As normal bulbs the halogen lamp has a tungsten filament and doesn't contain air. This is because if it did, the tungsten filament would burn up instantly. Therefore the oxygen is replaced with a mix of nitrogen, argon and halogen. To remove any oxygen that might be left, the filament is often layered with a phosphor compound(called 'getter') that reacts with the oxygen the first time the bulb is used.

When the current flows through the halogen lamps filament, it becomes white hot. At this temperatures the tungsten starts to vaporize. The tungsten atoms that flows away from the filament forms volatile compounds with the halogen. These compounds float around until they hit the filament, where the heat breaks up the compound, and the tungsten melts in to the filament again. This gives the halogen lamp longer a 'life'.

The halogen lamp should not be touched with bare hands since this deposits salts and fats on the glass. This is because the fat burns and makes dark spots on the glass. The salt fuses with the aluminosilicate glass of the bulb due to the high operating temperatures, and weakens it's integrity