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Health effects assessment of exposure to particles from wood smoke Summary
The number of residential wood burning devices has increased in Denmark during the latest years and it has been estimated that there in 2005 were about 551,000 wood stoves and about 48,000 wood boilers in Denmark. This has resulted in an increased exposure of the general Danish population to pollutants associated with residential wood smoke. New Danish monitoring results on particulate matter (PM) in ambient air have shown elevated PM levels in areas with many wood stoves, particularly 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. It has been estimated that about 17,665 tonnes PM2.5 per year (2005) in Denmark come from residential wood combustion. Therefore, there is an increasing concern that adverse human health effects might be associated with the increased exposure to residential wood smoke. This project has been set up in order to review the scientific literature concerning adverse health effects of pollutants associated with residential wood smoke with the main focus on particulate matter and to quantify and evaluate, if possible, the impact on human health of the increased exposure to particles in residential wood smoke. Human exposure to particles from wood smokeThere is only very limited information on population exposure to wood smoke particles in Denmark. Measurements during a 6-week winter period (2002 and 2003-2004) 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. 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. An increase in annual average PM2.5 of 1 µg/m³ is a best maximum estimate of the whole Danish 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 annual 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 Danish 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 sub-micrometer particles can easily penetrate into the indoor environment, especially if air filtration does not occur. A recent Swedish study 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. Human health effectsThe association of PM with adverse health effects has long been recognised, especially in relation to respiratory and cardio-vascular diseases and primarily in the elderly and in individuals with pre-existing respiratory and/or cardiac diseases. The experience is mainly based on epidemiological studies (cohort studies and time-series studies). A limitation with both study designs is the exposure characterisation, which is usually based on data from a single monitoring site in the area, and assumed to be representative for all individuals in the area. Another limitation is that most of the exposure information is on fine particles (PM2.5), or the sum of fine and coarse particles (PM10), whereas the information on ultrafine particles is limited. Human non-cancer health effects from exposure to particles from wood smokeThe emission of particles from residential wood burning and their impact on human health has received much attention lately. Several early studies (Table 5) have focused on the presence of a wood stove in the home as a risk factor. While these studies strongly suggest that there are adverse health impacts in form of more respiratory symptoms and diseases associated with wood smoke exposure, their crude exposure assessments preclude more specific conclusions. In addition, a number of studies (Table 7) have reported associations of adverse health impacts in the airways with use of biomass fuels. All these studies are observational and very few have measured exposure directly, while a substantial proportion have not dealt with confounding. As a result, risk estimates are poorly quantified and may be biased. A number of studies (Table 6) have evaluated adverse health effects from ambient air pollution in relation to residential wood combustion in communities where wood smoke was a major, although not the only, source of ambient air particulate. The studies indicate a consistent relationship between PM1, PM2.5 and/or PM10 and increased respiratory and asthmatic symptoms, and decreased lung function. The studies have mainly focused on children, but the few studies focusing on adults as well have shown similar results. There are also indications from several of the studies that asthmatics are a particularly sensitive group. The studies giving an indication of the dose-response relationship are summarised in Table 8 (section 7.2.2.1). The relative risk (RR) between an increase in ambient PM10 with 10 µg/m³ and different health outcomes varied between 1.01 and 1.12. An RR for increased asthma hospital admissions of 1.15 and 1.04 has been reported for an increase in ambient PM2.5 with 11 and 12 µg/m³, respectively. An RR for increased asthma symptoms in children of 1.17 has been reported for an increase in ambient PM1 with 10 µg/m³. Overall, these studies showed that an increased risk of experiencing adverse health effects in the respiratory tract from exposure to particles in wood smoke (RR 1.04-1.17) is associated with an increase in ambient PM (PM1, PM2.5 and PM10) of about 10 µg/m³. None of the available studies have indicated a threshold concentration for effects. However, it should be noted that due to differences in the statistical analyses and presentation of the results in the various studies, it is difficult to compare the results