Air quality: key behaviours report

7 Heating the home differently

7.1 Evidence on air quality impacts of home heating behaviour

The literature review highlighted impacts from home heating behaviour, specifically in relation to burning of solid fuels. There was a lack of literature on other types of domestic heating fuel in relation to air quality. Domestic burning of solid fuels is a significant source of air pollutant emissions. Burning solid fuels, which can include wood (biomass) burning as well as solid mineral fuel like coal, releases a range of pollutants in the form of PM, including ultrafine particles (with a diameter of ≤100 nm), fine particles (PM2.5), and larger particles (PM2.5-10), as well as gaseous pollutants like carbon monoxide (CO), and NOx (Hector et al., 2022; Font & Fuller, 2017). Domestic solid fuel burning has impacts on both outdoor and indoor air quality (Wood et al., 2023).

In Scotland in 2019, residential and other combustion sources (which exclude combustion in industry, energy production and transport) accounted for approximately 38% of the total emissions of PM2.5, 22% of PM10 emissions and 14% of the total NOx emissions (Hector et al., 2022). This makes residential and other burning the greatest source of PM2.5 and the second greatest source of PM10 (after industry) and NOx (after transport). It is not, however, clear from these data how much of these emissions are associated with solid fuel burning versus more prevalent types of home heating fuel like gas and oil. Residential emissions of CO have been on the increase since 2005, in line with increased wood burning at home, despite the otherwise downward trend in CO emissions (Hector et al., 2022). The major contribution of domestic fuel burning to PM2.5 in Scotland is a particular concern, as finer particles pose greater health risks than larger particles (Cowie et al., 2015). As with all emissions sources, the spatial distribution of emissions will affect the overall impact on air quality, and so geographical differences in solid fuel burning should also be taken into account. Although only around 1% of homes report using solid mineral fuel as their primary fuel type (Scottish Government, 2023), 79% of these households are located in rural areas (Scottish Government, 2021). Similarly, although the use of biomass as a primary fuel source is low (<1%) overall in Scotland, reliance on biomass is highest in rural areas, where the majority of homes are located off the gas grid (Scottish Government, 2023). Research examining the air quality and health impacts of domestic biomass burning in Scotland is currently underway as part of the Scottish Government's Strategic Research Programme^[6].

The international literature highlights that the use of solid fuels such as wood for cooking and heating is a principal source of air pollution at the global level (Chowdhury et al., 2023; Lelieveld et al., 2015). It is reported that for every 1 million households using solid fuels, emissions equivalent to 2.3 million diesel trucks are released (Chowdhury et al., 2023). Household burning overall has been estimated to contribute to 19-31% (depending on the study) of ambient PM2.5 globally, and at the European level household burning may be responsible for more than half of overall PM2.5 emissions (Chowdhury et al., 2023; Wood et al., 2023).

Domestic wood burning has increased in the UK in recent years, a trend that is reflected elsewhere in Europe (Chowdhury et al., 2023; DEFRA, 2023). Emissions of PM2.5 from wood burning at home increased by 124% between 2011 and 2021, and wood burning has now by far overtaken coal burning as a source of PM2.5, with wood burning accounting for three-quarters of PM2.5 emissions from domestic combustion (DEFRA, 2023). Heydon (2023) reports that of the 8% of UK households which use wood burning stoves, 95% have access to other sources of heat. Increases in wood burning in urban areas, where people are less likely to be reliant on wood as a primary heating source, is a particular concern as it is there that emissions from wood burning have the greatest potential to impact on ambient air quality and human health (Font & Fuller, 2017). Wood burning is estimated to be responsible for 23-31% of the urban-derived PM2.5 in London and Birmingham, and it is thought likely that the case will be similar for other cities (Font & Fuller, 2017). Domestic wood burning also varies in relation to seasonal and diurnal rhythms of heating. It primarily happens in winter and is greatest in the evening when people are more likely to be home. However wood burning behaviour correlates quite poorly with daily temperature, suggesting that a large part of wood burning might be related to taste and aesthetics rather than reliance on wood for heat (Font & Fuller, 2017). For example, Kantar (2020) found that wood burning fireplace is a broadly associated with homely and cozy feeling in rural households.

