I did a little reading on this at the Boeing website, where they have a
Cabin Air Systems page. Their Cabin Air Environment PDF document there has a lot of detail about the issues and how they try to tackle them.
During flight, the relative humidity in the cabin ranges from approximately 5 percent to 35 percent, with an average of 15 percent to 20 percent. This is similar to the dry summer climate of the southwestern United States or typical wintertime indoor levels. A low humidity environment has been shown to inhibit fungal and bacterial growth.
One of their claims for the 787 is that the composite construction will allow higher relative humidity in the cabin, because it is stronger and less susceptible to corrosion. We'll see.
By way of contrast, a relative humidity in the low 20s was the cause of complaints in the office I work in, also in winter: I had dry eye problems (with contacts or specs), some folks developed eczema and other skin problems. The 4% figure on the flight deck, quoted earlier in this thread, is one I've seen elsewhere too, which I don't think I'd like. Quoting that Boeing document again:
However, exposure to such an environment without sufficient fluid intake will dehydrate the body through perspiration and respiratory water loss. Dehydration can lead to headaches, tiredness and fatigue. In addition, low humidity can cause drying of the nose, throat and eyes, and it can irritate contact lens wearers.