- Video report by ITV News Science Correspondent Alok Jha
London is infamous for air pollution and has already breached internationally agreed safe levels this year.
But what effect is this having on children's health as they grow up?
One concerned mother agreed to measure the levels of air pollution her and her two-year-old son were exposed to do on their daily walk to playgroup for ITV News.
And the results were alarming.
Using two monitors to measure pollution, Sarah Bentley attached one to herself and one to her son Rowan's buggy for just one hour.
The results of Sarah's tests were then analysed by at Queen Mary's University in London.
Experts revealed the levels were remarkably high and went up at the same points in places.
But worryingly, they showed Sarah's son was exposed to more harmful levels of pollution than her as he was closer to the ground in his buggy.
Dr Abigail Whitehouse said the effects of pollution can start even before a baby is born.
She told ITV News: "The effect on your lung function, how well they function and how well they grow, is affected by your pollution exposure.
"The problem is, if you're exposed to that for the whole of your childhood, you never get your full lung growth. Your lungs never get to the right size."
Sarah said: "It makes me feel like bursting into tears.
"As a parent you do the best for your child, and even just living here you're exposing them to a lifetime risk of ill health."
Doctors say around 40,000 people die prematurely across the UK because of excessive pollution.
It can aggravate conditions such as asthma, bronchitis, heart disease and lung cancer, as well as increasing the risk of respiratory diseases.
And it is not just a problem in the capital.
Cities such as Birmingham, Manchester and Leeds have been found to be just as bad.
An Ultra Low Emission Zone is coming into force in London in 2020 which will charge - but not ban - more polluting vehicles.
But critics argue it only covers 300,000 people in the capital, not the three million living in polluted inner London boroughs.
However, London Mayor Sadiq Khan has proposed to extend the Ultra Low Emissions Zone to the North-South circular, covering inner London, and implement the plan a year early.