LA Times: Engine fumes in cabin air
 

"Known as bleed air, it is safe, unless there is a mechanical issue -- a faulty seal, for instance." "Airlines have been asking Boeing to install air sensors for years. But the company decided against developing the technology."

From a lot of passenger experience I question if it really happens "with alarming frequency across all airlines."

https://www.latimes.com/projects/tox...9-travel-woes/

I suppose it's a question of how many times you would need it to happen, worldwide, before you class it as "alarming".

This is a topic that crops up every year. I suspect that 'alarming' is needed to make the story but airlines should have installed the detectors because it is cheaper to do so than have court cases. But then, Boeing have only just learnt that lesson ...