As Jeremy mentioned, it is the humidity of the 'paper' that is important. The humidity of the room you are working in will have an impact on the paper (duh!). With the POP (printing out process) like Ziatypes, the humidity of the paper becomes more important.
This is also one of the reasons why, different papers behave different ways for different printers - plus it depends on geographic location. The way Jeremy's paper (and mine we live in the same part of TX) behaves will be different with each season, AND would be different than say someone working in Santa Fe, where the humidity is very low, vs someone living in Houston.
Some days you can do everything right and still not get a print - it is just the nature of alt printing.