Wood heater smoke and mortality in the Australian Capital Territory: a rapid health impact assessment

Abstract Objectives To estimate the number of deaths and the cost of deaths attributable to wood heater smoke in the Australian Capital Territory. Study design Rapid health impact assessment, based on fine particulate matter (PM2.5) data from three outdoor air pollution monitors and published exposure–response functions for natural cause mortality attributed to PM2.5 exposure. Setting Australian Capital Territory (population, 2021: 454 000), 2016–2018, 2021, and 2022 (2019 and 2020 excluded because of the impact of extreme bushfires on air quality). Main outcome measures Proportion of PM2.5 exposure attributable to wood heaters; numbers of deaths and associated cost of deaths (based on the value of statistical life: $5.3 million) attributable to wood heater smoke. Results Wood heater emissions contributed an estimated 1.16–1.73 μg/m3 to the annual mean PM2.5 concentration during the three colder years (2017, 2018, 2021), or 17–25% of annual mean exposure, and 0.72 μg/m3 (15%) or 0.89 μg/m3 (13%) during the two milder years (2016, 2022). Using the most conservative exposure–response function, the estimated annual number of deaths attributable to wood heater smoke was 17–26 during the colder three years and 11–15 deaths during the milder two years. Using the least conservative exposure–response function, an estimated 43–63 deaths per year (colder years) and 26–36 deaths per year (milder years) were attributable to wood heater smoke. The estimated annual equivalent cost of deaths was $57–136 million (most conservative exposure–response function) and $140–333 million (least conservative exposure–response function). Conclusions The estimated annual number of deaths in the ACT attributable to wood heater PM2.5 pollution is similar to that attributed to the extreme smoke of the 2019–20 Black Summer bushfires. The number of wood heaters should be reduced by banning new installations and phasing out existing units in urban and suburban areas.

1. Diurnal patterns of PM2.5 pollution Figures 2 and 3 illustrate the diurnal pattern of wood heater pollution at the three ACT monitoring sites, showing how wintertime PM2.5 pollution builds up in the evenings particularly at Monash and Florey, peaking after midnight.Emissions often increase when wood heater users turn down the air control for an overnight burn.A second smaller peak occurs in the morning when users reload or relight their heaters for the day.Lowest PM2.5 levels are in the early afternoon when mean PM2.5 during the wood heating season is lower than at other times of year.

Calculation of attributable mortality
As an example, we calculated deaths attributable to wood heater smoke in the ACT in 2021 by multiplying 90% of the 2160 total annual deaths (the 10% reduction accounts for deaths in early childhood, accidental deaths and suicides) with the attributable fraction (AF) of mortality associated with PM2.5.
AF was calculated from the relative risk (RRx) of mortality associated with long-term exposure to the wood heater PM2.5 contribution of 1.49 μg/m 3 (Box 3) using the formula: The relative risk RRx for a PM2.5 increment of x = 1.49μg/m 3 was calculated as: Where RR10 is the increase in natural cause mortality risk for 10 μg/m 3 increase in annual PM2.5 exposure.
Using the RR10 = 1.209 derived from an Australian study  PM2.5 = airborne particles of diameter smaller than 2.5 µm.* Assumes that 70% of the difference between the annual mean concentration and mean concentration for periods outside the wood heater season is attributable to wood heater emissions.** Assumes that 90% of the difference between the annual mean concentration and mean concentration for periods outside the wood heater season is attributable to wood heater emissions.

Figure 1 .
Figure 1.Monthly mean minimum temperature at Canberra Airport, by year*

Figure 2 .
Figure 2. Hourly mean PM2.5 concentrations at (a) Monash, (b) Florey, (c) Civic by hour of day, season (wood heating = April to September, Rest-of-Year = January to March and October to December) and year (2015-2023).Means for Rest-of-Year are omitted for 2019 and 2020 because of the impact of the Black Summer Bushfires.Years 2015 and 2023 are presented for illustration only and are not included in the mortality impact analyses because of missing data.The last digit of each year (5-9, 0-3) distinguishes the year.

Figure 3 .
Figure 3. Hourly mean PM2.5 concentrations at (a) Monash, (b) Florey, (c) Civic by hour of day, the transition months (April, May, or September) and non-wood heating season (January-March and October-December).Means for the non-wood heating season are omitted for 2019 and because of the impact of the Black Summer Bushfires.Years 2015 and 2023 are presented for illustration only and are not included in the mortality impact analyses because of missing data.The last digit of each year (5-9, 0-3) distinguishes the year.
hour of day at Monash, April vs Jan-Mar+Oct-

Table 2 .
Deaths attributable to wood heater pollution in the Australian Capital Territory, by year and exposure-risk function*

Table 5 .
Equivalent cost of mortality attributable to wood heater pollution in the Australian Capital Territory, by year and exposure-risk function** Assumes that 90% of the difference between the annual mean concentration and mean concentration for periods outside wood heater season is attributable to wood heater emissions.