Image Smear and other factors
Things were simplier in the silver halide days.
Photo Engineer is correct that image smear can be a significant problem in some ink jet materials exposed to humid conditions, along with concurrent density changes and hue shift. It is this combination of effects and their complex psychophysical (or perceptual) interactions that make designing a valid test for humidity effects so difficult.
An excellent paper, and the one I think Photo Engineer may be referring to, is by Hill, Suitor and Artz (researchers at Eastman Kodak) and was presented at the Non-Impact Printing Conference 16 (NIP16) organized by the Society for Imaging Science and Technology.
It demostrates how important it is to make sure that you correlate measurements made with instruments with human factor studies. Alas, such studies done right are expensive and some testing laboratories skip that critical step.
In the Hill, et. al., study it was found that either hue shift or smearing can dominate or they can interact when correlating changes with observer evaluations of print quality.
This also emphasizes that in the digital world more than ever one must consider all four environmental factors: heat, light, gases (largely, but not exclusively, ozone) and humidity--and that these factors must be in balance. Set your presumed light level much higher than real values and your test may mask out the real changes caused by, say, ozone--which has happened in some published test results.
Be suspicious of published tests results that show only, say, light, and list ozone or humidity only as "in progress". Check back to see if these values are ever filled in. Could it be that the product does not do well under those factors and the person who paid for the test is reluctant to have them published? And what does that tell you about the folks actually doing the test?