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Health effects assessment of exposure to particles from wood smoke 8 Conclusions
The number of residential wood stoves in Denmark has increased 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. Measurements of particulate matter (PM) levels in ambient air in areas with many wood stoves have consistently shown elevated levels, particularly during wintertime when wood burning is common. Recent results have shown that the particle emission from residential wood stoves is an important source of particles in ambient air pollution in Denmark. 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. 8.1 Exposure assessment, wood smoke PMAt present, the population exposure to wood smoke PM in Denmark cannot be estimated precisely as only few measurements have been conducted in selected residential areas with different kinds of heating. Based on the data from the two measurement campaigns in Denmark, an increase in annual average PM2.5 of 1 µg/m³ is a best maximum estimate of the whole population exposure. In addition to the measured PM2.5 levels, model calculations have estimated 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 as a best minimum estimate. Based on these data, the annual average PM2.5 exposure from wood smoke can be estimated to 0.2-1 µg/m³ as a preliminary estimate for the whole Danish population with a best estimate of about 0.6 µg/m³. The best estimate of 0.6 µg/m³ has been used as the exposure estimate in the risk characterisation. 8.2 Hazard assessment, wood smoke PMA number of epidemiological studies, which have evaluated adverse health effects from ambient air pollution in relation to residential wood combustion, indicate a consistent relationship between PM and increased incidences for different health endpoints. Overall, an increased risk of experiencing adverse health effects in the respiratory tract from exposure to particles in wood smoke was associated with an increase in ambient PM (PM1, PM2.5 and PM10) of about 10 µg/m³. Boman et al. (2003) have compared the results from five epidemiological 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. Based on this comparison, the authors concluded that there seems to be no reason to assume that the health effects associated with PM in areas polluted with wood smoke are weaker than elsewhere. 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) as well as the present report. None of the available epidemiological studies, on wood smoke PM as well as on ambient PM, have indicated a threshold concentration for effects. The available epidemiological studies do not provide a sufficient basis in order to evaluate the lung cancer risk due to exposure to particles from wood smoke, or to particles from other combustion sources. Some studies in experimental animals indicate that inhaled PM from wood smoke, similarly to PM in general, can cause adverse health effects in the respiratory tract and the lungs. Most 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. However, two recent studies have reported only minor respiratory effects following subchronic exposure to environmental levels (30-1000 µg/m³ total PM, particle size < 1 µm for the major part of the particles); the most recent study indicates that 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. The uncertainties about the actual contribution of PM from wood smoke to ambient PM preclude, for the time being, precise characterisations of specific dose-response relationships for the adverse health effects associated with exposure to wood smoke PM, and whether differences exist for dose-response relationships for wood smoke PM compared to the known dose-response relationships for ambient PM in general. Therefore, a more precise evaluation of the impact on human health of air pollution related to wood combustion is not possible for the time being. 8.3 Risk characterisation, wood smoke PMThe 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. From the estimated increase of 6% (95% CI: 2-11%) in the mortality rate for an increase of 10 g/m³ of PM2.5 as concluded by WHO (2005), the overall contribution of 0.6 g/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). From the estimated increase of 3.3% in respiratory hospital admissions (data from time-series studies) for an increase of 10 g/m³ of PM2.5 from the Sheppard et al. (1999) study, the overall contribution of 0.6 g/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. From the estimated increase of 14% for new cases of chronic bronchitis for an annual increase of 10 g/m³ of PM2.5 as assessed by the European Commission (2005), the overall contribution of 0.6 g/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. It should be noted, however, that 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 wood smoke 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 may result in underestimations as the dose-response relationship for mortality most likely is underestimated.
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