Review of literature on residential firewood use, wood-smoke and air toxics

Technical Report No. 4
Environment Australia, June 2002
ISBN 0 6425 4868 4

Executive summary

A review of over 350 articles dealing with residential firewood use and wood-smoke has demonstrated possible health impacts associated with wood-smoke concentrations now observed in Australian urban areas. The contribution that woodheaters and open fireplaces make to total particulate levels is difficult to calculate with much certainty due to a lack of Australian data on emissions from appliances when used under typical household conditions and uncertainty in the quantity of firewood burnt. Ambient monitoring of particles in several Australian cities has demonstrated a high proportion of particles from biomass burning. Laboratory testing associated with appliance certification demonstrates significant improvements in combustion technology have been achieved. However, insufficient data is available to assess the improvements, if any, realized through a range of actions aimed at reducing emissions from firewood use in people's homes.

  1. Households using firewood. The information base on the number of households using firewood as their main heating fuel is adequate. Australian Bureau of Statistics surveys provide periodic data allowing trends to be identified and absolute numbers of heaters used by State and Territory. The data suggest a decline, since about 1992/3, in the proportion of households choosing firewood as their main heating fuel. However, this proportional decrease is partially offset by the growth in the number of occupied households associated with population growth and fewer occupants per household. In absolute terms, this results in around 3000 to 4000 less households using firewood as their main heating fuel per year, out of about 1.5 million households using firewood for heating.
  2. Firewood consumption. The average quantity of firewood used per year per household is not accurately known. The quantity varies from one climate zone to another and also depends on the price and availability of firewood. Surveys of average annual firewood use show considerable variation within each region (for example firewood use is higher per firewood using household on the outskirts of larger cities than in the centres). Estimates of total firewood demand for Australia range from 3 to 5.5 million tonnes of air-dry wood per year.
  3. Firewood species. The preferred firewood species vary from region to region. Information on which species are preferred is available but the proportion of each species burnt is not accurately known. It is known that hardwood, almost all eucalyptus species, is the dominant fuel (over 90%). A small number of households purchase softwood (pine) firewood (1 to 2%), but of households that collect their own firewood about 10% collect some pine. It appears softwood is only around 5% of the total firewood supply. It is not known how much treated or painted wood is burnt. It is likely to be a very small proportion, but it could still be significant because of higher toxic emissions.
  4. Ecological impacts. Few studies of the ecological impacts of firewood collection have been conducted. Limited evidence available suggests that problems may be occurring in dryer regions that have already experienced widespread land clearing. The scale of the firewood industry suggests that more attention should be paid to this annual harvest of wood from Australia's forests and woodlands.
  5. Firewood moisture. Very limited data is available on the average moisture content of firewood used, or what proportion of firewood is wet (nominally above 20% moisture) either through inadequate seasoning or through rain-wetting.
  6. Wood-Burning Appliances. The proportion of woodheaters and open fireplaces used for residential heating is adequately known. However, the proportions of certified and non-certified woodheaters is not known. Estimates suggest only 20 to 25% of woodheaters now in use were manufactured after 1992 when heater emission certification began. It is not known to what extent retailers or householders have modified certified heaters to increase their burn times.
  7. Emission control technology. Woodheater manufacturers have been working towards lower emission appliances for about two decades. Some early success was achieved with catalytic devices built into appliances to encourage better combustion. Catalysts work well provided there is a regular supply of unburnt hydrocarbons to maintain catalyst temperatures, but as better design led to lower emissions from the primary combustion zone the catalysts became less effective. Most commentators, supported by recent emission studies, favor 'high-tech' combustion chamber design over catalysts as a means of reducing emissions. Other approaches to low emission design that have proved successful are high mass appliances, where combustion always takes place at high burn rates and heat is stored in the appliance for gradual release into the living space. Wood-pellet heaters, where fuel (wood-pellets) is fed into the combustion chamber continually have achieved significantly lower emission factors than batch-fed appliances. In the US, pellet heaters are popular. In Australia, neither pellet supply nor pellet heaters have been available until very recently. The literature review has not identified any add-on devices to reduce emissions from existing woodheaters.
  8. Woodheater certification. An Australian and New Zealand standard method for determining the emission factors for woodheaters is available (AS 4013). The method tests heaters under controlled conditions using air-dry firewood (hardwood is used for almost all testing although a few appliances have been tested with hardwood and softwood). The test method includes high, medium and slow burn rates. Two commercial laboratories are registered for testing to the standard and the Energy Information Centre in South Australia holds/compiles all results.