from different studies. Boman et al. (2003) compared the results from the five wood smoke studies in which residential wood combustion was mentioned as an important air pollution source with estimations for the association between PM and health effects in the general environment (Figure 3). All the included studies showed significant positive associations for respiratory symptoms evaluated. In comparison with the estimations concerning ambient PM and health effects in the general environment, the RR were even stronger in the studies in which residential wood combustion was considered a major PM source. A very recently published review (Naeher et al. 2007), which is based on an extended list of references, confirms the overall picture presented by Bomann et al. (2003). In conclusion, the available studies indicate that exposure to wood smoke PM is associated with the same kind of non-cancer health effects known from exposure to PM in general and that the health effects associated with PM in areas polluted with wood smoke are not weaker than elsewhere. However, the uncertainties about the actual contribution from wood smoke to ambient concentrations of PM preclude, for the time being, precise characterisations of specific dose-response relationships for wood smoke PM and whether differences exist compared to the known dose-response relationships from PM in general. Therefore, a more precise evaluation of the impact on human health of air pollution related to residential wood combustion is not possible for the time being. Human carcinogenic effects from exposure to particles from wood smokeThere is limited information regarding the human cancer risks associated with biomass air pollution, including wood smoke. The Chinese studies on an association between wood smoke exposure and lung cancer risk do not indicate an increased risk even after long-term exposure to very high levels of biomass smoke (PM10 22 µg/m³) from open-fire domestic cooking. Two more recent case-control studies from Mexico and Southern Brazil are suggestive of a small increased risk of lung cancer due to long-term exposure to wood smoke from cooking; however, these studies are limited by the lack of exposure assessments. The most recent case-control study found an increased risk for lung cancer among Canadian women in homes with wood stove or fireplace heating and with gas or wood stove cooking facilities. In conclusion, the available studies do not provide a sufficient basis in order to evaluate whether there is an association between wood smoke exposure and increased risk of lung cancer. However, cohort studies on cancer risk associated with particles in the general environment have suggested an excess risk of lung cancer associated with long-term exposure to PM. Known chemical carcinogens, e.g. PAHs, associated with the PM might be responsible for the excess risk of lung cancer; however, it can not be excluded that PM in itself is capable of causing lung cancer. It should be noted that the International Agency for Research on Cancer (IARC 2008) has recently evaluated that indoor emissions from household combustion of biomass fuel (primarily wood) are probably carcinogenic to humans (Group 2A). Data from studies in experimental animalsMost studies on adverse health effects of wood smoke particles in experimental animals have used relatively high exposure levels compared to the levels generally measured in the environment. In one study, minor but significant changes in the airways of rats (mild chronic inflammation and squamous metaplasia in the larynx; alveolar macrophage hyperplasia and pigmentation, and slightly thickened alveolar septae) were observed following exposure (whole-body, 3 hours/day, 5 days/week for 4 or 12 weeks) to 1 or 10 µg/m³ wood smoke particles (size distribution of 63-74% in the < 1 µm fraction and 26-37% in the > 1 µm fraction). A very recent study has summarised health effects of subchronic exposure to environmental levels of hardwood smoke in rats and mice exposed (whole-body, 6 hours/day, 7 days/week) for 1 week or 6 months) to dilutions of whole emissions based on particulate (30-1000 µg/m³ total PM, mass median aerodynamic diameter of approximately 0.3 µm). Exposure to these concentrations presented little to small hazard with respect to clinical signs, lung inflammation and cytotoxicity, blood chemistry, haematology, cardiac effects, and bacterial clearance, and carcinogenic potential. However, parallel studies demonstrated mild exposure effects on broncho-alveolar lavage parameters and in mouse and rats models of asthma. Lung carcinogenesis measured as either the percentage of young mice with tumours (incidence) or the number of tumours per tumour-bearing mouse (multiplicity) yielded no significant differences from the control group and there was no evidence of a progressive exposure-related trend. Risk characterisationIn order to assess the health impacts from the wood smoke PM, the dose-response relationship from the epidemiological studies on ambient PM in general (i.e., the relative risk RR) has been used as the available epidemiological studies indicate that wood smoke PM does not seem to be less harmful than ambient PM in general. The health impact of PM from wood smoke emissions in Denmark has been assessed for mortality, and for hospital admissions for respiratory and cardio-vascular