If you need to bring food into Shadesmar, why don't you need to bring air?
Y'know, we actually talked and thought about this. There are certain things I just decided for narrative reasons... I wanted Shadesmar to be travelable and I wanted it to be a real place, and so I just made air, I came up with kind of my own hacks. There are times I do this for narrative reasons.
Let me give you an easier example. In the Mistborn books, and I've told people this before, I was working on speed bubbles. Slowing down time, speeding up time in a small little bubble around you, right? I went to Peter and I'm like, "This is what I'm going to do, what are the problems with this?" And he's like, "Well, redshift." Which means that basically you would be irradiating everyone with the light coming from inside the speed bubble. I'm like, "alright, we're just going to say that doesn't happen." This is where the line between for me science fiction and fantasy exists. When I'm building my story, I do try to have one foot in science with things like this. But I tend to work backward... A lot of science fiction starts with what we have now and extrapolates forward to [an] interesting, plausible premise. For my fantasy works, I start with some cool idea. And then I work backward in plausibility, trying to justify it. And we kind of meet in the center, but at the end of the day I am breaking the laws of thermodynamics, right? Just straight-up breaking laws-- I mean, we have our whole Realmatic Theory and stuff like that, but at the end of the day, I am trying to tell stories where certain extreme situations exist. Like, I bent over backwards to make the science of Roshar work with the greatshells, but at the end of the day, we still have to have a magical solution, right. To get beasties as big as we want to do, it doesn't matter how high your oxygen content is, if you've got .7 gravity or not, all these concessions we've made: the square-cube law says those things crush themselves. You just can't have things this big. And so we built in a magical solution. The spren creating this symbiotic bond is making it so these things don't crush themselves.
And when I was looking at Shadesmar, there are a couple things-- what I want for the narrative is this place. I am going to work backward and try to make as many concessions and nods toward science as I can. But the air one, I just said "You know what? There's just gonna be air in Shadesmar. I am just gonna make it so that you can." I want you to be able to walk between the planets on Shadesmar, I don't want people to have to worry about bringing a Windrunner with them and plants or whatever to get oxygen. I'm just gonna make that the case. Your in-world answers, I'm like "Well, air kind of permeates and has escaped through and things," but really do we have an oxygen cycle there? We've got plants, but are they really--
The answer is, there is air in Shadesmar because I want there to be air in Shadesmar.