Childhood obesity is a growing problem worldwide, and its causes are complex and poorly understood. Researchers have previously shown how exposure to individual pollutants such as per- and polyfluoroalkyl substances and secondhand smoke contribute to obesity in children, yet few studies consider the multiple simultaneous exposures children experience in the real world. This makes it difficult to understand the significance of single risk factors against a background of confounding variables. Now, an international team lead by epidemiologists Martine Vrijheid of the Barcelona Institute for Global Health and Lida Chatzi of the University of Southern California have analyzed the individual and combined effects of more than 150 suspected risk factors (Environ. Health Perspect. 2020, DOI: 10.1289/EHP5975). The researchers looked at body fat measurements and exposure histories from more than 1,300 children aged 6 to 11 from six European countries. Their results confirm that children whose mothers smoked during pregnancy are more likely to be obese. This study is the first to show that indoor air pollution—specifically fine particulate matter and nitrogen dioxide—increases the likelihood of a child being overweight or obese. The association between indoor NO2 and childhood obesity was strongest in children whose mothers attended an institution of higher education. The researchers are interested in exploring that link.