Gas boilers have overtaken road traffic as the main contributor to nitrogen oxide (NOx) emissions in London, according to a new study led by Professor James Lee of the University of York.
The research, published in Environmental Science and Technology, shows that natural gas combustion for heating was responsible for approximately 72% of central London’s NOx emissions between 2021 and 2023. The findings are based on long-term pollution measurements and indicate a major shift in urban air pollution sources, as road transport emissions continue to decline due to improved vehicle exhaust technology.
While a 73% reduction in road NOx emissions is projected in London between 2016 and 2025, air quality remains a concern, with levels of nitrogen dioxide (NO₂), ozone (O₃), and particulate matter (PM2.5) still high. The study highlights the role of gas boilers in this trend and points to electrification – via technologies like heat pumps – as the most effective strategy for cutting both greenhouse gases and harmful air pollutants at a local level.
The authors also warn against over-reliance on low-carbon combustion fuels such as hydrogen and biogas. The study estimates that choosing hydrogen over full electrification could lead to annual environmental damage costs of up to £600 million in the UK, due to continued NOx emissions. It finds that electrification would yield far greater reductions in urban NO₂ and PM2.5 concentrations than alternative low-carbon fuel options.
Currently, there is no specific legislation in place to decarbonise the UK’s heating sector, although a ban on installing gas boilers in new homes is expected in the near future. The study notes that key policy decisions around the use of hydrogen and biogas have yet to be finalised. By contrast, in California and the San Francisco Bay Area, legislation has already been passed requiring all new boilers to produce zero NOx emissions from 2027 – a move expected to save up to $530 million a year in avoided damage costs.
Martin Lewerth, CEO of Aira Group, a heat pump manufacturer, has called on the UK government to remove bureaucratic hurdles to cleaner heating alternatives: “Despite being one of the most congested cities in the world, the single biggest source of air pollution in central London is no longer traffic — it’s gas boilers. This research makes it clear: we need to move away from gas boilers in favour of clean heat solutions, such as heat pumps, to improve air quality.
“While schemes like the Boiler Upgrade Scheme are a step in the right direction, we need the government to act decisively and rebalance the cost of electricity relative to gas, provide implementation dates for heat pump planning permission changes and revisit the gas boiler ban to accelerate the clean energy transition. These changes need to be handled with care, but failure to progress only causes more harm to people’s health, environment and finances. Now is the time for change.”
