Exposure to ambient air pollution is associated with low birth weight, preterm birth, and other adverse birth outcomes. For years, scientists have puzzled over the mechanism behind these negative effects. Now, new research has uncovered an important clue: small particulate matter can cross over the placenta (Nat. Commun. 2019, DOI: 10.1038/s41467-019-11654-3). The study focused on black carbon—minute particles produced by incomplete combustion of fossil fuels and other organic substances. Black carbon particles are especially harmful because they are often coated with toxic substances like heavy metals, says Hasselt University biophysicist Hannelore Bové, who led the study. The researchers took placental biopsies from 28 mothers. When illuminated with short, fast laser pulses, the black carbon particles embedded in the placenta glowed white. Those particles were detected on both the maternal and fetal sides of all placentas sampled. The researchers also found that black carbon concentrations were higher in the placentas of women who lived in more highly polluted regions. Bové and coworkers now plan to track the development and health of the children as part of a long-term longitudinal study.