ISO 8573-1:2010 is the part of the ISO 8573 series that states how much contamination is allowed in each cubic metre of compressed air. It grades three contaminants separately: solid particles, water and oil. A specification is written as three digits in that order. For example, ISO 8573-1:2010 Class 1.2.1 means particle class 1, water class 2 and oil class 1 [1].
“Class 0” is the most misunderstood entry in that table. It is not a number printed in the standard. This page explains what the table contains, what Class 0 requires, and what to write on an RFQ to a Chinese oil-free compressor plant.
The ISO 8573-1:2010 purity classes
Class 0 has no figures in the table; it is explained in the next section. The numbered classes are:
| Class | 0.1–0.5 µm | 0.5–1 µm | 1–5 µm |
|---|---|---|---|
| 1 | ≤ 20,000 | ≤ 400 | ≤ 10 |
| 2 | ≤ 400,000 | ≤ 6,000 | ≤ 100 |
| 3 | — | ≤ 90,000 | ≤ 1,000 |
| 4 | — | — | ≤ 10,000 |
| 5 | — | — | ≤ 100,000 |
Maximum number of solid particles per m³, by particle size.
| Class | Water: pressure dew point | Total oil, mg/m³ |
|---|---|---|
| 1 | ≤ −70 °C | 0.01 |
| 2 | ≤ −40 °C | 0.1 |
| 3 | ≤ −20 °C | 1 |
| 4 | ≤ +3 °C | 5 |
| 5 | ≤ +7 °C | — |
| 6 | ≤ +10 °C | — |
Classes 6 to 9 also cover particles by mass concentration and liquid water content; Class X means above the highest numbered limit [1]. The oil column is a total figure: liquid oil, oil aerosol and oil vapour together [1][2].
Class 0 is a written limit, not “zero”
The standard defines Class 0 only as “as specified by the equipment user or supplier and more stringent than Class 1” [1][2]. Two consequences follow:
- Class 0 does not mean zero contamination. It means a limit tighter than Class 1 — 0.01 mg/m³ total oil for the oil class — that the buyer and the supplier have agreed in writing [1].
- “Class 0” without the agreed figure is incomplete. The agreed specification has to appear on the documentation, and it has to be measurable with the test methods in ISO 8573 Parts 2 to 9 [1][2].
Some compressor makers use “Class 0” to describe air from an oil-free compressor. That is a supplier usage. For a purchase, ask which agreed limit and which test the claim refers to.
Which part of ISO 8573 tests what
| Part | Test method for |
|---|---|
| ISO 8573-2 | Oil aerosol content |
| ISO 8573-3 | Humidity |
| ISO 8573-4 | Solid particle content |
| ISO 8573-5 | Oil vapour and organic solvent content |
| ISO 8573-6 | Gaseous contaminants |
| ISO 8573-7 | Viable microbiological contaminants |
| ISO 8573-8 | Solid particles by mass concentration |
| ISO 8573-9 | Liquid water content |
A total-oil claim needs both an aerosol result (Part 2) and a vapour result (Part 5). A single “oil-free” label does not show which was measured.
Oil-free compressor, Class 0 and the air at your machine
An oil-free airend keeps injected lubricant out of the compression chamber. It does not control what the compressor draws in. The cited guide notes that a compressor tested under clean-room conditions can show minimal outlet contamination, while the same machine in an urban plant room delivers air that depends on its intake. A compressor-outlet Class 0 claim therefore does not automatically hold at your point of use, and purification is still needed in the compressor room and at the point of use [1].
For that reason, state where the class applies: package outlet, after the dryer, or at the point of use after a sterile filter.
What to write on the RFQ
- The standard and edition: ISO 8573-1:2010.
- The class for each contaminant separately (particles, water, oil), for example 1.2.1, or an agreed Class 0 limit stated as a number.
- The measurement point: package outlet, after the dryer, or point of use.
- The test methods and who performs them: Part 2 and Part 5 for oil, Part 3 for humidity, Part 4 for particles.
- The treatment the plant includes: dryer type, filter grades, a sterile housing if needed, and the drain arrangement.
- The installation room's ambient and intake conditions, 50/60 Hz and voltage.
The cited guide notes that many breathing, medical and food uses are specified at Class 2.2.1 or 2.1.1 at the point of use [1]. Check your own sector code or customer requirement instead of defaulting to Class 0.
How CompressorB2B treats test papers
- ISO 9001 on a factory is a quality-management certificate, not an ISO 8573-1 air-quality result.
- CE marking on a package is not a Class 0 result.
- A report for another machine, another sample point or another pressure is not your machine's result.
Ask for the report number, the laboratory, the ISO 8573 part used, the sample point and the operating condition, and attach them to the quotation. Compare oil-free packages in the oil-free catalogue and issue the duty sheet.
Sources
Checked 29 October 2026.
- [1] Parker domnick hunter, *Introduction to ISO Air Quality Standards* — ISO 8573-1:2010 class table, ISO 8573 Parts 2 to 9, Class 0 notes.
- [2] Ken Korane, “What is Class 0 air?”, Fluid Power World, 30 August 2018.
For a current quotation or application-specific documents, send the duty sheet to the manufacturing plant.