Scottish Air Quality Advisory Group advice note 2, March 2026

Could there be unintended consequences for air quality in implementing any of these policies?

The impact of vehicle electrification in reducing PM₂.₅ concentrations will be lessened if there is not an associated reduction in overall vehicle kilometres and numbers of journeys undertaken. Although electrification will address tailpipe emissions, and regenerative braking is likely to counteract the effect of heavier vehicles increasing brake wear emissions, there will still be a substantial contribution from tyre wear and resuspension of road dust. The harms arising from these particles remain to be established.

Depending on the energy source for electric vehicles, NOx emissions may be at least partially displaced rather than eliminated – although this would reduce population exposure to direct emissions and secondary chemical products. As NOx emissions decrease in urban areas, there will be an increase in ozone concentrations in these areas which will need to be addressed.

In the case of hydrogen, whether the source is grey, blue or green has different implications for air quality, as does whether it is combusted or utilised in fuel cells. Combustion in domestic boilers, for example, has the potential to increase NOx emissions in comparison with the use of natural gas. Much of the future trajectory of emissions will depend on the emissions standards developed for these new replacement technologies.

For domestic fuel combustion, the whole life cycle should be taken into account when considering whether wood is a renewable fuel source. A further point in this context is that aesthetic use of woodburning stoves will not be reduced simply by alternative heating sources, without proactive public awareness campaigns to drive action and to communicate both the benefits of low or zero emission heating and the health and environmental harms associated with the practice. This is compounded by anecdotal evidence suggesting that stoves are used as backup for heat pumps and the fact that stoves are still permitted as a primary heating source in new domestic buildings.

Energy from waste has the potential to generate air pollutant emissions similar to any other type of combustion unless these are abated at source.

Widespread uptake of ammonia as a fuel, along with using it as a medium for transporting hydrogen and as a coolant for larger scale heat pump systems, increases the potential for leakages in the supply chain (shipping, pipelines and as slip during combustion or cracking) with possible negative consequences for sensitive habitats and designated sites which are already under nitrogen stress, and for formation of secondary PM₂.₅. Additionally, flaring of ammonia could result in emissions of several toxic nitrogen compounds.

Introduction of sustainable aviation fuels (SAFs) is likely to have a negligible impact on NOx emissions, but initial evidence suggests they may lead to lower black carbon (although additives based on aromatic compounds would reduce the benefit) and ultrafine particle emissions depending on the feedstock. There is at present much uncertainty around the volatile organic compounds (VOCs) impacts of SAFs. Any performance additives could also negatively impact on emissions, depending on type. Environmental impact associated with SAF production remain to be explored.

With carbon capture and storage (CCS), the use of amines as an integral part of the process may contribute to particle nucleation and UFP formation and to formation of toxic nitramines and nitrosamines and constitute a significant additional contribution of nitrogen deposition, at least locally.

A common theme relevant to all of the above is the need to engage early with industry and regulators around emerging technologies and to develop emission standards before these technologies are rolled out at scale.

Finally, policy changes could potentially lead to spatial changes in pollutant dispersal patterns, which will need to be considered in any policy scenario assessments.