Airports can pose a far bigger threat to local air than previously recognized, thanks to the transformative power of sunlight.

In the first on-tarmac measurements of their kind, researchers have shown that oil droplets spewed by idling jet engines can turn into particles tiny enough to readily penetrate the lungs and brain.

Allen Robinson of Carnegie Mellon University in Pittsburgh and his team collected the pollution spewed from a plane powered by one of the most common types of commercial jet engines as it operated at different loads. Though jet engines operating at...