Whether carbon or labyrinth seal(s) and regardless of how they work or leak:
Up at 10km height in the (still) air, the air pressure is regulated to an altitude of about 2,400m, i.e. 500m lower than the mountain ‚Zugspitze’ in the Bavarian Alps. This means that the air pressure is only 75% of what you are used to on the ground. As a result, the oxygen content is also significantly lower, to which human organisms react differently. You can read about it in the "Compendium of Aviation Medicine", on page "1-27". (not translated yet).
For example, foreign substances are metabolized differently. This means that the body tries to break down a substance which is not its own and which it does not recognise, or does not like very much, into other substances. At least those bodies that work.
This happens for each person in different ways - one faster, the other slower. With some substances this does not work at all, e.g. regarding deadly poisons, or other toxic substances.
In a completely different and compressed air environment as described above, medication can also react differently. Sometimes not at all. Or just at a fraction.
For passengers, a flight is a temporary thing. Flights longer than 13 hours (e.g. Frankfurt - Buenos Aires) are rare. As a rule, they are significantly shorter.
It's different for the crew. For them the cabin or cockpit is their workplace. The toxic body load is thus potentially higher. Significantly higher. And that's potentially where the problems for crews begin.
When a so-called leakage occurs and contaminated air flows into the cabin and/or cockpit, it is called a "Fume Event". This does not always have to be associated with smoke and/or bad smell (fumes). It depends on the substances that get into it: burnt oil or its additives (such as TCP), hydraulic fluids, de-icing agents in winter (which are designed for a temperature of 80°C) and much more. So far, about 200 such chemical substances have been identified. Not much is yet known about them, never mind about the interactions between them.
We analyze why this is the case in: Who, how, what and why. If you don't do research, you stay stupid. - Casuistic in occupational medicine (to be translated).