The Astrodome actually was designed as an emergency shelter, though.
And that ventilation systems has been upgraded at least three times that I can think of. The big problem it has is that it can't be turned off - so they keep it at a nice 72F inside 24/7/365.
The last upgrade was in the early 90s. Also, they limited people in the Astrodome to 12K, sending the rest to Reliant and the convention center.
12K people can camp out in the Astrodome for quite a while without taxing it too much.
Good to know. Thanks