ganic elements, K, Zn and Cl, accounted for less than 6%
of the total mass concentration [138], thus soot and
organic carbon seemed to dominate the PM inhaled in
this study rather than inorganic ash. The geometric mean
diameters were 42 and 112 nm in the two different rounds
of wood smoke exposure. Blood and urine measurements
suggested that wood smoke may be associated with sys-
temic inflammation (the acute phase protein serum Amy-
loid A and to some extent serum C-reactive protein),
blood coagulation (Factor VIII) and lipid peroxidation
(urinary excretion of the isoprostane 8-isoPGF2α) [7]. In
addition, wood smoke exposure increased markers of
inflammatory effects on distal airways (alveolar nitric
oxide and Clara cell protein in serum) [8]. Several of these
biomarkers are cardiovascular risk factors. The oxidative
DNA damage and related repair capacity in peripheral
blood mononuclear cells was investigated in the same
study. Although wood smoke exposure was followed by
significant up-regulation of the repair gene hOGG1, no
direct genotoxic effects were observed [139].
Recently, another human inhalation study was performed
by a Swedish interdisciplinary group at Umeå University,
Umeå University Hospital and Lund University, investi-
gating the effects of wood smoke from an adjusted resi-
dential wood pellet burner under low temperature
incomplete combustion conditions in a human chamber
study [120,140]. The exposures were performed at 224 ±
22 μg PM1/m3 where the PM was dominated (~90%) by
carbonaceous matter [68]. The preliminary human expo-
sure effect data indicate a moderate response, including
increased levels of glutathione which indicates that the
antioxidant defense was activated, possibly due to oxida-
tive stress [120,140].
Future human inhalation studies should be designed to
compare the effects induced by wood smoke from differ-
ent combustion conditions, as comparative studies would
be a useful tool in the process of targeting strategies for
reducing human wood smoke exposure to the appropriate
particle fractions.
In vivo animal studies
In vivo wood smoke studies in animal models may be
divided into exposure conditions relevant for a) fire-fight-
ers or fire victims (studies using high doses and short
exposure time) and b) ambient residential wood smoke
exposure in developed countries (studies using lower con-
centrations and acute, intermediate or long-term expo-
sure). The majority of the in vivo animal studies using low
exposure conditions were performed at the Lovelace Res-
piratory Research Institute (LRRI, New Mexico, US)
[75,141-145]. A conventional wood stove was applied to
generate the smoke, using a three-phase burn cycle (kin-
dling, high and low burn rate). Since > 70% of the com-
bustion was performed with a low burn rate, the particles
used in these inhalation studies were most likely domi-
nated by spherical organic carbon particles, as supported
by the high OC content reported in these studies (90-94%
of total carbon content) [143]. However, one early study
used particles that were dominated by carbon aggregates
(soot) [75]. In light of the discussion in the sections con-
cerning wood smoke exposure, these studies applying par-
ticles with very low content of soot (EC/TC ratio < 0.06)
may not be fully representative for wood smoke exposure
in general in developed countries. Wood smoke-induced
effects in mice and rats reported in the studies performed
at LLRI include exacerbation of allergic airway inflamma-
tion, decreased lung function, mild lung inflammation
and toxicity, systemic immunotoxicity and increases in
platelet levels [75,141-145]. In contrast to these studies,
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Table 4: Experimental studies of wood smoke toxicity
Stove/combustion
conditions
Dominating
particle class
Model system Biological response Comparison of
combustion
conditions
References
Human inhalation
studies
Conventional wood
stove
organic carbon/soot inhalation, human - inflammation in
distal airways
- systemic
inflammation
- blood coagulation
- lipid peroxidation
- increased oxidative
stress ?
- [7,8,138,139]
Pellets burner/
incomplete combustion
organic carbon/soot inhalation, human - increased oxidative
stress ?
- [120,140]
In vivo animal
studies
Conventional wood
stove/mixed burn-cycle
organic carbon/soot inhalation, rat - mild chronic
inflammation
-[75]
Conventional wood
stove/incomplete
combustion
organic carbon inhalation, mouse/rat - allergic airway
inflammation
- decreased lung
function
- mild lung
inflammation
- systemic
immunotoxicity
- increases in platelet
levels
- [141-145]
Conventional wood
stove/high-temperature
incomplete combustion
soot footpad immunisation
model, mouse
- enhanced allergic
sensitisation
-[146]
In vitro studies
Old boiler, modern
boiler, pellets boiler
epithelial cell line,
human
- genotoxicity
- inflammation
no large differences [147]
Thermolysis of bark/
incomplete combustion
organic carbon macrophage-like cell
line, mouse
- DNA damage
- oxidative stress
- inflammation
-[148]
Conventional wood
stove/high-temperature
incomplete combustion
soot epithelial and
monocytic cell lines,
human
- DNA damage - [149]
Modern boiler,
conventional wood
stove/normal and poor
combustion conditions
inorganic ash soot,
organic carbon
fibroblast cell line,
hamster
- chromosome
breakage
- cytotoxicity
organic carbon
> soot > ash
[53]
Conventional masonry
heater/normal and poor
combustion conditions
macrophage-like cell
line, mouse
- cytotoxicity
- inflammation:
TNF-α
MIP-2
poor > normal
poor < normal
poor > normal
Salonen et al. in [120]
Conventional wood
stove/high-temperature
incomplete combustion
soot epithelial and
monocytic cell lines,
human
- inflammation - [150,151]
Large biomass
combustion plant
inorganic ash epithelial cell line,
human
- inflammation - Bellman el al. in [120]
The table summarizes the studies discussed in the text. Only the endpoints or biological effects that were influenced during exposure to wood
smoke particles are listed in the table, not all the endpoints investigated in each study.
