Oops - I used the wrong word. I should have said replenished or replaced (as part of the pressurisation system's method of pumping new air into the cabin and bleeding off stale air).

One of the features of the 787 is that it not depends on bleed air from first phases of engine compressors. While bleed air is considered safe, there are studies claiming that it can have a small degree of contamination.

There's actually a debate about whether bleed air is safe in certain failure scenarios. In fume events, some of the compounds are alleged to be neurotoxic, leading to the term "aerotoxic syndrome."

I do not know whether or not such information is accurate, but there is concern about it on the internet.

Sounds like https://www.businessinsider.com/british-airways-flight-smoke...

I had a monitor with me on an international flight recently. Once with hit our cruising altitude, co2 was only 800 ppm. Higher than you might want, but lower than a lot of office buildings.

Humidity, however, was 10%, and the cabin pressure would also have been lower than at ground level.