About 17 million babies worldwide live in areas where outdoor air pollution is six times the recommended limit, and their brain development is at risk, the UN children's agency (UNICEF) has said.
The majority of these babies, more than 12 million, are in South Asia, it said, in a study of children under one-year-old, using satellite imagery to identify worst-affected regions.
"Not only do pollutants harm babies' developing lungs, they can permanently damage their developing brains and, thus, their futures," said UNICEF executive director Anthony Lake.
Any air pollution above the World Health Organisation's recommended limit is potentially harmful for children, and risks grow as pollution worsens, UNICEF said.
Air pollution is closely associated with asthma, pneumonia, bronchitis and other respiratory infections, it said.
Scientific findings about the links with brain development are not yet conclusive, but rapidly growing evidence is "definitely reason for concern", UNICEF's Nicholas Rees, the report's author, told the Thomson Reuters Foundation.
Brain development in the first 1,000 days of a child's life is critical for their learning, growth and for them "being able to do everything that they want and aspire to in life", he said.
"A lot of focus goes on making sure children have good quality education, but also important is the development of the brain itself," he added.