Interesting that the next two guys expressed the importance of wetting the floor...
From Brisk1961's article...
"5. Make certain it is bone dry where you are going to paint. Helpful old souls will tell you to wet the floor down to keep the dust down ... say "Thank you for that idea," and then, whatever you do, DO NOT WET THE FLOOR IN THE PAINTING AREA. All urethane is moisture-cure material. Any humidity, fog, steam, cloud, water vapor, passing from the floor into the sky as it evaporates will pass through the spray mist from your spray gun. At the end of the spray gun, as you pull the trigger, the temperature of the paint drops many degrees Fahrenheit or Celsius, as you wish ... this cures some of the paint before it ever hits the car!!! "Gee, I got a lot of dirt in my finish ... duh. " No, actually, you PUT A LOT OF "DIRT" IN THE FINISH. DRY! DRY! DRY! Always paint in a dry place. "
I know that at the body shop I worked in, there was a policy of wetting down the filters and floor to reduce dust, but that was always done with enough time to dry before pushing the car in the booth. A humidity guage is often used because most paints have a temperature range and humidity range that must be adhered to for best results.