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Samuelsen et al. (2008) used particles from incomplete
high-temperature combustion in a conventional wood
stove (soot dominated) to investigate the allergy adjuvant
effect in mice, and observed enhanced allergic sensitisa-
tion after wood smoke exposure [146]. The applied model
system, a footpad immunisation model, differed consid-
erably from the model systems used in the studies per-
formed at LLRI, with respect to both exposure route and
analysed biological endpoints. This precludes a compari-
son of the results from these studies.
In vitro studies
The mutagenic potential of wood smoke particles has
been relatively well documented in bacterial systems and
seems to depend on the PAH content, which is influenced
by the combustion conditions [5]. Wood smoke particles
have also been reported to induce DNA damage in human
monocytic and epithelial cell lines and in a murine mac-
rophage cell line [147-149]. Surprisingly, particles from
three different combustion appliances (old boiler, mod-
ern boiler and pellets boiler) with varying content of
organic carbon showed a similar genotoxic potency [147].
On the contrary, the combustion conditions were found
to have great influence on the ability of wood smoke par-
ticles to induce chromosome breakage, when investigated
by the micronucleus test in a lung fibroblast cell line from
Chinese hamsters; particles generated during incomplete
combustion conditions induced much higher levels of
chromosome breakage than particles generated during
more complete combustion conditions [53].
Particles emitted from a variety of stoves and combustion
conditions have been reported to increase the release of
pro-inflammatory cytokines in different in vitro model
systems [120,147,148,150,151]. However, only one study
compared the influence of the combustion conditions on
the inflammatory response. Particles from normal com-
bustion conditions in a conventional masonry heater
were found to induce a slightly higher release of the pro-
inflammatory cytokine tumour necrosis factor (TNF)-α
from a murine macrophage cell line than particles from
poor combustion conditions (Salonen et al. in [120]). The
latter were, however, more potent inducers of macro-
phage-inflammatory protein (MIP)-2, the murine ana-
logue of IL-8. One study compared particles emitted from
three different combustion appliances, a modern wood
pellet boiler, a pellets burner and an old boiler, but the
reported differences in inflammatory potential were small
[147]. This study only used one concentration and time
point in their experiments, which limits the reliability of
the presented data, as the relative responses induced by
the different wood smoke samples could change with par-
ticle concentration and exposure time.
Particles from large biomass combustion plants from
combustion of waste wood or bark, consisting mainly of
inorganic salts, were found to induce an inflammatory
response in a human epithelial cell line, but the same par-
ticles did not induce an influx of inflammatory cells to the
lungs of rats (Bellmann et al. in [120]). The authors sug-
gested that this may be due to rapid clearance of soluble
constituents in the in vivo model systems, whereas clear-
ance was not possible in vitro.
Particles from incomplete high-temperature combustion
were found to induce low cytotoxicity in human mono-
cytic and epithelial cell lines [150,151]. In another study,
soot from a poorly operated stove exhibited much higher
cytotoxicity than particles from normal combustion con-
ditions in a fibroblast cell line, whereas inorganic particles
from complete combustion conditions were even less
toxic [53]. Similarly, particles from incomplete combus-
tion conditions induced greater increases in cytotoxicity
and programmed cell death (apoptosis) in a murine mac-
rophage cell line than particles from normal combustion
conditions (Salonen et al. in [120]).
The organic fraction of wood smoke particles has been
suggested to be involved in the release of inflammatory
mediators and DNA damage [149-151]. Klippel and Nuss-
baumer compared the toxicity of the condensable organic
matter collected during poor, normal and complete com-
bustion conditions [53]. Interestingly, the condensable
organic matter from the three different combustion con-
ditions had a similar toxicity when compared on equal
mass concentration, but the amount of condensable
organic matter emitted increased with decreasing combus-
tion efficiency. Kubatova et al. (2006) applied a novel
method for fractionation of organic extracts in combina-
tion with chemical analysis to determine the groups of
organic compounds that contribute to cellular oxidative
stress. Mid-polarity and non-polar compounds, including
oxy-PAHs, were identified as inducers of oxidative stress
in a macrophage cell line [44]. Further similar studies are
necessary to determine how these groups of compounds
or other organic compounds influence a wider range of
biological endpoints, but also to determine the influence
of varying combustion conditions.
