On the international space station for example. People are up there for a long long time, where does all the air come from? It's not like they do air deliveries every so often, so how do they keep breathing? Surely there isn't enough space on the space station to store enough air to last them for months.
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Scrubbers recycle the carbon dioxide breathed out and turn it back into breathable oxygen. [Edited on 01.05.2013 8:51 PM PST]