What is pure air?
Air is composed of numerous gas, among which Nitrogen (79%) and Oxygen (20%). Various other gas are present in small quantities, such as Argon (1%), CO₂ (that we expire), Neon, …
Gas are more or less dangerous depending on their quantity. For instance, Ozone (O₃) is rather a useful gas in small quantities (it destroys microbes), and natural air contains some of it. But when it is too dense, it irritates eyes and respiratory ways. Also, CO₂ at a “normal” rate (0.04%) is perfectly acceptable, but when it reaches 0.1%, it becomes disturbing: it induces a sleep of low quality, etc.