It sounds like the humidifier might be causing the problem. When I took my MacBook Pro out in the sun when it was very humid I got a "hazy" affect on the screen and things weren't very clear as you describe them to be. What I figured out was that the haze was coming from a thin layer of condensation between the glass and the LCD panel. I believe that because the humidifier is close to the iMac that it is causing a layer of condensation to form. I would try and take the humidifier out of the room to see if it got rid of the haze.
Hope this helps.