diseases based on the known dose-response relationships for ambient PM in general as assessed from the available epidemiological studies as well as from the data from the few studies where wood smoke was indicated to be a major source. The increase in the RR for a health endpoint related to ambient PM in general is used to estimate the increase in RR for the specific health endpoint due to the contribution from wood smoke PM. Then this RR is used to estimate the number of cases for this specific health endpoint, which is associated to wood smoke PM. From the estimated increase of 6% (95% CI: 2-11%) in the mortality rate for an increase of 10 µg/m³ of PM2.5 as concluded by WHO (2005), the overall contribution of 0.6 µg/m³ of PM2.5 from wood smoke to the annual population exposure is estimated to be associated with an increase in the mortality rate of 0.36% (95% CI: 0.12-0.66%). This increase in mortality rate corresponds to about 200 deaths each year (95% CI: 66-360). It should be noted, however, that the most recent studies by Jerret et al. (2005) and Laden et al. (2006) found a nearly 3 times higher increase in the mortality rate per 10 µg/m³ increase in PM2.5 than the dose-response used by WHO and thus, the use of the WHO estimate may result in an underestimation of the mortality rate. From the estimated increase of 3.3% in respiratory hospital admissions for an increase of 10 µg/m³ of PM2.5 from the Sheppard et al. (1999) study, the overall contribution of 0.6 µg/m³ of PM2.5 from wood smoke to the annual population exposure is estimated to be associated with an increase in respiratory hospital admissions of 0.2%. This increase in respiratory hospital admissions corresponds to about 156 cases each year. It should be noted, however, that using an estimate for increase in respiratory hospital admissions from a short-term time-series study in relation to an annual increase in PM most probably will underestimate the actual health impact. From the estimated increase of 14% for new cases of chronic bronchitis for an annual increase of 10 µg/m³ of PM2.5 as assessed by the European Commission (2005), the overall contribution of 0.6 µg/m³ of PM2.5 from wood smoke to the annual population exposure is estimated to be associated with an increase of 0.84% for new cases of chronic bronchitis. This corresponds to about 60 new cases of chronic bronchitis each year. No epidemiological data on wood smoke exposure in relation to cardio-vascular diseases are available neither from time-series studies nor from cohort studies. However, the high increased risks for cardiovascular events (coronary heart disease, cerebro-vascular disease, myocardial infarction, coronary re-vascularisation, stroke) and cardiovascular mortality found in a very recent study by Miller et al. (2007) in a subgroup of women (above the age of 50 years and without previous cardiovascular disease) in relation to long-term particle exposure (general ambient air PM) indicate that even an annual increase of 0.6 µg/m³ PM2.5 would have a significant health impact for the Danish population with respect to cardiovascular events and mortality. However, due to the specific design of this study a general health impact assessment for the whole population cannot be made. It should be remembered that this preliminary health impact assessment of woodsmoke PM is hampered by the limited exposure data available as well as by the absence of specific dose-response relationships for the selected health impacts due to long-term exposure to wood smoke PM. It should also be noted that the approach taken to assess the selected health impacts for wood smoke PM is in general considered to underestimate the health impacts as the dose-response relationship for mortality most likely is underestimated. RecommendationsMore information is needed for a more precise health impact assessment of wood smoke PM in Denmark, particularly in relation to assess the exposure to wood smoke PM in the general population as well as to assess the effects from long-term exposure of wood smoke PM on morbidity and mortality including a characterisation of the specific dose-response relationships for the various adverse health effects related to wood smoke PM exposure. It might thus be recommended to conduct one or more studies in a selected residential area with many wood stoves aiming at 1) assessing the exposure to wood smoke PM including measurements of chemical markers for wood smoke in order to evaluate the contribution of wood smoke PM to the ambient general PM, 2) characterising wood smoke PM as well as wood smoke composition, 3) investigating indoor exposure to wood smoke, and 4) investigating the relationship between exposure to wood smoke PM and the health outcomes including a characterisation of the specific dose-response relationships for the various adverse health effects related to wood smoke PM exposure. In addition there is still a need for further experimental studies aiming at understanding the effects and underlying mechanisms of wood smoke exposure in order to evaluate the implication in relation to human health risks as well as investigating chemical and physical particle properties and health effects in relation to the different phases of the combustion cycle and type of combustion device including modern stoves.
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