Air

The air that we breathe is a mixture of solid and liquid particles suspended in a gaseous medium, which is in turn composed of:

oxygen (O2), (20.95% of volume) almost entirely of biological origin, produced by auto-trophic organisms (plants), thanks to the function of the chlorophyll in the photosynthesis;
carbon dioxide (CO2) (in varying proportions) of natural and anthropogenic origin (i.e. generated by man’s activities: for instance combustion processes);
water (H2O), (in varying proportions) that may be present in the atmosphere as steam or in a liquid or solid state;
nitrogen (N2), equal to 78% of the volume and argon (A) equal to 0.93% of the volume, both inert gases;
other gases such as neon, krypton, xenon, hydrogen and others that taken together make up just 0.01% of the volume of the atmosphere. Solid and liquid particles (dust, spores, pollen, etc.) are present in limited amounts and their size, physical characteristics and chemical properties are extremely variable.