A new study has revealed a significant shift in the sources of nitrogen oxides pollution in central London, with gas boilers used for heating now identified as the primary contributor.

The research found that the combustion of natural gas in boilers accounted for 72% of nitrogen oxides emissions within a few kilometres radius of London’s BT Tower between 2021 and 2023. 

The findings highlight the success of transport interventions, such as the London Low Emission Zone and the Ultra Low Emission Zone, in significantly reducing nitrogen oxides emissions from vehicles.

Nitrogen oxides emissions – the name for a group of different nitrogen oxides – from vehicles in central London are estimated to have fallen by about 73% between 2016 and 2025. In addition, the increasing prevalence of Euro 6 vehicles and electric cars has also contributed to this decline.

Nitrogen dioxide in outdoor air is associated with adverse effects on our health, including reduced life expectancy.

The study is published in the journal Environmental Science & Technology, was led by Professor James Lee, at the National Centre for Atmospheric Science and University of York. 

He said:  “Our research highlights how much nitrogen oxides emissions from transport have improved, but also the task still ahead in getting combustion out of cities. Whilst talk of switching from gas to heat pumps is mainly framed around the decarbonisation benefits, it would be a major gain for air quality too.”

Dr Sam Cliff, who carried out the research while completing their PhD at York, said: “Without action on gas boilers it could be hard for central London to meet the World Health Organisation 2021 guideline values for nitrogen oxides, no matter how clean the vehicle fleet.”

The study pinpointed the source of pollution by analysing the ratio of nitrogen oxides to carbon dioxide which differs between traffic and boilers, using measurements taken from the BT Tower.

The BT Tower Observatory provides the only long-term measurement of nitrogen oxide emissions from a megacity in the world. It is operated by the National Centre for Atmospheric Science.

At 190 metres tall, the observatory filters and analyses air as it moves upwards from the streets and buildings below. A gas inlet mast on top of the tower pumps air into the 35th floor laboratory, collecting pollution from the nearby region which is dominated by roads and commercial residential buildings, but also includes some urban parkland.

The researchers say while the decarbonisation benefits of switching from gas to heat pumps are widely discussed, the study emphasises the significant air quality improvements this transition could also deliver.

The researchers noted that industrial and non-domestic boilers, which are larger than those in private homes, are the main contributors in central London, which is not a highly residential area. 

While lower emission boilers have been introduced, Professor James Lee believes that a transition to heat pumps is likely the most effective long-term solution, although he acknowledges the current cost challenges associated with widespread adoption.