What is the air filled in to a bulb?

Oxygen can rapidly oxidize metals and other materials in the presence of heat, so it is not compatible with the heated filaments or ionized gases within a light bulb.

The development of partial vacuums and use of inert gases (mostly nitrogen) allowed early light bulb filaments to become hot enough to glow without being burned up. Because metals conduct current better than their oxides, oxidation of filaments and conductors is also avoided.