The air quality impacts of solid fuel burning can be reduced through using properly installed and maintained stoves rather than open fires, and the use of modern, more efficient stoves since stoves sold in the UK now have to conform to strict emissions standards (Burki, 2018; Kantar, 2020). The use of properly dried and seasoned wood with a water content of 15-20% (compared to up to 50% in freshly cut firewood), or smokeless coal rather than standard house coal, can also reduce the smoke and therefore pollutant emissions produced from burning (Burki, 2018; Kantar, 2020). In a survey of UK householders who burn solid fuels at home, half of those who burned wood said that they bought their wood pre-dried or seasoned, and a quarter seasoned it themselves (Kantar, 2020). Of those who burned wood, 20% used wood classed as wet (either seasoned for less than 12 months or not at all) (Kantar, 2020). The survey also found that 31% of respondents burning solid fuels at home used an open fire, 26% used stoves installed prior to 2010, and 9% did not know how old their stove was (Kantar, 2020). Two-thirds of respondents with an appliance installed before 2000 said they were not at all likely to replace their appliance, and a further 21% said it was fairly unlikely (Kantar, 2020).

In light of the evidence reviewed, we have identified two key behaviours relating to home heating for prioritisation:

Heating the home differently

Burning less at home

Ensuring good practice when burning solid fuel (including use of efficient appliances)

7.2 Factors influencing home heating behaviour

C-O-M factors influencing key home heating behaviours (burning less; ensuring good practice when burning) identified from the evidence review are presented below in Table 9 and Table 10.

Table 9: Capability, Opportunity and Motivation factors influencing burning solid fuels at home

Burning less at home

Capability

Lack of knowledge of impacts

Indoor wood burning significantly increases health risks for individuals. However, the absence of direct sensory experience diminishes people's awareness. In turn, this reduces their motivation to shift away from solid fuel burning (Heydon & Chakraborty, 2022). Even when indoor air quality monitors are provided the information they convey is not well understood (Heydon & Chakraborty, 2022).

Opportunity

Access to cleaner energy

To reduce reliance on solid fuels, access to different types of cleaner energy source is necessary. There are some differences between urban and rural areas with respect to the variety of different options available. The majority of rural areas in Scotland lack access to a natural gas network (National Gas, 2023) and there are fewer opportunities for district heating schemes in areas where there is a lower heat demand density (Element Energy, 2020). However, other options for low carbon heating such as heat pumps and electric heating exist in off-gas-grid rural areas. The suitability of different types of low carbon heat technology will also vary depending on the specific characteristics of individual properties (Element Energy, 2020).

Policy and regulatory environment

The existing legislation concerning domestic combustion emissions is rooted in the Clean Air Act of 1993, which is being reviewed (Scottish Government, 2020). The recent passage of the Heat Network Bill by the Scottish Parliament, along with the ongoing development of large-scale heat infrastructure (Scottish Government, 2022), presents a favourable opportunity for households currently utilising solid fuels for heating to consider changing their heating method. At the time of writing, proposals on a heat in buildings bill, addressing clean heating in existing buildings, are in development.

Motivation

Cost of switching energy source

Costs of switching to a cleaner energy source may be a barrier to reducing reliance on solid fuel. Direct electric heating can be costly to operate, and installation of low-carbon heat technology not already present in the home has upfront cost implications (Element Energy, 2020).

Environmental motivations

Despite its contribution to high levels of air pollution, wood burning is often regarded as a carbon-neutral method of energy acquisition (Acton et al., 2023; Clean Heat, 2016).

Comfort and aesthetics

The primary motivations individuals cite for indoor burning are often to create a cozy, homely atmosphere, and in some cases, purely for aesthetic reasons (Kantar, 2020).

Table 10: Capability, Opportunity and Motivation factors influencing adoption of good practices when burning solid fuel

Ensure good practices when burning solid fuel (including use of efficient appliances)

Capability

Knowledge

To mitigate particulate emissions, greater emphasis should be placed on optimizing wood log burning conditions (Fachinger et al., 2017). This requires knowledge of good practice by users. There is a general lack of education on the right burning appliance and good burning techniques (fuel, installation/maintenance of devices) (Scottish Government, 2020). Burning practices are rarely questioned, and there is a lack of knowledge regarding the specifics of which type of fuel to burn and the necessary seasoning requirements. These factors can vary significantly, e.g., individuals often assume that the wood they purchase will already be properly seasoned (Kantar, 2020).

Motivation

Costs

While there is not a quantified cost associated with upgrading outdated home burning appliances (as it greatly depends on the local market), it is evident that households will incur additional financial costs to enhance or replace outdated heating equipment (Kantar, 2020). Switching to less polluting solid fuels may also add additional costs for households (Masey et al., 2023).

Effort

Good practice includes ensuring wood is thoroughly dried and cutting it into smaller pieces before burning (Fachinger et al., 2017) which may require extra effort and inconvenience.