The available literature concerning wood smoke exposure
in human volunteers and animal model systems is not
sufficient for comparison of the effects induced by parti-
cles from different combustion conditions or to discuss
the influence of the physicochemical properties on the
biological response. However, the number of in vitro stud-
ies that compare wood smoke particles generated under
varying combustion conditions is currently increasing. As
discussed above, particles from different combustion con-
ditions seem to induce differential pro-inflammatory
response patterns, whereas particles from poor combus-
tion seem to have greater effects on both cytotoxicity and
DNA damage than particles from more complete combus-
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tion conditions. However, in vitro model systems have
several limitations. For instance, the particle exposure
does not mimic the conditions during in vivo exposures
and these models also lack the cellular interactions and
neurological signals that are of importance in animals.
The physicochemical properties of collected particles may
also be altered compared to the properties of particles
deposited directly from the gas-phase, which occurs dur-
ing human exposure. In in vitro experiments, different par-
ticle samples are usually compared on an equal mass
basis. However, the pulmonary deposition and retention
of particles partly depends on the physicochemical parti-
cle properties [25,26]. Thus, during inhalation of equal
concentrations of wood smoke particles with different
physicochemical properties, pulmonary cells may be
exposed to different particle concentrations due to differ-
ences in deposition efficiency. This was recently demon-
strated for biomass combustion aerosols generated under
different combustion conditions [69]. Particles generated
during complete combustion conditions (inorganic ash)
and particles generated during incomplete combustion
conditions (soot/organic carbon) showed relatively low
respiratory tract deposition compared to traffic-derived
particles due to their size and hygroscopicity [69]. This
demonstrates the importance of considering the depos-
ited dose when estimating the toxicological potential of
air pollution particles.
In order to target the strategies applied to reduce wood
smoke emissions, it is crucial that future toxicological
studies provide information about how physicochemical
properties, combustion conditions and the type of fuel
and combustion appliance influence the toxicity of the
emitted particles. We suggest that future toxicological
studies perform a minimum of physicochemical charac-
terisation, i.e. determine the fractions of organic carbon,
soot and inorganic ash, and perform further characterisa-
tion of the organic fraction. We also emphasize the need
for further studies comparing wood smoke particles from
different combustion conditions generated from the same
stove, particularly in in vivo model systems.
Summary and conclusion
Summary
Wood smoke particles were divided into three classes
based on their physicochemical properties; spherical
organic carbon particles, soot particles and inorganic ash
particles. These particle classes differ with respect to prop-
erties that are likely to influence their toxicity, such as size,
morphology, internal microstructure, solubility, hygro-
scopicity, organic chemistry and content of inorganic
compounds (Figure 1). Emissions from various appli-
ances often contain several of the defined particle classes.
The reviewed studies of ambient, indoor and personal
exposure applied various markers to estimate the wood
smoke exposure. These markers are usually only repre-
sentative for one class of residential wood combustion
particles, and therefore provide limited information
regarding the physicochemical properties of the particles
we are exposed to. However, by considering the physico-
chemical properties of emissions from different types of
combustion appliances (Table 2), and their emission fac-
tors (Table 3) and estimates, soot and organic carbon were
suggested to be the dominating classes in wood smoke
exposure in European countries. Inorganic ash particles,
primarily emitted from pellets burners and modern wood
log boilers, were found to make a small contribution to
ambient concentrations at present, although their contri-
bution is likely to increase in the future.
Only a few experimental studies have compared the bio-
logical effects induced by particles from different combus-
tion conditions (Table 4). The conducted in vitro studies
suggested that the biological potential varied with the
combustion conditions; particles from poor combustion
induced more severe effects on both cytotoxicity and DNA
damage than particles from more complete combustion
conditions. However, the current in vivo data concerning
biological effects of particles from varying combustion
conditions is scarce, and further investigations are neces-
sary.
Conclusion
Epidemiological and experimental studies provide
increasing evidence for an association between wood
smoke exposure and various health outcomes such as
decreased lung function, reduced resistance to infections
and increased severity/incidences of acute asthma. More-
over, inhalation studies have demonstrated that wood
smoke exposure may induce systemic effects, providing a
possible link to cardiovascular effects. The influence of the
physicochemical properties of wood smoke particles, and
of the combustion conditions, on various biological end-
points is presently largely unknown, although in vitro
studies suggest that particles from incomplete combus-
tion conditions are more toxic than particles generated
under more complete combustion conditions. In order to
establish targeted strategies to reduce wood smoke emis-
sions in developed countries, more research is needed
concerning the physicochemical properties of the wood
smoke particles we are exposed to and the influence of
these properties on the induced biological effects. To
achieve this, there is need for a stronger collaboration
between the different fields of research including combus-
tion science, aerosol science, epidemiology and toxicol-
ogy.
Particle and Fibre Toxicology 2009, 6:29 http://www.particleandfibretoxicology.com/content/6/1/29
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Competing interests
The authors declare that they have no competing interests.
Authors' contributions
AKB planned and coordinated the study. All authors pro-
vided essential contributions to the manuscript and were
involved in drafting the manuscript or revising it critically.
All authors read and approved the final manuscript.
Additional material
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Additional file 1
Different types of wood combustion appliances. The table provides a
description of the four main types of wood combustion appliances men-
tioned in the text.
Click here for file
[http://www.biomedcentral.com/content/supplementary/1743-
8977-6-29-S1.doc]
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