Toddlers in buggies are breathing in harmful diesel soot pumped out by car exhaust pipes, a cabinet minister has warned.
Energy Secretary Ed Davey said parents had raised fears with him that the pollution from busy roads was damaging their children's health.
In an interview with the Evening Standard, the Lib Dem minister urged London Mayor Boris Johnson to step up the battle against dirty air with a quicker switch to electric vehicles.
And he called on the Mayor to put 'smog alerts' on bus stops to alert families to the danger of venturing out.
Mr Davey stressed children were particularly at risk from traffic pollution because they are so much closer to exhaust pipe emissions than adults.
He said: "If you had electric vehicles, they would not be exposed. If you had low emission vehicles, they would not be exposed.
"But at the moment they are vulnerable if they are being pushed down congested high streets. In a buggy they are at near exhaust pipe level.
"It's vital for the health of our children that we take this issue far more seriously."