Clean air provides the only effective safe guard against dust and bacteriological contaminated particles airborne in the atmosphere, which can all too easily impair critical processes in industry, medicine or research.
Contamination exists naturally in 'fresh air'; occurring when particles are released from the human body by movement and breathing, and also generated by processes and machinery.
The naturally occurring contamination of 'fresh air' is approximately one million particles above 0.5µ per cubic foot.
These photos indicate the sizes of common particles:
Pollen 50 microns diameter
Human hair 120 microns diameter