Seventeen million babies under the age of one are breathing toxic air, putting their brain development at risk, the UN children's agency has warned.
Babies in South Asia were worst affected, with more than 12 million living in areas with pollution six times higher than safe levels.
A further four million were at risk in East Asia and the Pacific.
Unicef said breathing particulate air pollution could damage brain tissue and undermine cognitive development.
Its report said there was a link to "verbal and non-verbal IQ and memory, reduced test scores, grade point averages among schoolchildren, as well as other neurological behavioural problems".
The effects lasted a lifetime, it said.
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"As more and more of the world urbanises, and without adequate protection and pollution reduction measures, more children will be at risk in the years to come," Unicef said.
It called for wider use of face masks and air filtering systems, and for children not to travel during spikes in pollution.