Is the increased humidity on jets such as the 787 and A350 artificial? Or is it a direct result of a greater cabin pressure?
A Reuters article confirms the humidity just comes from the passengers, on the 787 at anyway:
(...) the air is dry and moisture comes mostly from passengers.
And this Flight Global article:
(...) the system does not add moisture through active humidifiers.
This APEX article has no discussion of higher cabin pressure being an influence on humidity.
Lastly, this Business Insider article says Boeing indicates the higher pressure alleviates the feeling of mountain sickness that can come from being at 8,000 ft vs 6,000 ft.
[S]ince there isn't a perfect one-to-one correlation between altitude and jet lag, Boeing has taken additional measures to mitigate the symptoms. These measures include an increase in cabin humidity as well as a new air-filtration system.
So, it's real humidity, and not really an effect of cabin pressure.
There may be two factors at work. First, there may be less air exchange, which would leave more moisture in the aircraft from people. Reducing air exchange reduces the power required to pressurize the aircraft. As I recall the 787 is a no-bleed system, with electrical compressors utilized, resulting in a claimed 3% fuel efficiency. Accordingly, I would expect that a system goal would be to minimize air exchange, which would in turn minimize power consumption. This would result in more retained water vapor.
Secondly, atmospheric pressure impacts dew point, which relates to our sensation of humidity. A psychometer reading is dependent upon atmospheric air pressure, and is often adjusted for ambient pressure at altitude (for example in cities at higher elevations).
I haven't worked out the numbers, so this is a matter of unsubstantiated but dangerously informed opinion.
Coming with some ECS/Pressurization background in engineering tech support, the only factor I can think of where higher pressure differential would influence humidity, which is all coming from passengers, would be related to leak rate of the pressure hull (they all leak, mostly through door seals etc). I might expect higher differential to result in a higher leak rate, which would result in more inflow to compensate, which could reduce the humidity level some amount. However, I suspect that this effect, from say going from 8 to 10 psi (or whatever the 787 uses), is pretty small and if the 787 is using a sophisticated control system that manages inflow/outflow to control humidity as well as pressure, the point is probably irrelevant.