The air you breathe on airplanes comes directly from the jet engines. Known as bleed air, it is safe, unless there is a mechanical issue — a faulty seal, for instance. When that happens, heated jet engine oil can leak into the air supply, potentially releasing toxic gases into the plane.

For decades, the airline industry and its regulators have known about these incidents — called fume events — and have maintained that they are rare and that the toxic chemical levels are too low to pose serious health risks.

And yet there’s a lot of pushback to measuring just how bad the problem is.