The US Food and Drug Administration (FDA) has published Federal Regulation 21 CFR Part 820, Quality System Regulation. This regulation follows ISO 9000. More specific to the clean areas and cleanrooms of interest are the standards and drafts under ISO 14644. A good overview of these standards is available on the IEST web site. Similarly, European Standards provide European Nations with guidelines for GMPs.
Why do these GMPs matter with regard to air filtration in pharmaceutical, bio-pharmaceutical, and medicinal products manufacturing processes? Because GMPs require that the quality of these products be assured, including the control of product contamination during manufacturing and packaging. Generally speaking, the control of contamination is most critical for injectable or parenteral preparations as compared with topically applied or orally ingested preparations. Eye drops, which can be considered a topically applied preparation, are an exception to this rule as they eye is quit susceptible to infection.