A report released today, prepared by Emission Impossible Ltd in conjunction with Resource Economics, examines the health impacts of combustion appliances – adding to a growing body of research in New Zealand on household energy use. 

The independent study looks at gas stoves, unflued gas heaters, and wood burners of all types. Commissioned by EECA (the Energy Efficiency and Conservation Authority), the study models fine particulate (PM2.5) and nitrogen dioxide (NO2) emitted indoors from using different types of appliances for heating and cooking.  

“Open fires and gas appliances (both gas stoves and unflued gas heaters) are identified as resulting in the highest indoor air pollution costs per household,” said EECA Lead Advisor Efficient Homes, Dr Gareth Gretton.   

“This is in contrast to the results for outdoor air pollution, established in previous work, which shows significant impacts from open fires and wood burners, but no impact from gas burning appliances.”  

The report does not consider flued gas space heaters, which vent all combustion products to the outside. Similarly, outdoor gas water heater califonts and gas barbeques were out of scope. 

Indoor air pollution impacts identified in the report are those that occur for the household with the combustion appliance. These are additional and distinct to the impacts of outdoor air pollution which also impacts air quality in your home. 

“The clearest example of the distinction between indoor and outdoor air pollution is that from wood burners,” said Gretton.  

“Most of the particulate emissions from any wood burner will go up the flue. These contribute to outdoor air pollution, and some of this will enter homes through ventilation. The additional indoor air pollution identified in the present study occurs when lighting the fire, or refuelling, which is when particulate emissions enter the house directly.” 

The report puts estimated national annual cost to the healthcare system from indoor air pollution at $102 million*. Wider social costs are modelled at $5.6 billion (social costs include the statistical value of a life). The impacts in the report are expressed using Treasury’s standard measures [the Value of a Statistical Life (VOSL) and the Value of a Life Year (VOLY)] – in 2025, the central VOSL figure is set at $15 million**. 

“Health impacts from indoor air pollution connected to different appliances will now be included in EECA’s cost benefit analysis – alongside financial and environmental outcomes,” said Gretton.  

Indoor air quality is an emerging area of interest globally. “This data set is the first to quantify the health costs from the release of indoor combustion products in New Zealand and so we really welcome feedback and discussion from across the wider industry,” said Gretton.   

“The study is best understood as part of wider analysis which considers all the impacts of different appliance choices.”  

The results are based on real-world testing in New Zealand homes, supplemented by and benchmarked against international evidence. The study has been peer reviewed from health and economic perspectives, and a further international peer review from an indoor air quality scientist is being scoped.  

Read the full report [PDF 1.2 MB]

Additional information  

* $102 million of healthcare cost is calculated from the per-appliance fiscal costs in Table 12, and the number of appliances which are given in Tables ES-3 and 16. Costs are driven by respiratory and cardiovascular hospitalisations, and childhood asthma prevalence and hospitalisations, and are given in Table 7.  

** These figures, which originated from studies on what people are willing to pay to reduce road fatalities, are now widely applied across government.     

Assumptions – for a full list please review report methodology.  

  • This data inherently includes ventilation, window use, and extractor fan use – in line with the real-world efficacy of these mitigations.  
  • The methodology includes seasonal variation to this mitigation by assuming lower NO2 exposure in summer, due to higher window use and resulting ventilation. In the central scenario, summertime exposure is 25% of winter.
  • The level of NO2 exposure was estimated using New Zealand evidence, benchmarked against more recent international evidence. The annual exposure level used in this study is in line, or lower, than estimated in these recent international studies - which reflect the current mix of recent appliances.
  • The methodology does not factor in “flooding events” in which exposure briefly reaches a very high level immediately after cooking, instead focusing entirely on the long-term low increase in exposure from the regular use of gas stoves. 
  • The report assumes a very low number of houses use open fires. EECA’s omni survey in 2025 found that 7% of respondents owned an open fire which they had used in the past 12 months. This would equate to 129,000 open fires – well above the 1973 used for the harm estimates in this paper.  
  • The report assumes all fixed gas heaters are flued. In the 2023 census, 150,000 people indicated they had a fixed gas space heater.