you mean those glass bead filled glass circles where the plasma makes lightening like paths around the beads? used on star treck for the borg ship props and a thousand other interesting places on tv and in the movies?
If you're talking about what I think you're talking about then Carbine15 is right, the answer is no, they are perfectly safe.
They will emit an electrical field and probably a rather strong RF field close by. I'd keep them away from your computer or other sensitive electronics as it's possible for them to interfere with it. The high voltage power supplies in these things tend to be cheap too, I wouldn't plug it into the same circuit as your computers or other such things as they can also send noise down the powerlines and confuse electronics.
But as far as people are concerned they are a non-issue. Dont radiate enough power to do anything to you. If the powersupply were say, 100 times more powerful you might get some x-rays out of one
But until you mod the powersupply it's very safe
