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Health effects assessment of exposure to particles from wood smoke 3 Human exposure
3.1 Exposure from ambient air3.1.1 ParticlesWood burning devices contribute to outdoor air pollution. The contribution of wood smoke particles to ambient air pollution has recently been summarised by Zelikoff et al. (2002). On a moderately cold winter day, 51% of the respirable air particulates in the Portland, OR, area were from residential wood combustion sources (Cooper 1980). Investigations examining other parts of the northwest America reported that residential wood smoke in the Olympia, WA, area accounted for 50% (on clear days) to 85% (on polluted days) of airborne PM and it was concluded that wood smoke represented a more significant source of ambient PM than the sum of total of all industrial point sources in the state of Washington (Koenig et al. 1988). Additional studies in the same geographic area have demonstrated that 80-90% of the PM measured in the ambient air was due to use of wood burning devices during night time hours (Larson et al. 1992). Larson & Koenig (1994) have summarised a number of studies, which have documented the outdoor concentrations of airborne particles resulting from wood burning. The studies indicated that wood smoke may account for up to 90% of the airborne particle concentrations during the winter. In cases where wood smoke contributed predominantly to the particulate mass (> 80%), PM10 concentrations up to about 150 µg/m³ and PM2.5 concentrations up to about 85 µg/m³ have been reported. In a more recent review (Boman et al. 2003) including nine studies (see Table 6 in section 5.1.2.2) in relation to residential wood combustion, 24-hour PM10 concentrations up to 187 µg/m³ were reported with average values (mean) of about 20-30 µg/m³ in most studies. One American study has reported a 24-hour mean PM2.5 of 16.7 µg/m³ (Sheppard et al. 1999) and another American study has reported an average concentration of PM2.5 of approximately 12 µg/m³ for a 15-month period (Norris et al. 1999). In a recent Swedish study (Molnár et al. 2005), outdoor levels of PM2.5 were measured in the winter 2003 in a residential area where domestic wood burning is common; the mean value was 13.7 ± 8.0 µg/m³. Data from Nordic countries have shown that emissions from wood stoves depend very much on the combustion conditions and technologies (Sternhufvud et al. 2004). Old stoves are still the most important PM emission source compared with newer types of stoves with improved technology. In 2005, there were about 551,000 wood stoves and about 48,000 wood boilers in Denmark (Evald 2006). Recent results have shown that the particle emission from residential wood burning stoves is an important source of particles in ambient air pollution in Denmark. According to Illerup & Nielsen (2004), about 10,000 tonnes PM2.5 per year or about half of the total particle emission in Denmark come from residential wood combustion. In 2005, the total particle emission was about 27,787 tonnes PM2.5 in Denmark of which about 17,665 tonnes PM2.5 were from residential wood combustion (MST 2007). Another important source is road traffic, which contributes with about 20% of the total emission of PM2.5 (Palmgren et al. 2005). There is only very limited information on population exposure to wood smoke particles in Denmark. Only data from two measurement campaigns can be used to estimate the particle contributions from local wood combustion. Measurements in a Danish residential area with no district heating and many wood stoves during the winters 2002 and 2003/4 have shown that the vast majority of PM2.5 pollution stems from three sources: long-range transport, traffic and wood combustion. The highest contribution to PM2.5 was from long-range transported pollution, mainly salts. Measurements during a 6-week winter period showed that the contribution from wood combustion to ambient PM2.5 was comparable to the contribution from a heavily trafficked road to PM2.5 at the sidewalk. The 24-hour range of PM2.5 in ambient air was about 12-19 µg/m³ (it is important to note that the applied measurement method underestimates PM2.5 by about 30% compared to the reference method). The average PM2.5 concentration was elevated by about 4 µg/m³ compared to background measurements during the six winter weeks (Glasius et al. 2006). Measurements in the Danish residential area also showed increased ambient air concentrations of dioxin and PAH. (Vikelsøe et al. 2005, Palmgren et al. 2005). In another residential area with natural gas combustion as the primary heating source and wood combustion as a secondary heating source, the average PM2.5 concentration was elevated by about 1 µg/m³ compared to background measurements during four winter weeks (Glasius et al. 2007). Based on these limited datasets, the general population exposure can only be estimated with very large uncertainties. In addition to lack of measurements, there is presently a lack of information on e.g. the actual geographic distribution of the wood-combustion appliances and the number of people living in the vicinity. An increase in annual average PM2.5 of 1 µg/m³ is a best maximum estimate of the whole population exposure based on the data from these two measurement campaigns showing an increase in average PM2.5 of 4 µg/m³ during winter in a residential area with no district heating and many wood stoves and of 1 µg/m³ during winter in a residential area with natural gas combustion as the primary heating source and wood combustion as a secondary heating source. Model calculations have been used to estimate the PM2.5 levels resulting from wood combustion in Denmark. The total Danish PM2.5 emissions from wood-combustion were assumed to be distributed evenly over the whole area of Denmark, and the results showed an increase in PM2.5 of 0.4 µg/m³ during winter (October-March) corresponding to an increase in annual PM2.5 of 0.2 µg/m³ (Palmgren et al. 2005). An increase in annual PM2.5 of 0.2 µg/m³ is a best minimum estimate of the whole population exposure. In conclusion, the annual average PM2.5 exposure from wood smoke is roughly estimated to be 0.2-1 µg/m³ for the whole Danish population with a best estimate of about 0.6 µg/m³. The contribution to PM2.5 from wood smoke of 0.2-1 µg/m³ should be seen in connection with the overall PM2.5 levels in Denmark. Measurements of PM2.5 in the centre of Copenhagen and in urban background showed PM2.5 concentrations of 19.8 µg/m³ in Central Copenhagen and 14.6 µg/m³ in urban background (Jensen et al. 2004). 3.1.2 PAHThe following data are not specifically addressing wood smoke PAH but pertain to PAH ambient air measurements with contribution from all sources. In a recent study (Prevedouros et al. 2004), atmospheric monitoring data for selected PAHs have been compiled from remote, rural and urban locations in the UK, Sweden, Finland and Arctic Canada. Urban sites included London and Manchester as well as the semi-rural site Hazelrigg, rural locations were in Rörvik (Sweden) and Pallas (Finland), and the remote site was near Alert (Arctic Canada). Table 4 gives the typical ranges for the compounds at each site for 1996 as a reference year. The sites differed substantially in PAH concentrations and represented a range along an urban, rural and remote gradient, i.e. a ‘dilution’ of ambient air concentrations at sites further away from major source regions. Urban centres in the UK had concentrations 1-2 orders of magnitude higher than in rural Europe and up to 3 orders of magnitude higher than in Arctic Canada. The concentrations from the semi-rural site Hazelrigg may have been influenced by the proximity of a major highway and this could be the source to the observed elevated concentrations of some PAHs (most notably phenanthrene) at this site. Seasonality, with winter concentrations being higher than summer concentrations, was apparent for most PAHs at most sites; high molecular weight compounds (e.g. benzo[a]pyrene) showed this most clearly and consistently. Strong winter>summer seasonality is linked to seasonally dependent sources, which are greater in winter, and photolytic degradation during summer. This implicates inefficient combustion processes, notably the diffuse domestic burning of wood and coal. However, sometimes seasonality can also be strongly influenced by broad changes in meteorology and air mass origin (e.g. in the Arctic Canada). Table 4. Ranges of the PAH air concentrations (gas and particle) at the selected sites (in ng/m³). Reproduced from Prevedouros et al. (2004).
