Above 15 kV, all products that operate in a vacuum, including vacuum relays, can produce X-rays that can be considered hazardous. This is one reasons we recommend using GIGAVAC's SF6 gas filled relays because the electrons collide with the gas molecules and are unable to accumulate sufficient energy to make significant radiation.
When vacuum relays are used over 15kV, the equipment should be shielded with lead that is at least 16 mm thick. If shielding is not possible, then appropriate X-ray warnings should be posted and a radiation X-ray monitoring program should be implemented. Contact you local regulatory agency regarding your specific situation.