Is there a way to manipulate lung capacity through .NET?
Title. I can't seem to find anything in Player.Character that has to do with oxygen.
Probably not. Check the native database.
What are you trying to do?
For example, if player or npc is underwater you can simulate lung capacity by increasing or decreasing health,
Same if you're hiking up a hill.
If you want to simulate breathing disorders link them to smoking.
Really depends on whether you can easily trap event and trigger, but unless you want to check Tom Brady's exercise capacity specifically for oxygen, use health.