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One in five cases of asthma in kids could be linked to traffic pollution, according to new research.

Globally, that means four million children develop asthma every year as a result of air pollution from vehicles, that's the equivalent of 11,000 new cases a day.

Nitrogen dioxide, which comes from vehicle exhausts, has been described as a "substantial" risk factor for the condition, meaning busy UK cities such as London and Manchester are not good for lung health.

The research was published in the journal of Lancet Planetary Health and is the first global assessment of the impact traffic fumes has on childhood asthma.

Their work suggests that 29% of new childhood cases in London can be blamed on nitrogen dioxide pollution - While in Manchester, the figure is at 23%.

Out of the 194 countries studied, the UK had the 24th highest new cases of childhood asthma that could be blamed on traffic pollution.

However, South Korea topped the chart, with nearly a third of new cases linked to car fumes.

It is thought that pollution from traffic makes it harder to breathe because it's bad for children's airways. This means that if asthma runs in someone's family they're more likely to get symptoms in areas where there are lots of vehicles.

One of the scientists involved in the research, Dr Ploy Achakulwisut, from the George Washington University, in America says that in countries where governments have tried to reduce traffic pollution they have seen an improvement in the breathing health of children.

In Shenzhen in China, they have electrified their entire bus service, while in London has created a Ultra-Low Emission Zone.