Indoor Air Pollution from Residential Stoves: Examining the Flooding of Particulate Matter into Homes during Real-World Use Rohit Chakraborty, James Heydon, Martin Mayfield and Lyudmila Mihaylova

This is a brief summary of the research paper published here. The article is open access - https://doi.org/10.3390/atmos11121326

Summary

This study concerns the levels of particulate matter (PM2.5 and PM1) released by residential stoves inside the home during ‘real world’ use. Focusing on stoves that were certified by the UK’s Department of Environment, Food, and Rural Affairs (DEFRA), PM sensors were placed in the vicinity of 20 different stoves over four weeks, recording 260 uses. The participants completed a research diary in order to provide information on time lit, amount and type of fuel used, and duration of use, among other details.Multivariate statistical tools were used in order to analyse indoor PM concentrations, averages, intensities, and their relationship to aspects of stove management.

The study has four core findings. First, the daily average indoor PM concentrations when a stove was used were higher for PM2.5 by 196.23% and PM1 by 227.80% than those of the non-use control group. Second, hourly peak averages are higher for PM2.5 by 123.91% and for PM1 by 133.09% than daily averages, showing that PM is ‘flooding’ into indoor areas through normal use. Third, the peaks that are derived from these ’flooding’ incidents are associated with the number of fuel pieces used and length of the burn period. This points to the opening of the stove door as a primary mechanism for introducing PM into the home. Finally, it demonstrates that the indoor air pollution being witnessed is not originating from outside the home.

Taken together, the study demonstrates that people inside homes with a residential stove are at risk of exposure to high intensities of PM2.5 and PM1 within ashort period of time through normal use. It is recommended that this risk be reflected in the testing and regulation of residential stoves.

Key Findings

Wood burners increase average levels of indoor particulate matter

The findings indicate that average indoor PM2.5 (mean = 12.21 µg/m3 and PM1 (mean = 8.34 µg/m3) levels are about three times higher for PM2.5 and about four times higher for PM1 during the period in which stoves are lit compared to the period in which they are not lit (PM2.5 levels: mean = 4.12 µg/m3, and PM1 levels: mean = 2.54 µg/m3). The average duration of use was approximately 4 hours, with most households using their stove between 6 pm and 10 pm.

For reference, the World Health Organisation limit for exposure to PM2.5 over a 24 hour period is mean = 25 µg/m3. As of yet, there have been no safe levels determined for PM1.

In terms of understanding your exposure, we recommend thinking about the increased average exposure seen during stove use in the context of other exposures during your day. For instance, from cooking, sitting in traffic, or going walking on a day where outdoor air pollution levels are high. They all contribute.

Wood burners cause spikes of particulate matter indoors

The findings indicate that the ‘flooding’ of indoor space with particulate matter occurs as a result of the stove door being opened for refuelling.

The peak hourly concentrations are often higher by a minimum of 250% and a maximum of 400% when participants have refuelled their stove more than once during a usage compared to one refuel or none at all. This means some users are experiencing exposure to very high short term 'bursts' of particulate matter. This is in line with the findings from other studies into stove use in other parts of the world.

When looking across all participants, the hourly peak mean PM2.5 (27.34 µg/m3) and PM1 (19.44 µg/m3) is significantly higher than the daily mean PM2.5 (12.21 µg/m3) and PM1 (8.34 µg/m3).

For reference, the World Health Organisation limit for exposure to PM2.5 over a 24 hour period is mean = 25 µg/m3. As of yet, there have been no safe levels determined for PM1. Again, we recommend taking your exposure in the context of your whole day.

For those wondering if this particulate matter is leaking from their stove, a ’leakage’ would result in a more uniform shape to the rug plots seen in the published paper (Figure 10). This echoes the findings from other studies and provides further support for the theory of opening doors being the cause of the indoor air pollution seen rather than a leakage. Indeed, the literature suggests that leaking is more common to open fires than ‘closed’ stoves.

Overall exposure varies but there is a baseline of risk

There exists high variation in exposure concentrations, concerning both short peaks and averages over the period of use. This characteristic is related to the "real-world" nature of the study and the different ways in which people use their stoves.

On average, participants used 9.58 pieces of solid fuel and 8.32 pieces of kindling per use. The number of fuel pieces used in a single use varied between a minimum of 7 to a maximum of 40, while kindling varied between a minimum of 1 and a maximum of 32. All participants used dried and seasoned logs, but the sizes and type of wood varied. There was also a diversity of kindling used, taking the form of firelighters, newspapers, balls of paper, twigs, sawdust, packing cardboard, greeting cards and empty egg boxes.

Echoing the findings of existing studies, this means that the same wood burner may emit different levels of air pollution depending on the quantity and type of fuel and kindling used. However, longer usage is associated with greater numbers of fuel pieces used. Therefore, instead of fuel type, the results support the stove door explanation for the ’flooding’ phenomenon observed - higher short-term peak concentrations are seen during longer periods of use because these periods are sustained by more refueling actions. This accords with existing studies that also found the lighting and refueling aspects of stove management form the main indoor pollutant-generating phases of operation.

While it appears to be less relevant to indoor air pollution, other studies show that the type of fuel used does influence the levels of PM2.5 and PM1 emitted outdoors.

Key Limitations

The study cannot tell us about the impact of stoves on outdoor air quality

The placement of the outdoor monitors means that we could only take general readings of ambient air quality and not determine the levels of PM coming from the chimney. This has allowed us to comment on whether the indor peaks/averages seen indoors are coming from outdoors (they are not), but have not allowed us to say anything about the outdoor PM coming from each stove. For this, a sensor would need to be placed in the flue or the stack itself and so this is recommended for future studies. Similarly, the influence of sensor data on participant stove management practice has not been explored in detail. This will be drawn out more fully in a separate paper.

Recommendations

On the basis of the above results, it is recommended that DEFRA testing standards be modified in order to account for the normative exposure risks posed by the stoves. The PM that is released into the home is not an aberration from normal use, but results directly from it. This is because real-world operation cannot occur without opening the stove door. It may be that with regulatory encouragement stove designs can be modified in a way that limits such instances. In the meantime, or in the event that appropriate modification cannot be achieved, it is also recommended that new residential stoves be accompanied by a health warning at the point of sale in order to indicate the normative exposure risks posed to users. At the least, this should alert people to the need to minimise the time the door is opened.

Considered in the context of World Health Organisation 24h averages, some may see this as an over-reaction. However, many people operate log burners with the door open, they do not know about the need to minimise the time in which it is open during relighting or refuelling, and many have young, old or vulnerable people in the home who may be particularly susceptible to the higher short term intensities witnessed during operation. Furthermore, taken across a 24 hour period, use of a burner in an evening may push a person's exposure rate over the recommended limit. Recall also that PM1 does not yet have a limit for which it is considered 'safe'. Finally, if technological modifications can be encouraged in order to minimise 'flooding' events, or the time in which the doors can be opened, then it follows that a recommendation should be made on this basis.

Questions?

If you have any questions about the above quantitative data please contact Rohit Chakraborty at Rohit.Chakraborty@Sheffield.ac.uk.