Cloud The Cloud is a massive , blood - red blanket of toxic smog , spewed over the dead city of the Sierra Madre by the air conditioning and climate control systems inside the Sierra Madre Casino . Background . < section begin = " background " / > 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 - Doc s . 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 . < section end = " background " / > Characteristics . 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 ) . Appearances . The Cloud appears in the " " " Dead Money " and is mentioned in a Y - 17 medical facility terminal in " Old World Blues " .