Not at all, at least not for humans. About this period, oxygen levels were probably around 4-5 percent, so we'd be nowhere near the 21 percent we're used to inhaling. The air would have mostly comprised nitrogen and greenhouse gases. Again, you'd most likely suffocate if you didn't have a breathing device. Carboniferous air is a common theme in fossil fuels. Before we burned all the forests, they supplied us with lots of carbon dioxide that now forms part of our atmosphere. That's why there are trees on Earth today: because their predecessors weren't killed by lack of oxygen. Now, plants still use carbon dioxide during photosynthesis, but it's the green plants that fix the gas and make more organic material out of it that get turned into soil when they die. So actually, we can breathe carboniferous air! This article was written by Sarah Parcak, PhD candidate at George Mason University. She is currently writing a book about how fossils are used as time markers by scientists to understand changes in the environment through time.