Approximately 93% of the world’s children under the age of 15 — about 1.8 billion children — are breathing air that threatens their health and development, according to a new report released by the World Health Organization.

Between the lines: Children are more vulnerable to air pollution because they are still developing, but they also breathe more rapidly than adults and are therefore exposed to more pollutants, the authors write. Children are also physically closer to the ground, and therefore often take in pollutants at higher concentrations.