There are all sorts of ways for the environment to become toxic; loads of chemical reactions consume oxygen [0], gases leaking from the ground could displace th

There are all sorts of ways for the environment to become toxic; loads of chemical reactions consume oxygen [0], gases leaking from the ground could displace the oxygen eg Xenon gas, etc. Even without those over time maintenance crews will consume the air in an area through both respiration and equipment work. [0] One relatively common danger to bulk cargo ships is rusting of transported cargo consuming the oxygen in a enclosed space then crews becoming incapacitated when they enter for maintenance or other activities.