The quality of air inside and outside primary schools must be improved to protect children's health, campaigners have demanded.
A study by the Health and Environment Alliance found levels of nitrogen dioxide and carbon dioxide in a study of seven schools Lambeth showed they were among the most polluted in London.
Their report, Healthy Air, Healthier Children, says this highlights the urgent need to further tackle traffic pollution and improve ventilation.
It says 40,000 deaths are caused every year by exposure to air pollution and children are particularly at risk.
Polluted air can increase the risk of a child developing asthma especially if a child lives close to a busy road.
Jonathan Grigg, professor of paediatric respiratory and environmental medicine at Queen Mary University, London, said: 'This report highlights that exposure of children to toxic compounds in the air not only occurs outdoors - but also in schools.
'More research is therefore urgently needed on the health effects of these exposures, and health-based indoor exposure limits must be developed for school children.'