Technical Report No. 5
J. Gras, C.Meyer, I. Weeks, R. Gillett, I. Galbally, J. Todd, F. Carnovale, R. Joynt, A. Hinwood, H. Berko and S. Brown.
Environment Australia, March 2002
ISBN 0 6425 4867 6
9. Relevance of standards
Wood-heater emission limits are set against specific standard measuring procedures. Together these have a dual function of allowing comparison of different heater designs and ensuring that emissions from new appliances meet a minimum level of performance. Operating conditions expressed in standard procedures usually only reflect generalized aspects of normal heater operation. Standard tests aim to minimize sources of variation external to heater design including fuel type (hardwood, softwood), moisture, density, fuel loading, etc. Predictably, national emission standards have strong regional characteristics and are potentially less applicable outside the regions for which they were designed. Given this evolution of different national standards, one important issue to be considered is the relevance of tests tailored for Northern hemisphere conditions, heaters and fuels to Australian circumstances.
Test specifications which vary widely between different standards include:
fuel types and burning regimes,
the species used to assess emission performance, e.g. mass, CO and VOC,
physical parameters measured (e.g. heating efficiency), and
the number of duplications.
Even a single measurement parameter such as (aerosol) mass can be expressed in different ways, such as emission mass per mass of fuel (wet or dry), mass per MJ power produced and mass per MJ effective power into the room. All of these approaches are legitimate and have associated benefits and limitations. Efforts to establish unified or harmonized international test procedures have so far foundered.
The use of any of mass, CO or VOC emission factors as indicators of the overall emissions is somewhat arbitrary, since these and other species are formed by similar pyrolysis and combustion processes and therefore tend to co-vary. For example an appliance with low VOC emissions can be expected to also have low mass and benzene emissions. There are exceptions including nitrogen oxides and dioxins, which form in different combustion conditions or in narrower temperature ranges and which consequently tend to behave differently to the more commonly used indicator species.
Qualitative comparison between different standards is certainly possible, although quantitative comparisons are severely limited. This was shown clearly by the round-robin experiment of Skreiberg et al. (1997), where large variance was observed for emissions from similar heater operating modes under different national standard test procedures. AS4013 was specifically designed to test heater designs for the hardwood fuels and climate of southern Australia and must be considered the most appropriate of the various international standards available for that purpose.
A second issue is the relevance of emissions standards for air quality management and epidemiological health issues. Wood-smoke aerosol is a major component of the urban aerosol, at least seasonally, in a number of regions across southern Australia (e.g. Gras et al. 1992; Gras 1996; Ayers et al. 1998). Epidemiological studies that have examined the relationship between aerosol mass and various health outcomes, specifically acute effects on mortality and morbidity, consistently show a positive relationship with no lower (effect) threshold (Gras 1996). Chronic effects of other toxic components in wood-smoke may occur with sufficient exposure, specifically from the many carcinogenic species including benzene, some PAHs and dioxins. Absence of a lower effect threshold means that there is no simply defined safe limit for exposure. Thus, establishing an acceptable value for an emissions standard is a political process which must balance the relative risk through community exposure to wood-smoke against community benefits from, or reliance on, wood burning as a heating source.
Broadly speaking, Australian standards, as well as international standards, tackle the issue of controlling emissions in general through the measurement and control of one to several species. Such species are selected to be representative of total emissions, and in the absence of specific risk analyses for individual species and to enable testing to be reasonably cost effective, this is a reasonable approach. Generally for urban aerosol, acute health effects related to bulk measures such as mass are given greater weight than the chronic or long-term effects of minor species (see Gras 1996). AS4013 does directly address the issue of possible acute health effects of aerosol mass but does not specifically address any potential longer-term toxic effects of emissions.
A third issue is whether emissions standards have, or perhaps have had, an impact on air quality and associated health issues and whether other additional approaches may be necessary. Evolution of emission standards is certainly encouraging the adoption of more technologically-advanced combustion methodologies, for example in the USA, Europe and Australia, although not rapidly enough for some critics (e.g. Gilmour and Walker 1995). Additionally, not all of the available management tools have been taken up. For example there is no current method for auditing the performance of production model heaters and no mechanism or program for assessing in-service performance of installed heaters. There are no controls on the sale of used heaters and some states have not enacted legislation to enforce the sale of only AS4013-compliant new heaters. Many local jurisdictions could be more effective in dealing with nuisance smoke emissions, and accurate risk assessments would strengthen the case for such actions.
Overall, the present work suggests that the existing Australian standards and procedures are a relevant management tool for reducing exposure to toxic components in wood-smoke. However, a major shortcoming in the procedure of managing emissions is that there has not been, and still is no program for assessing whether the introduction or tightening of emission standards has translated into reduced exposure and risk. A logical development from the present study would be to use the outcome to establish the relative risks of the toxic components in wood-smoke to the community.