ISO 8573 comes up in every conversation about compressed air for food, pharmaceutical and electronics applications. It also comes up in plenty of general industrial conversations where the standard is cited without much understanding of what it actually means. This post is an attempt to demystify it.
What the Standard Covers
ISO 8573 defines purity classes for three main contaminants in compressed air: particles, water and oil. Each is given a separate classification number, and you specify all three together.
So when someone says they need "ISO 8573 Class 1.2.1 air," they mean:
- Particles: Class 1 — maximum 20,000 particles per cubic metre at 0.1–0.5 microns
- Water: Class 2 — maximum pressure dewpoint of -40°C
- Oil: Class 1 — maximum total oil content of 0.01 mg/m³
In practice, most applications only need to define the classes relevant to them. A general workshop application might only care about water class. A food-grade application will care about all three.
The Classification Numbers
Particles run from Class 0 (specified by user, stricter than Class 1) to Class 6 (no requirement). Class 1 is suitable for pharmaceutical filling lines and electronics manufacturing. Class 4 covers most general industrial use.
Water is measured as pressure dewpoint. Class 1 is -70°C dewpoint (very dry, expensive to achieve). Class 4 is +3°C — achievable with a basic refrigerant dryer. Class 6 specifies no requirement for liquid water, but no dewpoint control.
Oil runs from Class 0 (oil-free, user-specified) to Class 5 (no requirement). Class 1 requires an oil content below 0.01 mg/m³. This requires an oil-free compressor or an oil-injected machine with high-efficiency coalescing filters and activated carbon.
Where People Go Wrong
Over-specifying on oil class is common. Many customers specify Class 1 oil because it sounds most stringent, without realising it either requires an oil-free compressor (significant cost premium) or an extensive filtration train with activated carbon that must be maintained rigorously. For most general industrial applications, Class 3 (1 mg/m³) is more than adequate.
Misunderstanding dewpoint catches people out regularly. A refrigerant dryer delivers around +3°C pressure dewpoint — that's Class 4. If your specification says Class 2 (-40°C dewpoint), you need a desiccant dryer. Desiccant dryers cost more to buy and significantly more to run. Make sure you actually need the dewpoint you're specifying.
Ignoring installation losses is a hidden issue. The air quality at the compressor outlet degrades through the distribution system. Old steel pipework, a dirty receiver, or poorly maintained filter elements will bring the quality down. ISO 8573 classification should be measured at the point of use, not the outlet of the treatment train.
Practical Class Recommendations
For most manufacturing applications without specific cleanliness requirements, 3.4.3 or 4.5.3 is appropriate and achievable without specialist equipment.
For food contact applications, check with your food safety auditor — typically 1.2.1 is required for direct contact, 2.4.2 for indirect.
For painting and powder coating, moisture matters most — Class 3 or better on water prevents bloom and adhesion failures.
If you're specifying for a new project, get a clear brief on what the air actually contacts before committing to a purity class. The difference in capital and running cost between Class 4 and Class 1 on water is substantial.