Composition of air

Peraps the best way to have clean air is to avoid buring anything unessesarily and to have lots of plants around. As we know plants provide Oxygen while humans exhale carbon dioxide. I recall the early space programs used pure oxygen in the cockpits of spacecraft until a small ignition burned the entire craft. From then they are using a composition similar to natural air which is, I think, about 30% Oxygen and 70% Nitrogen (there may be traces of other gases...) hope to hear your views too. Cheers:)