Parents should use pram covers to protect babies and young children from harmful air pollution during the school run, researchers suggest.
Traffic lights and bus stops were pollution hotspots and there were higher levels of pollution in the morning compared with the afternoon, the University of Surrey study said.
Young children are more at risk because their bodies are small and developing.
And they could be breathing in damaging substances, the study said.
A recent World Health Organization report said that 570,000 children under the age of five died every year from illnesses that could be linked to pollution.
The Surrey research, published in Environmental Pollution, was based on measurements from air monitoring equipment inside toddlers' prams being pushed to and from schools during drop-off and pick-up times.
Scientists looked at pollution levels in prams during 64 trips, covering about 50 miles (80km), in Guildford.
Bus stops and traffic lights were the worst places for being exposed to the tiny particles from exhaust fumes and tyres that can get into the bloodstream.
And there were more of these harmful particles in the air during morning drop-off times, when traffic was at its busiest.
Dr Prashant Kumar, lead study author from the University of Surrey, said his findings were a warning to parents.
"Young children are far more susceptible to pollution than adults, due to their immature and developing systems and lower body weight," he said.
"Essentially, children could be at risk of breathing in some nasty and harmful chemical species such as iron, aluminium and silica that form together the particles of various size ranges."
Dr Kumar said the best way to stop this happening was to use a barrier between children in prams and the emissions from vehicle exhausts, especially at traffic lights, crossroads and bus stops.
He said his research team was also trying to find a way to clean the air around children sitting in their prams.