Air Exchange

The Air Exchange, also known as "AirEx" or "Air Conditioner", is a chapter cut from Half-Life 2. It was set after the Depot chapter, at the end of the Wasteland chapters. Prototyped and having all its maps made, it was never finalized to let the team concentrate on areas more fruitful for gameplay.

The AirEx was last worked on in February 2002; the last AirEx map to be modified, the map "temp3.vmf" (an overview of the AirEx) was last modified on that date.

Features
The AirEx was to be a Combine facility guarded by Combine Soldiers that was to replace the air by noxious, murky gases with its main reactor powered by an unknown blue energy source, probably the predecessor of the Dark Energy. Most of the air of the region was to be highly toxic, and most of the City 17 inhabitants were to wear gas masks.

In this chapter, Gordon Freeman was to reach the top of the highest tower and battle a Gunship and then jump into a pool of industrial solvents at its base. Then he was to go inside the main reactor of the facility, protected and operated by Stalkers, and sabotage its core, making the Air Exchange completely nonoperational and triggering the human uprising in City 17. Then Gordon was to reach the shore on foot and proceed to the Borealis with Odell on a tugboat.

Like the Citadel and the Depot, the Air Exchange was to be one of the many Air Exchanges spread on Earth by the Combine.

Fate
Blowing up the AirEx to trigger the uprising was merged with the Depot levels to make the Nova Prospekt levels; reaching the top of the highest tower and blowing up the core of the whole facility was recycled in the Citadel levels, as long with the factory-like themes.

It is uncertain whether or not the AirEx exists in the final storyline of the game, but as there are no in-game references to it, it is quite likely that it does not. This is especially true as its effects were to force the remaining humans to wear gas masks apparently constantly, as seen in other related early concepts. The fact that no humans are witnessed wearing said masks means that the idea was almost definitely removed completely.