Scientists say the fumes may damage airways, leading to inflammation and the development of asthma in children who are genetically predisposed to it. In the first study of its kind, they estimate that four million cases of childhood asthma worldwide are attributable to traffic pollution each year, accounting for 13 per cent of all cases. This includes 19 per cent of diagnoses in the UK, rising to 29 per cent in London and 23 per cent in Manchester. Of 194 countries analysed, the UK ranked 24th for the highest proportion of pollution-related asthma cases, above the US (25th) and India (58th). Although levels of other pollutants in India are among the highest in the world, NO2 appears to be lower than in many other countries.

South Korea was the worst for NO2 exposure, with 31 per cent of childhood asthma cases linked to it. It was followed by Kuwait, Qatar and the United Arab Emirates, all on 30 per cent.

Nine in 10 pollution-related asthma cases occurred in areas where average NO2 levels were below the World Health Organisation’s safe guideline limit.

And researchers said that the true scale of the problem may be far greater as the condition often goes undiagnosed in poorer countries.

Study author Dr Susan Anenberg, of George Washington University in the US, said: “Nitrogen dioxide pollution appears to be a substantial risk factor for childhood asthma incidence in both developed and developing countries, especially in urban areas.”

Dr Anenberg added: “Our findings suggest that the World Health Organisation guideline for NO2 concentrations might need to be revisited, and that traffic emissions should be a target to mitigate exposure.”

Lead author Ploy Achakulwisut, also at George Washington University, said strict policies to reduce emissions could help improve children’s health.

The researchers cited London’s introduction of a new ultra-low emission zone and the Chinese city of Shenzhen’s electrification of its entire bus fleet as examples of urban authorities taking action.

Trigger Commenting on their findings, British expert Professor Stephen Holgate, of the University of Southampton, said: “It has long been known that outdoor air pollution can trigger worsening asthma, but only recently has it been linked to the causation of new asthma.

“This startling discovery mandates that NO2 as a toxic pollutant is taken more seriously and more stringent limits introduced to protect the generations to come.”

The results of the study were published in the medical journal The Lancet Planetary Health.

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