One in five new childhood asthma cases in the UK could be linked to traffic pollution, new research suggests.

Exposure to nitrogen dioxide, which is mainly emitted from road transport, appears to be a "substantial" risk factor for the condition, according to a study in journal The Lancet Planetary Health.

The researchers estimate that four million new cases of childhood asthma globally - 13 per cent of those diagnosed - could be attributable to nitrogen dioxide pollution every year.

In the UK, they suggest 19 per cent of new childhood asthma cases every year are attributable to nitrogen dioxide pollution.

This figure rises to 23 per cent in Manchester and 29 per cent in London.

The most polluted towns and cities in the UK Show all 25 1 /25 The most polluted towns and cities in the UK The most polluted towns and cities in the UK 25. Middlesbrough 11 micrograms per cubic metre. A report from the World Health Organisation has given figures for fine-particle air pollution across the world. Here follow the 25 most polluted towns and cities in the UK The most polluted towns and cities in the UK 24. Derry 11 micrograms per cubic metre Getty The most polluted towns and cities in the UK 23. Hull 11 micrograms per cubic metre (estimate) Getty The most polluted towns and cities in the UK 22. Coventry 11 micrograms per cubic metre Getty The most polluted towns and cities in the UK 21. London 11 micrograms per cubic metre PA The most polluted towns and cities in the UK 20. Stoke-On-Trent 12 micrograms per cubic metre Getty The most polluted towns and cities in the UK 19. Sheffield 12 micrograms per cubic metre (estimate) Getty The most polluted towns and cities in the UK 18. Sandy, Bedfordshire 12 micrograms per cubic metre Google The most polluted towns and cities in the UK 17. Royal Leamington Spa 12 micrograms per cubic metre Getty The most polluted towns and cities in the UK 16. Prestonpans 12 micrograms per cubic metre Richard Webb The most polluted towns and cities in the UK 15. York 12 micrograms per cubic metre Getty The most polluted towns and cities in the UK 14. Plymouth 12 micrograms per cubic metre Getty The most polluted towns and cities in the UK 13. Nottingham 12 micrograms per cubic metre Getty The most polluted towns and cities in the UK 12. Eccles 12 micrograms per cubic metre (estimate) Rept0n1x The most polluted towns and cities in the UK 11. Grays 12 micrograms per cubic metre (estimate) Getty The most polluted towns and cities in the UK 10. Liverpool 12 micrograms per cubic metre Getty The most polluted towns and cities in the UK 9. Leicester 12 micrograms per cubic metre Getty The most polluted towns and cities in the UK 8. Leeds 12 micrograms per cubic metre Getty The most polluted towns and cities in the UK 7. Chepstow 12 micrograms per cubic metre Getty The most polluted towns and cities in the UK 6. Carlisle 12 micrograms per cubic metre The most polluted towns and cities in the UK 5. Gilllingham 13 micrograms per cubic metre Philip Halling The most polluted towns and cities in the UK 4. Swansea A view of Swansea centre taken from Kilvey Hill January 2016 Getty The most polluted towns and cities in the UK 3. Manchester Manchester, UK - November 24, 2016: View of Manchester city center. Tall buildings an be seen on either side of the road and a tram can be seen passing. People can be seen walking on the pavements. Getty The most polluted towns and cities in the UK 2. Gibraltar Wide view of the rocks of gibraltar from the observation deck Getty The most polluted towns and cities in the UK 1. Scunthorpe Scunthorpe, UK - October 19, 2015. TATA steels' Scunthorpe plant in England which has recently been saved from closure after being bought by the investment firm Greybull Capitla for Â£1. Getty

"Our study indicates that policy initiatives to alleviate traffic-related air pollution can lead to improvements in children's health and also reduce greenhouse gas emissions," lead author Dr Ploy Achakulwisut, from George Washington University, in the US, said.

"Recent examples include Shenzhen's electrification of its entire bus fleet and London's Ultra-Low Emission Zone congestion charges."

It is thought that pollution from traffic may damage airways, leading to inflammation and the development of asthma in children who are genetically predisposed to the condition.

While it is not clear which pollutant in traffic air pollution is responsible, previous research has suggested exposure to nitrogen dioxide is key.

Traffic emissions can contribute up to 80 per cent of ambient nitrogen dioxide in cities.

The researchers used global data on nitrogen dioxide concentration and asthma incidence to estimate the number of new cases in children aged one to 18 years old which could be related to traffic pollution.

Out of the 194 countries studied, the UK had the 24th highest proportion of new childhood asthma cases which could be attributable to traffic pollution.

South Korea topped the list, with nearly one third (31 per cent) of new diagnoses linked to nitrogen dioxide exposure.

The researchers said that 92 per cent of cases of childhood asthma attributable to exposure to traffic pollution occurred in areas with average nitrogen dioxide concentrations below the World Health Organisation (WHO) guideline of 21 parts per billion.

"Nitrogen dioxide pollution appears to be a substantial risk factor for childhood asthma incidence in both developed and developing countries, especially in urban areas," senior author Dr Susan Anenberg, from George Washington University, said.

"Our findings suggest that the WHO guideline for annual average nitrogen dioxide concentrations might need to be revisited, and that traffic emissions should be a target to mitigate exposure."

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Dr Penny Woods, chief executive of the British Lung Foundation, said: "We used to think the only real danger roads posed to children was the threat of a car accident.

"However now we can see there's an equally deadly risk; breathing in illegal levels of air pollution and getting a respiratory disease like asthma, or growing up to have smaller, weaker lungs.

"Air pollution is invisible, so it's easy to ignore - but studies like this make it clear toxic air is a pressing threat, and we need to act."