HV+vacuum = Xrays
HV + no vacuum = no Xrays...
The Xrays come from accelerating a particle to a high speed and then
having it hit something and stop: bremsstrahlung (braking radiation..
braking as in like the brakes on your car)
If the HV is in air, an ionized particle runs into other gas molecules
so it can't get moving fast enough.
Yes, one can make Xrays in air in some cases:
1) Get a discharge under high enough pressure and hot enough so that
the it's emitting X-rays as black body radiation. Typical free
burning arcs are in the 5000-7000K range, and their black body
radiation is, as we've all observed, peaked in the visible, towards
the blue end. In free burning air arcs, the air and arc expands to
limit the temperature. Since radiation cooling goes as T^4, making it
twice as hot takes 16 times the dissipated energy. And you need a lot
more than twice as hot to get Xrays
2) With a discharge that has very, very high di/dt (rate of change of
current), you can get xrays. Essentially what's going on is that the
air heats fast enough that it can't expand, nor does it lose energy by
radiation quickly enough. I can't recall the critical di/dt, but it's
pretty high (10 to the multiple digits)
So, practically speaking, your ionizing radiation hazard from HV in
air is ultra violet radiation.