Parents are being urged to ditch the car for the school run and walk or cycle with their children along quiet routes to protect them from air pollution.
Children in cars stuck in traffic are exposed to double the level of pollutants than youngsters walking along even busy roads, raising their risk of health problems including asthma and potentially reducing the growth of their lungs.
And their exposure to pollution is 2.5 times lower if they take a route along quiet back streets than if they walk along busy roads, research for environmental charity Global Action Plan has revealed.
On the street, young schoolchildren are exposed to 30% more pollutants such as particulate matter because they are shorter than adults and nearer the exhaust pipes of vehicles.
So to mark Clean Air Day on Thursday, parents are being urged to take their children to school and back by foot or on bicycles on quiet roads where possible to reduce their exposure to air pollution as much as possible.
If more people left their cars at home it would also reduce pollution on streets, and lead to fewer health problems, Global Action Plan said.
For the research, children and adults across four UK cities - Manchester, Leeds, Glasgow and London - carried an air pollution monitor for 10 minutes while walking along a quiet route, a busy road, a journey by car in traffic and on a bus.
Chris Large, senior partner from the campaign group, said: "Global Action Plan's research found that the millions of children in the UK that are walking to school along busy roads are potentially being exposed to 30% more pollution than their parents.
"A simple solution for parents would be to choose quieter back routes to walk or cycle their children to school, away from the traffic, thereby reducing their exposure to unnecessarily high levels of damaging air pollution."
Separate analysis by Unicef UK suggests one in three UK children live in areas with unsafe levels of air pollution, which could leave them with long-term health problems.
Around 4.5 million children, including 1.6 million youngsters aged five and under, are living in areas with levels of particulate matter or PM2.5 pollution above what the World Health Organisation deems is safe.
The children's charity is calling on the Government to prioritise and fund measures which target the most polluted areas and protect children from toxic air in the places where they live, learn and play.
Amy Gibbs, Unicef UK's director of advocacy, said: "Worryingly, one third of our children could be filling their lungs with toxic air that puts them at risk of serious long-term health conditions.
"We wouldn't make our children drink dirty water, so why are we allowing them to breathe dirty air?"
Joe Farrington Douglas, from Asthma UK, said: "It is really shocking that young children are breathing in toxic air pollution on their way to school.
"It could be particularly perilous for the 1.1 million children in the UK with asthma, as air pollution is a common cause of asthma attacks."
He urged the Government to protect the next generation from toxic air by bringing in a new Clean Air Act to clean up pollution hotspots.