High Voltage Insulators: Gases
Air is the most common insulator for high voltage. Cost is certainly a big advantage. It can contain a variable amount of moisture. I would consider it to be “dynamic” in that there is movement such as convection currents, and can sometimes contain unwanted particles or ions. Thus, its dielectric strength varies with time.
Sulphur Hexafluoride (SF6) is a great gaseous insulator, however, it is extremely hazardous, so it must be handled carefully, and used in a sealed chamber. One hazard is that poisonous gases are generated if there is an arc. Second, it is denser than air. Thus, if it is breathed in, it will settle in the bottom of the lungs, and displace the air, causing suffocation.
A good compromise may be nitrogen. It is not as hazardous as SF6, does not contain moisture, and is homogeneous.