This study explores the stricter cap on the sulfur content in marine fuel implemented in seas in north-western Europe in 2015. The analysis is built on a difference-in-differences design that compares outcomes in coastal areas before and after the tightening of the sulfur cap, to the outcomes in control areas over the same period. The results show that the new sulfur cap reduced annual average ambient concentrations of sulfur dioxide (SO2) in coastal areas by 22 percent and levels of fine particulate matter (PM2.5) by 6 percent, compared to control areas. In the subsample with available air quality readings, the sulfur cap led to a 7 percent reduction in the risk of low birthweight in coastal areas. The estimated improvements provide a benchmark for assessing the benefits of tightening sulfur standards in other settings. © 2021 The Author(s)