Unless you are sterilizing everything entering the cleanroom, that is simply not possible. Fungal spores, bacteria, and viruses are everywhere. If one of your workers is a smoker, they can spread Tobacco Mosaic Virus, for example.
Clean rooms are not perfectly clean. I've always wondered if Intel, which is famous for wanting all of their fab cleanrooms to be the same, also infects them with the same bacteria and mites...