Initial results showed children's lungs in London were smaller in capacity than those in Luton, which is less polluted than London but has a similar mix of pollutants.

At the end of the study the children's lung capacity reached near identical levels in both groups.

"While we would expect children's lungs to grow year on year, our results indicate that the London children's lung growth had accelerated during the four years to 'catch up' with the control group in Luton to reach similar levels of lung capacity," researchers say.