Some national standards bodies or libraries may provide free or low-cost access to ISO standards, including ISO 18593. However, be aware that these may be limited to viewing only, and you may not be able to download a copy.
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ISO 18593 is an international standard for "Cleanrooms and associated controlled environments - Part 1: Vocabulary" and also provides guidance on the measurement of airborne particle concentration in cleanrooms and clean zones. iso 18593 download