What is International Organization of Standardization ISO 8573-1 CLASS 0 ？

What is International Organization of Standardization ISO 8573-1 CLASS 0 ？
International Organization of Standardization (ISO) has developed ISO 8573-1 standard to examine the air compression quality by considering solid, water, oil, and gas particle within 1 meter cube. The ISO 8573-1 Class 0 standard is using the strictest method of test currently available and simulates the environment as if onsite. Air compressing must possess less than 0.01 mg/m3 of the liquid, fog, or steam in total.

How can I know that I have received ISO 8573-1 Class 0 compressed air?
Understanding the long-term benefit of oil-free centrifugal air compressors as FS-Elliott strive to design air compressors that provide clean air according to ISO8573-1 Class 0 (2010)

To ensure your confidence in us, all our compressors conform to the requirements of oil-free air ISO 8573-1 Class 0 Standard as we are certified by TÜV (Technishe ÜberwachungsVerein – technical inspection association) who is an independent leader in world stage – ensuring the highest scrutiny while testing the quality and safety of the product. FS-Elliott oil-free centrifugal air compressor is ensured with “Class 0”, so it is guaranteed that it will be decreasing the most risk while providing the high-quality air.