N/a: Not analysed. 3.2 Indoor penetrationThe transport properties of particulate air pollution strongly depend on the particle size distribution. Sub-micrometer particles can easily penetrate into the indoor environment, especially if air filtration does not occur. Although wood smoke levels in outdoor air are important, most people spend a majority of their time indoor, especially at night in residential areas. Thus indoor penetration is an important variable when interpreting the exposure assessment. Indoor exposure can occur not only from infiltration of outdoor air, but also from emissions into the home from a wood burning appliance. It has been estimated that approximately 70% of the fine particles from the outside air penetrate into the home. For an outdoor concentration of 20 µg/m³ of wood smoke particles, there is an effective infiltration rate of 1 mg/hour of fine particle mass if 7/10 of the volume of air in the room is exchanged with outside air every hour. Higher outdoor concentrations or more rapid air exchange rates would give larger infiltration rates. For most studies of fine particle mass in homes with airtight stoves, the indoor-outdoor ratios are at or below 1.0 implying that infiltration is important even in homes with stoves. (Larson & Koenig 1994). The number concentration of ultrafine and fine particles was measured simultaneously indoor and outdoor in some rural and urban areas of Sweden and Denmark (Matson 2005). The results revealed that the outdoor-generated particle levels were major contributors to the indoor particle number concentration in the studied buildings when no strong internal source was present. The determined indoor-outdoor ratios varied between 0.5 and 0.8. In residential buildings, the indoor number concentration was strongly influenced by several indoor activities, e.g., cooking and candle burning. In the presence of significant indoor sources, the indoor/outdoor ratio exceeded unity. In a Danish study, 15 one-week samples of PM1, PM2.5, inhalable dust (PMinh) and 16 polycyclic aromatic hydrocarbons (PAHs) were collected inside and outside of an uninhabited 4th floor apartment at Jagtvej in central Copenhagen during winter, spring and summer in 2002 (Jensen et al. 2005). Similarly, urban background samples were collected at a 2 km distant 4th floor high rooftop. The particulate air pollution was dominated by fine particles. Approximately 70 wt% of the PM2.5 consisted of PM1 at all sites. The average PM2.5 content in PMinh was 54 and 69 wt% at Jagtvej and in the urban background, respectively. Indoor PMinh consisted almost entirely of PM2.5. Correlation analysis showed a strong relationship between PM1, PM2.5 and PMinh at Jagtvej and in the urban background. However, PM at Jagtvej exceeded the urban background concentrations. Indoor PM correlated well with PM in both the street and the urban background. However, indoor-outdoor ratios below unity (0.77±0.21 for PM1 and 0.77±0.24 for PM2.5) were only achieved using PM concentrations measured in the street at Jagtvej. In PM2.5 samples, the total concentrations of 16 PAHs were 15-284 ng/m³ indoor, 46-235 ng/m³ outdoor, and 2-105 ng/m³ in the urban background. The concentrations were probably underestimated due to extraction recovery below 100%, breakthrough, and reaction with ozone and nitrogen oxides during sampling. The real concentrations may be up to two times higher than observed. Urban background, traffic and indoor sources contributed to the overall concentration of PAHs in the uninhabited apartment. Traffic in the Jagtvej street canyon and indoor sources appeared to be the most important sources for PAHs indoor. Indoor-outdoor measurements of levoglucosan (a chemical marker for wood smoke) have been carried out in two single-family detached houses in Denmark (Randers), one with and one without a wood stove (Glasius et al. 2007). Measurements of levoglucosan showed that the house with a wood stove had increased levels compared with outdoor levels; the increased levels may thus be associated with use of a wood stove in the house. Levoglucosan was also measured in the house without a wood stove indicating that particles from outdoor are transported into the house. 3.3 Exposure from other settings with wood smokeExposure to biomass air pollution occurs in many settings. The highest concentrations of particles have been measured in indoor air in developing countries where wood and other biomass is used as a cooking and heating fuel. In terms of exposure, domestic cooking and heating with biomass clearly presents the highest exposures since individuals are exposed to high levels of smoke on a daily basis for many years. Exposures during cooking with biomass fuel have been reviewed by Smith et al. (2000). Particulate concentrations in Nepal were as high as 200-8200 µg/m³, while measurements from India showed particle concentrations as high as 3600-6800 µg/m³. Measurements of particle concentrations in kitchen areas in developing countries have shown values