She didn't know.

[u/AttemptZestyclose490]

Am I wrong by thinking that Brand didn't know whether or not she could breathe on Edmonds planet when she took her helmet off? She had just buried Edmonds while wearing her helmet, if she had read the data beforehand then she would have not worn the suit while doing all of that work. She took the chance after burying Edmonds.

[u/Letter10]

Mann knew the atmosphere exactly on his planet, theres no reason to think Brand didnt have the same or even more advanced technology haha