Air pollution is a big killer. Researchers estimate that smog—particularly the tiniest particles in the mix—contributes to the early deaths of up to 7 million people worldwide each year. Harm to fog-filled lungs is an obvious concern, yet air pollution is notably linked to cardiovascular events, including heart attacks and stroke. And researchers have puzzled over why.

A leading theory is that the teeny particles may cause systemic inflammation and cellular stress that can wear on the cardiovascular system and muck up heart rhythms. But a new study suggests that the nanoparticles have a more direct role in the disease.