I can see easily half a dozen problems you can wreak havoc with with ashes on an airplane. Ashes or any dust, actually. Remember, it's essentially a closed, hermetic, environment.

People have this idea that air on an airplane is constantly being recycled. I'm guessing that is where your objection comes from.

In reality the engines are constantly producing a stream of hot compressed air which is bleed off for various subsystems. One of these is the air conditioning systems. These cool that air and filter it so that it's clean and at a reasonable temperature for passenger comfort. The air is added to the cabin at a pretty much fixed rate and the pressure is regulated by a dump valve which dumps excess pressure overboard. There is no real recycling of air.

I'm genuinely curious,

How do they regulate oxygen levels?

Or are those levels more or less the same as on the ground but it's the pressure that incommodes?

The pressure changes with altitude, not the gas makeup. So at 35kft/10.6km the air is still 21% oxygen. The gp is right, the engines bleed off some of the fresh air from the compressor stages in the engine, before the combustion stage, and use that to supply the cabin. This is called Bleed Air - https://en.wikipedia.org/wiki/Bleed_air

What about sand? Or talcum powder. I think the concern is less about cleanliness or hazard, and more about finding a rational, normative basis for seeing something unusual on an X-ray.

Questionable items with no obvious cash value simply go away. Off the plane.

It probably is the smell, the charcoal and trace chemicals that mess with swabs and dogs.

embers

...you can make caustic lye...