The Cloud is not a natural occurrence; it is an artificially created toxin produced in Big MT not long before the war. It was deployed in the Sierra Madre as part of a greater experiment, using the casino and the Villa as testing grounds - with Sinclair completely unaware of this. The hazmat suits worn by the ghost people were a follow-up experiment to the Cloud, something many researchers in Big MT found disturbing. It is also pointed out in terminals at the Big MT research facilities that Sinclair was aware of the experimental state of the suits, but not of the Cloud.
The toxins were seemingly meant to force the usage of the newly created Auto-Docs. During the construction of the villa, construction crews discovered the gas backed up in the Sierra Madre's ventilation system and attempted to inspect it. Even trace amounts of the gas proved to be harmful. Hazmat suits that were also developed in the Big MT were ordered by the workers with the hopes that it would protect them. Unfortunately, this wasn't the case; the workers were still exposed to the Cloud and the fastenings of the suits were corroded, trapping the exposed workers inside to the point where they had to be cut free with a cosmic knife. The exposed workers were sent to the medical district, and concerns were raised about the gas building up beneath the streets.
By the time the Courier arrived two hundred years after the War, a number of the fans in the main ventilation system had gone offline, and the Cloud had completely engulfed the casino and villa, somehow preserving it despite the decades of disuse. Elijah sees the potential in this, and wants to use the Cloud in his new world, using it to preserve and cleanse Old World technology. He also plans to use it as offensive technology, unleashing it upon the Mojave, cutting it off from the rest of the world. He could then use modified REPCONN rockets to wipe out the New California Republic (NCR) at Hoover Dam, and finally let traces of it drift west to NCR territory, killing outlying communities there. He would then presumably reset the fans of the ventilation system to remove most or all of the Cloud around the Sierra Madre.
The Cloud slowly depletes the health of visitors to the Sierra Madre (during Hardcore mode), and concentrated pockets of it will deplete health far more quickly. Due to this toxic atmosphere that surrounds the Sierra Madre, it is impossible to sleep on any beds exposed to the outside air. Only the ghost people walk the Cloud, in their ancient hazmat suits fused to their bodies. Though the exact composition of the Cloud isn't mentioned, it's implied to consist of heavy metals: Elijah mentions that it contains copper and sulfur, and Dog/God will often remark on the cloud as "tasting like copper, or old world gold." The rebreather or any power armor will offer no protection from the Cloud. In addition to corroding metal, the Cloud has a tendency to stain the walls and streets of the town. These patches of Cloud residue appear as patches of damp, dark red powder, and can be collected for crafting.
During the quest Mixed Signals, parts of the Cloud can be dispersed in the streets of Puesta del Sol by reactivating the ventilation fans (Christine will remind the player character to do this). Most of the Cloud in Salida del Sol South can also be cleared. At the right of the entrance to the Switching Station in Salida del Sol South, where Dog/God is stationed to discharge the fireworks for the Gala Event, there is a monitor used if necessary to lock Dog/God in the Switching Station. If this monitor is activated after the conclusion of Dead Money, it will show three sub-menus: Substation Status, Ventilation System Status, and Reset Ventilation System. Clicking on Ventilation System Status will bring up a display that indicates that two of the five main fans for Salida del Sol South, Fan 2 and Fan 5, are offline and need to be reset. This can be done via the Reset Ventilation System link on the main screen, and then all five fans are shown as back on line, and the Cloud in the streets of Salida del Sol South disperses (though it remains in pockets in some rooms, and the area outside the neighborhood is unaffected).
- The Cloud is able to decay even Saturnite.
- Upon completion of the final quest in Dead Money, the Cloud's presence is significantly diminished, allowing further exploration of the Sierra Madre without the constant drain on health. The high concentration toxic clouds, however, will still poison the player character.
- While Dean Domino is a companion, one will gain the Unclean Living perk, which grants a short period of time (delay five seconds by default) before taking damage inside a toxic cloud as well as reducing the amount of damage taken inside the toxic cloud by 25%.
- Having the Heartless perk will not reduce the amount of damage taken while in the Cloud, even though it states that the player character cannot be poisoned. This is because the Cloud is corrosive as opposed to poisonous.
- The default damage of Cloud is 3 HP per 20 seconds; the toxic Cloud deals a default damage of 40 HP per second.
- Due to the corrosive nature of the Cloud, the Courier cannot wait, sleep, or fast-travel in the outside areas. However, it is possible to wait and sleep indoors where the Cloud does not have a strong presence.