Over-specifying air quality costs real money. A customer who specifies Class 1 oil content for a general fabrication application is paying for an oil-free compressor or a full activated carbon filtration system that their process doesn't need. Under-specifying it in a food or pharmaceutical environment creates product contamination risk and potential regulatory liability.
ISO 8573 is the framework for getting this right, but it gets cited on specifications far more often than it gets understood.
What the standard actually specifies
ISO 8573 defines purity classes for three contaminants: particles, water and oil. Each gets its own class number, and you specify all three together as a set.
"ISO 8573 Class 1.2.1" means:
- 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
Most applications only need to define the classes relevant to their process. A general workshop might only care about water. A food-grade packaging line needs all three.
The classification numbers: what they mean in practice
Particles run from Class 0 (user-defined, stricter than Class 1) down to Class 6 (no requirement). Class 1 suits electronics manufacturing and pharmaceutical filling lines. Class 4 covers most general industrial use without complaint.
Water is specified as pressure dewpoint. Class 1 is -70°C, expensive to achieve and maintain, requiring high-specification desiccant drying. Class 4 is +3°C, which is what you get from a standard refrigerant dryer. Class 6 specifies no requirement for liquid water but no dewpoint control.
Oil runs from Class 0 (oil-free, user-defined) to Class 5 (no requirement). Class 1 at 0.01 mg/m³ requires either a true oil-free compressor or a very well-maintained filtration train with coalescing and activated carbon stages. Class 3 at 1 mg/m³ is achievable with a quality coalescing filter on an oil-injected machine.
The three mistakes that cost the most money
Specifying Class 1 oil when Class 3 would do. I see this regularly on specifications that have been copied from a previous project without checking whether the application actually requires it. Class 1 oil either means oil-free capital cost (roughly double an equivalent oil-injected machine) or a filtration train that needs rigorous maintenance and regular activated carbon replacement. If your process doesn't need it, you're paying for nothing.
Specifying Class 2 water without understanding what that requires. A +3°C dewpoint from a refrigerant dryer is Class 4. If your spec says Class 2, you need a desiccant dryer. Desiccant dryers cost considerably more to buy and to run, heatless regeneration models consume 15-20% of the compressor's output just in regeneration air. Make sure the process actually requires it before committing.
Measuring at the compressor outlet and calling it done. ISO 8573 classification should be verified at the point of use, not at the outlet of the treatment train. Old steel pipework, a dirty receiver, or a poorly maintained filter element will degrade quality through the distribution system. A site that passes at the outlet and fails at the machine is a real scenario I've encountered more than once.
Practical class recommendations
For most manufacturing applications without specific cleanliness requirements, a specification of 3.4.3 or 4.5.3 is appropriate and achievable without specialist equipment or unusual maintenance.
For food contact applications, the usual starting point is 1.2.1 for direct contact and 2.4.2 for indirect, but confirm this with your food safety auditor, since specific requirements vary by certification scheme and customer audit.
For painting and powder coating, moisture class matters most. Class 3 or better on water prevents moisture-related adhesion failures and surface bloom. Oil class is also relevant if using waterborne coatings.
If you're specifying for a new project, establish what the air actually contacts before picking a class. The capital cost difference between Class 4 and Class 1 on water is significant, typically £8,000-15,000 on a mid-sized installation when you include the drying equipment and ongoing operating costs. That difference needs to be justified by an actual process requirement, not a conservative assumption.