About four million babies in East Asia and the Pacific under the age of one are breathing toxic air, putting their brain development at risk.
This is according to a report by the UN children's agency.
Babies in South Asia were worst affected, with more than 12 million living in areas with pollution six times higher than safe levels.
Unicef said breathing polluted air 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".
Unicef says the effects lasted a lifetime.
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