Vans have become the biggest road source of a deadly air pollutant in central London as black cabs have become more environmentally friendly.

Nitrogen oxides (NOx) are a group of toxic gases linked to stunted lung growth in children and increased risk of asthma attacks. Nationally, they are mostly produced by diesel vehicles.

Six years ago, taxis were the biggest source of NOx emissions from road transport in central London, at 28.4 per cent. Diesel vans amounted to 21.7 per cent.

However, the move to plug-in hybrid electric black cabs and other fully electric cabs, and the growth in package deliveries for online orders, have resulted in taxis and vans swapping places.

By 2022 — the latest year of data available — vans accounted for 30.2 per cent of NOx emissions in the capital’s centre, and taxis 21.7 per cent. The rest of the emissions came from, in order, cars, buses and coaches not run by Transport for London (TfL), TfL buses and heavy goods vehicles.

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Vans are also forecast to be the biggest source of NOx from road transport in 2030, according to Clean Cities.

The campaign group analysed the most recent annual figures from the London Atmospheric Emissions Inventory, considered the most authoritative database tracking the city’s air pollution.

The spotlight on the negative side effects of diesel vans mirrors national concerns over a sluggish switchover to electric vans. Electric car sales have increased in line with government targets, to 21.3 per cent of new car sales this year.

However, the market share of new electric vans stalled last year, remaining at 6.3 per cent. The Climate Change Committee, which advises the government, recently expressed concern that the slow transition could endanger the country’s carbon targets.

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The Clean Cities campaign is urging Sir Sadiq Khan, the mayor of London, to rethink TfL’s plans to reduce congestion charge exemptions for electric vans next year, which would result in a £9 daily cost.

Joined by almost 100 charities and small businesses, the group wrote: “Some [van owners] may even have to shift back to diesel to make ends meet. This financial strain will also discourage the future uptake of electric vans, undermining your health and environmental targets.”

The Ultra Low Emissions Zone was expanded London-wide two years ago last week. A report found that the measure had cut air pollution most steeply in the outer boroughs, where some local authorities had fought its introduction.

Another type of air pollution, ozone driven by methane emissions from energy, waste and farming, is causing about 1,500 lung-related deaths each year, researchers have estimated.

Sunny and still conditions exacerbate the formation of ground-level ozone, and the government issued ozone pollution alerts on August 12 and 13. The Green Alliance think tank said the government should deliver an ambitious action plan for methane emissions to curb the formation of ozone.

The Mayor of London’s office was contacted for comment.