A production possibilities frontier, sometimes called a production possibilities curve, is a simple economic model that shows the possible combinations of goods or services that can be produced by an economy, country or firm. Economists choose products or types of products and plot the possible combinations of output. This way you can see the opportunity cost, which is what is given up, when the production is changed from one point to another. The output (goods and services) are created using the given resources, land, labor and capital. Land (natural resources), labor (human resources) and capital (reusable, human made resources) are all known as inputs. Because of this, bread, wheat and medical services are all considered output and can be placed on either axis of a PPF. Land and air, however are not made by producers and can not be placed on the axis of a PPF.
I suppose in a science fiction situation where air had to be manufactured on a different planet, air could be considered an output, but in modern times on Earth, it is not.