Evaluation of the effects of PM2.5 due to domestic solid fuel burning on health outcomes in Scotland

6 Air quality impacts of policy options

6.1 Modelling assumptions

For each shortlisted option the anticipated change in annual average PM_2.5 concentrations, mapped across Scotland, has been quantified using the data and modelling developed through the UKCEH (2025). A set of behavioural response modelling assumptions has been developed for each option, drawing on the results of the literature review and the expert judgement of the team, which has been agreed with the Steering Group.

As discussed in UKCEH (2025), the new emission factors from the EFDSF study included some unexpected results that affect the model implementation and may point to some uncertainties that need to be considered when interpreting the results. In particular, (i) for most mineral fuels modern stoves (reflecting Ecodesign) showed emissions that were no lower than those from the medium age stove tested, (ii) emissions for low-sulphur (smokeless) MSF were not much lower than for high-sulphur MSF (but anthracite, another smokeless fuel, burnt much cleaner) and (iii) SO₂ emissions from MSF (both low and high-sulphur) were higher than those for house coal. These results have large impacts on our estimates of the potential impacts of fuel swapping and stock improvements. It is possible that the fuel types and stove models chosen for the EFDSF study were unrepresentative for their category. For example, the medium age stove may already have met Ecodesign emission standards even though that standard did not exist at the time.

Policy scenario 1 the impacts were assumed to be modest, but widespread, and therefore a uniform 1% reduction of solid fuel burning across all fuels and areas in Scotland was applied. The population-weighted mean concentration contribution from Scottish solid fuel burning was thus reduced by 1% to represent this scenario. This assumption is based on similar assumptions regarding the impact of information campaigns made in: the Scottish Government’s ‘Impacts of the sale of house coal and the most polluting manufactured solid fuels: report’^[35] and Defra’s Impact Assessment for the ‘Proposed regulation of the sales, distribution and marketing of: Wet wood (>20% moisture) sold in units up to 2m³, Bituminous house coal and Banning manufactured solid fuels with sulphur content over 2%’^[36].

Policy scenario 2 was assumed to result in a shift of solid fuel burning away from the most polluting fuels, without changing the energy content of fuel used. In particular, for the three fuel types considered:

• Wet wood was replaced with seasoned wood;
• All manufactured solid fuels (MSF) were changed to have the emissions of smokeless (low sulphur) MSF; and
• All house coal, peat and lignite burning was replaced with smokeless MSF. This was calculated as an adjustment to the coal contribution, in order to retain the underlying spatial distribution of previous coal burning.

Here the EFDSF emission factors for low-sulphur fuels were used, which, unlike the UKCEH (2025) baseline, does not differentiate between low and high sulphur MSF and applies the average emission factor to any MSF. The population-weighted mean concentration contribution of primary PM_2.5 from each fuel type was scaled by the change in emitted PM_2.5, while the corresponding contribution of secondary PM_2.5 was scaled by the change in emitted SO₂. The scaling factors are given in Appendix Table 9‑9. Scaling factors were sufficiently similar for rural, SCA and non-SCA urban areas to warrant this simplified approach.

Policy scenario 3 was assumed to result in the upgrade of all open fireplaces and ‘old’ stoves to ‘modern’ stoves (at least Eco-Design standard) within the six targeted Local Authorities. The proportion of ‘old’ stoves in the baseline stove population was 32%, while around 1/3 of coal burning and 1/5 of wood burning is associated with open fireplaces. For wood, the emission factors generally reduce with the appliance upgrades, with slightly different scaling in smoke control areas, urban non-smoke control and rural areas. For coal, primary PM_2.5 and Volatile Organic Compound (VOC) emissions reduce, but NO_X and SO₂ emissions increase. The scaling factors are given in Appendix Table 9‑10 and the changes in emissions relative to baseline are mapped in Figure 9‑6. For MSF, the emissions were left unchanged, as the emission factors for MSF for modern stoves would have been larger than those for older stoves. This is one of the outcomes of the EFDSF study that is deemed particularly uncertain. A new gridded emission file for EMEP was created, applying these changes within the targeted Local Authority areas, and the EMEP model re-run to calculate the new concentration distribution.

6.2 Impacts on air quality

The PWMC of PM_2.5 associated with the three policy scenarios are shown in Table 6‑1. Corresponding values for primary and secondary PM_2.5 separately are given in Table 9‑11 and Table 9‑12.

* Coal category in Policy 2 includes impacts of existing anthracite burning and previous coal, peat and lignite burners switching to smokeless MSF. Overall, this reduces primary PM_2.5 emissions but increases SO₂ emissions and thus secondary PM_2.5 concentrations.

The magnitude of change is fairly small for all scenarios, but the largest change is found for Policy 2 (Burn better - fuel quality improvements), driven by reductions from wood and MSF. Appendix Table 9‑13 presents a wider range of statistics of the concentration distribution across Scotland for the baseline, Policies 2 and 3. This shows that Policy 2 also reduces the maximum PM_2.5 concentration contribution from solid fuel substantially, from 5.76 to 4.43 µg m^‑3. The primary PM_2.5 emissions and SO₂ emissions from MSF are reduced by this scenario, but SO₂ emissions from wood are slightly increased, and SO₂ emissions are substantially increased for former coal burners, reflecting the relatively high emissions of SO₂ even from the ‘low sulphur’ MSF investigated in the EFDSF study. A greater magnitude of change for Policy 2 could be achieved if the change from coal to MSF maintained or reduced both primary PM_2.5 and gaseous (SO₂) emissions, such that the secondary PM_2.5 contribution of former coal burners is not increased.

The magnitude of changes for Policy 3 are limited by the initial proportions of old stoves and open fireplaces, along with the increase in gaseous emissions and thus secondary PM_2.5 from coal burning. Policy 3 has minimal effects on all PM_2.5 concentration statistics at national scale.

Maps of the PM_2.5 concentrations throughout Scotland due to Policies 2 and 3 are shown in Figure 9‑7, with the corresponding concentration changes in Figure 9‑8 and detailed maps of southern Scotland in Figure 9‑9 for policy 2 and Figure 9‑10 for Policy 3. Primary PM_2.5 concentrations due to solid fuel burning under Policy 2 show reductions both in peak and lower concentration areas across Scotland. The largest concentration changes from Policy 2, reductions of up to 1.3 µg m^‑3 are predicted to occur in a range of smaller urban areas, including Bathgate, Cowdenbeath, Fort William, Ayr and Dumfries. The PM_2.5 concentration changes from policy 3 are predicted to be much smaller in magnitude than from policy 2, with reductions up to 0.14 µg m^‑3 in North and South Lanarkshire and Fife (Dunfermline and Kirkcaldy). The concentration changes from policy 3 are primarily located within the six targeted local authorities, but small reductions are also found in neighbouring local authorities including East Dunbartonshire, West Lothian, Stirling and Clackmannanshire. This emphasises the transboundary aspect of PM_2.5 pollution, especially for secondary PM_2.5, which is formed in the atmosphere over moderate timescales after emission (hours – days). Bigger impacts within the targeted local authorities might be achieved by extending interventions to include predominantly ‘upwind’ Local Authorities, for example Renfrewshire, East Renfrewshire, North Ayrshire and East Ayrshire for the City of Glasgow.