The other way is from oxygen gas stored in a tank on the outside of the Space Station. Scientists at NASA are working hard to find better ways to make and store oxygen and to clean the air in the Space Station. The gas called carbon dioxide is breathed out by the astronauts, just like we do here on Earth. There is lots of air around us here where it can go. But on the Space Station it is taken out of the air by a machine.
People also give off small amounts of other gases in other ways.
Gases also come from our sweat, and when food is digested in our stomachs. These gases have names like methane, ammonia, acetone, methyl alcohol and carbon monoxide. They also have to be taken out of the air on the Space Station, which is done with filters made of charcoal. Cleaning the air on the Space Station can be even harder to do because there will be dozens of chemicals that are used in science studies that are done there. If they get loose, some gases from the experiments could mix in strange ways, and possibly make people feel bad or get sick.