Researchers have long warned that tiny air pollutants — 30 times smaller than the width of a human hair — are linked to preterm births. PM2.5 is behind an estimated 10% of all global preterm births, defined as babies born before 37 weeks of pregnancy, and preterm births account for 17% of deaths for children under 5 years old around the globe. For those who survive those early years, being born too early can lead to health issues including cerebral palsy and respiratory problems. They estimated PM2.5 exposure based on where the women lived. Women experiencing these molecular changes were more likely to have early and preterm births.