Air pollution both inside and outside is contributing to the deaths of 9,500 Londoners each year.
A new report by the Royal College of Physicians and the Royal College of Paediatrics and Child Health shows the major health impact of outdoor pollution, but also the less understood impact of indoor pollution.
When outside - it's nitrogen dioxide, an invisible toxic gas, which is of most concern. You can't see it, but its there - given off by diesel vehicles.
Air pollution is a particular concern for Shazia Ali Webber, her asthmatic children go to a school next to a busy road in Hackney.
The increase in nitrogen dioxide pollution is down to the increase in diesel vehicles.
Successive governments have made them cheaper to tax because they release fewer carbon emissions. But they're worse for our health, and there's more of them on our roads than ever before.
In the 1980s, fewer than 10% of new cars were diesel. By 2000 it was 14% but now, half of the cars on London's roads are diesel.
Wherever you are in London air pollution is high, and it's been found to reduce children's lung capacity, affect the development of unborn babies, and increase the chances of someone suffering from a heart attack.
The study also found that the air quality inside homes can be low due to things like cleaning products. So what should people do if they live next to a busy road?
The report calls for the government and local councils to do more to improve the air we breathe.