between 200 and 9000 µg/m³. Park & Lee (2003) measured the particle exposure and size distribution in 23 houses with wood burning stoves in Costa Rica. Daily PM2.5, PM10 and particle size distribution were simultaneously measured in the kitchen. Average daily PM2.5 and PM10 were 44 and 132 µg/m³, respectively. All houses had a particle size distribution of either one or two peaks at around 0.7 and 2.5 µm aerodynamic diameters. The maximum peak levels ranged from 310 to 8170 µg/m³ for PM2.5 and from 500 to 18900 µg/m³ for PM10. Forest fire-fighters comprise an occupational group with high exposure to biomass smoke. In a recent study, the average daily personal exposure to fine particles was 882 µg/m³ with an interquartile range of 235 µg/m³ to 1317 µg/m³ (Slaughter et al. 2004). Concentrations of acrolein, formaldehyde, and carbon monoxide were similarly elevated. Exposures of this population are seasonal (4-5 months per year) and highly variable depending upon the number of fires per season, the intensity of the fires and specific job tasks. Fire-fighters are normally among the most physically fit in the population and do not normally suffer from any pre-existing health conditions. Accordingly, the absence of health impacts among this group does not indicate that health impacts will not be observed in the general population. As biomass combustion associated with forest fires is a special situation with high temperatures resulting in the emission of high levels of various compounds, the health effects of fire-fighters will not be discussed further in this report. 3.4 Summary of human exposure to particles from wood smokeMeasurements of PM levels in areas with many wood stoves have consistently shown elevated levels of PM emissions during wintertime when wood burning is common. Due to the size distribution of wood smoke particles essentially all will be contained in the PM2.5 fraction. Studies from North American communities have reported 24-hour PM10 levels of up to 165 µg/m³ with average values (mean) of about 20-30 µg/m³ in most studies (see Table 6). One American study (Sheppard et al. 1999) has reported a 24-hour mean PM2.5 of 16.7 µg/m³ and another American study (Norris et al. 1999) has reported an average concentration of PM2.5 of approximately 12 µg/m³ for a 15-month period. The most recent American study (Schreuder et al. 2006) has reported a mean 24-hour PM2.5 of 10.6 µg/m³ (95% CI: 8-86 µg/m³). In one study from New Zealand (Hales et al. 2000), the 24-hour PM10 levels were up to 187 µg/m³ with a mean of 28 µg/m³. In a recent Swedish study (Molnár et al. 2005), the mean winter outdoor level of PM2.5 was 13.7 µg/m³ in a residential area where domestic wood burning is common. There is only very limited information on population exposure to wood smoke particles in Denmark. Measurements during a 6-week winter period (2002 and 2003/4) in a Danish residential area with no district heating and many wood stoves showed that the contribution from wood combustion to ambient PM2.5 was comparable to the contribution from a heavily trafficked road to PM2.5 at the sidewalk. The average local PM2.5 contribution from wood combustion was about 4 µg/m³ (Glasius et al. 2006). In another residential area with natural gas combustion as the primary heating source and wood combustion as a secondary heating source, the average PM2.5 concentration was elevated by about 1 µg/m³ compared to background measurements during four winter weeks (Glasius et al. 2007). An increase in annual average PM2.5 of 1 µg/m³ is a best maximum estimate of the whole population exposure based on the data from the measurements in these two residential areas. Based on the total particle emission from residential wood burning, model calculations have been used to estimate the contribution to the PM2.5 levels. The results showed an increase in annual PM2.5 of 0.2 µg/m³ (as a best minimum estimate) for the whole population exposure (Palmgren et al. 2005). In conclusion, the annual average PM2.5 exposure from wood smoke is roughly estimated to be 0.2-1 µg/m³ for the whole Danish population with a best estimate of about 0.6 µg/m³. The sub-micrometer particles can easily penetrate into the indoor environment, especially if air filtration does not occur. A recent Swedish study (Matson 2005) has revealed that the outdoor-generated particle levels were major contributors to the indoor particle concentration when no strong internal source was present and the determined indoor-outdoor ratios varied between 0.5 and 0.8. Recent Danish indoor-outdoor measurements of levoglucosan (a chemical marker for wood smoke) showed increased indoor levels compared with outdoor levels in a house with a wood stove indicating that the increased levels may be associated with use of a wood stove in the house. Levoglucosan was also measured in a house without a wood stove indicating that particles from outdoor are transported into the house. (Glasius et al. 2007).
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