Well, the penetrating depth depends upon the voltage, and there needs to be space for the electrons to accelerate (this means vacuum or air, dialectric insulati

Well, the penetrating depth depends upon the voltage, and there needs to be space for the electrons to accelerate (this means vacuum or air, dialectric insulation in the air gap cuts it down). Anything above around 15 kV has 'potential' for x-rays which can penetrate beyond your skin and do some damage. We were working with 180 kV power supplies, with a 10 inch vacuum-filled free fall distance for the electrons, so we got a goodly amount of rays. You can get x-rays off of Tesla coils, spark gaps, etc., but usually the dose rate is fairly low. It is worth measuring though, especially if you will be exposed for a long time or its close to your body. Typically it will be below around 1/100th of your yearly exposure limit per hour. So, if you are playing with one for an hour, no biggie. If you are planning on putting one on your 40 hour/week work desk, think again. I once made a helmet for Halloween with a pair of Tesla Coils, one on each side of my head. The helmet was conductive, and it would look like the sparks from the coils were going through my head. This was not what I would call 'safe' (body-mounted high voltage can go badly wrong), but it was a lot of fun for one night. I made sure to keep the voltage below 20 kV, and reduced my exposure time by only running it when I wanted to impress people (mainly cute girls).