MUMBAI, India — About 300 million children in the world breathe highly toxic air, the United Nations Children’s Fund said in a report on Monday that used satellite imagery to illustrate the magnitude of the problem.

The vast majority of these children, about 220 million, live in South Asia, in places where air pollution is at least six times the level that the World Health Organization considers safe, Unicef said.

The agency said the children faced serious health risks as a result.

“Children are uniquely vulnerable because their lungs are still developing,” said Nicholas Rees, the author of the report.

“Early exposure to toxic air has lifelong consequences for them,” he said.

Among the most dangerous pollutants are air particles known as PM2.5, which are a small fraction of the width of a human hair.