  9. Particulate emission factors. A good data set of particulate emission factors and efficiency is available for woodheaters that have been tested and certified to the standard. This indicates that the average emission factor for all models tested is 3.3 g/kg and the average efficiency is 62%. The tests show emission factors for Australian certified heaters range from 5.5 g/kg to 0.8 g/kg, a factor of almost 7. [The emission factor relates the mass of particulates expressed in grams to the mass of wood (corrected to oven-dry weight) expressed in kilograms.] No data on emission factors is available for woodheaters when used in people's homes. The laboratory test data is based on correct heater operation with dry fuel. These conditions will not always apply in 'real-world' heater use. Emission inventory studies use 'real-world' emission factors of 10 to 15 g/kg, but these are derived more through guesswork than scientific process, although estimates are similar to US figures where field measurements have been made in people's homes.
  10. Open fireplace emission factors. No previous Australian studies have been carried out to measure emission factors for masonry open fireplaces. US data has been used in emission inventories.
  11. Carbon monoxide emission factors. Very few measurements of carbon monoxide from woodheaters have been made in Australia. The standard emission test is for particulates and does not require simultaneous CO measurement. No previous measurement of CO from open fireplaces has been identified in the Australian literature. It is likely that woodheaters and, to a lesser extent, open fireplaces contribute significant quantities of CO to urban air-sheds. International literature suggests emission factors for CO of around 70 g/kg.
  12. Air toxics emission factors. Few measurements of toxic compounds emitted by woodheaters and open fireplaces have been made in Australia prior to the current study. Emission inventories rely on international studies. Even these have been very limited for some important toxic compounds such as dioxins. Estimates of emissions of individual air toxic species are possible, but large uncertainties will result. Woodheater emission factors are reported for 47 air toxic species, including 29 PAHs.
  13. Mutagenicity of wood-smoke. The international literature reports on the mutagenicity of wood-smoke, noting changes in mutagenicity as the smoke undergoes aging in the atmosphere. Most authors suggest wood-smoke mutagenicity is less than that of vehicle emissions and 'moderate' compared to cigarette smoke. Mutagenicity is reported to increase in the presence of NOx, O3 and sunlight.
  14. Ambient air wood-smoke measurement. Particulate measurements throughout Australia will be influenced to some extent by wood-smoke. In summer, and warm climate regions, bushfire and hazard reduction burn smoke may be present. In cool and temperate regions, residential use of firewood will contribute to particulate loads, especially in winter. Industrial combustion of wood waste occurs throughout the year. Emission inventories suggest some air-sheds will have winter particulate levels dominated by wood-smoke. In Launceston, Tasmania, for example, some studies have suggested that as much as 95.6% of PM10 particles can be attributed to wood-smoke, although this estimate might be too high. Chemical analysis of particle composition in air-sheds such as Launceston, confirm a high proportion of new carbon (from biomass) compared to old carbon (from fossil fuels). While there is considerable uncertainty about the precise proportion of PM10 particles contributed by residential firewood use, there is good evidence that woodheaters and open fireplaces are significant contributors in all cool and temperate regions.
  15. Indoor exposure to wood-smoke. Several international studies have measured increased particulate and air toxics concentrations in homes using open fireplaces and woodheaters, although most studies indicate open fireplaces are the main source. Some studies have linked childhood respiratory problems to homes with firewood use while others have not demonstrated any correlation.
  16. Health impacts. Epidemiological studies in several countries have established convincing correlations between PM10 (and PM2.5) and near-term mortality and morbidity. Australian studies in Sydney and Brisbane show similar correlations, but one study in Melbourne found less significant correlations. The studies suggest particle composition is less significant than the physical properties of particles. The link between PM10 and human health has been a determining factor in setting the PM10 goal for Australia as 50 µg/m3 (24 hour average). The epidemiological studies also suggest that there is no lower cut-off point for health impacts of particles. Thus, it is desirable to aim for minimum particulate concentrations, irrespective of maximum 24-hour goals, and reducing wood-smoke emissions is one means of doing this.

    Longer-term links between wood-smoke and health have been inferred because of the chemical similarities of wood-smoke and cigarette smoke. The presence of many PAHs, including benzo(a)pyrene, suggest possible health impacts through prolonged exposure to wood-smoke. Some studies in Mexico and New Guinea suggest people with a history of extremely high indoor wood-smoke exposure do suffer more respiratory illnesses later in life.
  17. Emission reduction strategies. Policy documents from all States and Territories, together with national policy statements, suggest a similar range of possible control options:
    • enforce compliance with the emission standard and, possibly, reduce the maximum allowable emission factor;
    • encourage existing woodheater users to change to other forms of heating or to lower emission woodheaters through incentive programs (southern states);
    • target households apparently emitting more smoke than is considered reasonable;
    • regulate the moisture content of wood sold as firewood (southern states);
    • educate woodheater users on methods of minimizing smoke emissions, possibly targeting households seen to be emitting more smoke than average;
    • regulate minimum flue heights to ensure better dispersion and minimize local nuisance problems; and
    • improve the thermal performance of homes to reduce the need for heating.
  18. Further research. The existing information base is able to inform certain policy decisions, but more information is needed for comprehensive management of the wood-smoke problem. Information gaps have been identified.

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