What are some examples of combined gases?
What about the air we breathe now? What gases comprise this mix?
There are speciality gases used in medicine, and speciality gases used in scuba-diving, and speciality gases used in in welding. All of these are purpose chosen to use in a particular role. Helium is used by deep-sea divers in that its supreme unreactivity makes the gas UNLIKELY to be dissolved in blood, and thus less of a risk in decompression; as a bonus it makes you sound like Donald Duck. You will have to look these up....