ISO 8573-1 Air Purity Classes Explained: Particle, Water and Oil Tables, and Class 0
HCEM Engineering TeamShare
ISO 8573-1 is the international standard that grades compressed air purity by three contaminants: solid particles, water and total oil. Each gets a class number, written in that order, for example ISO 8573-1:2010 [1:2:1]. The lower the number, the cleaner the air; Class 0 is stricter than Class 1, but only at a limit the user or supplier writes down. The current edition is ISO 8573-1:2010, and every class limit below is from that edition.
This guide gives the class tables for particles, water and oil, explains what Class 0 really means, why an oil-free compressor is a good start but not the whole answer, and which parts of ISO 8573 are used to test the air.
How do you read an ISO 8573-1 class like [1:2:1]?
The three numbers in the brackets always come in the same order:
- A, particles: how many solid particles of each size are allowed per cubic metre.
- B, water: the pressure dew point (or, for the wetter classes, the liquid water content).
- C, total oil: liquid oil, oil aerosol and oil vapour together, in mg/m³.
So ISO 8573-1:2010 [1:2:1] means particle class 1, water class 2 (pressure dew point −40 °C or lower) and oil class 1 (0.01 mg/m³ or less). Each contaminant is graded on its own. "Class 1 air" on a purchase order means nothing until all three numbers and the edition year are written out.
All limits refer to air at the standard's reference conditions: 20 °C, 1 bar absolute (100 kPa) and a relative water vapour pressure of 0. Measured values are converted to these conditions before they are compared with the tables.
What are the ISO 8573-1 particle classes?
Classes 1 to 5 count particles in three size bands. Classes 6 and 7 use mass concentration instead.
| Class | 0.1 < d ≤ 0.5 µm | 0.5 < d ≤ 1.0 µm | 1.0 < d ≤ 5.0 µm |
|---|---|---|---|
| 0 | As specified by the equipment user or supplier, and stricter than class 1 | ||
| 1 | ≤ 20,000 | ≤ 400 | ≤ 10 |
| 2 | ≤ 400,000 | ≤ 6,000 | ≤ 100 |
| 3 | not specified | ≤ 90,000 | ≤ 1,000 |
| 4 | not specified | not specified | ≤ 10,000 |
| 5 | not specified | not specified | ≤ 100,000 |
| 6 | Mass concentration 0 < Cp ≤ 5 mg/m³ | ||
| 7 | Mass concentration 5 < Cp ≤ 10 mg/m³ | ||
| X | Mass concentration Cp > 10 mg/m³ | ||
Counts are maximum numbers of particles per cubic metre at reference conditions. Particles come from the intake air, from wear inside the compressor, from desiccant dust and from rust and scale in the pipework, so the class at the point of use can be worse than the class at the compressor outlet.
What are the ISO 8573-1 water and dew point classes?
Classes 1 to 6 are set by the pressure dew point: the temperature at which water vapour in the compressed air starts to condense, at the working pressure. Classes 7 to 9 apply when liquid water is present and are set by its concentration.
| Class | Limit | Usually reached with |
|---|---|---|
| 0 | As specified by the equipment user or supplier, and stricter than class 1 | Specified case by case |
| 1 | Pressure dew point ≤ −70 °C | Desiccant dryer |
| 2 | Pressure dew point ≤ −40 °C | Desiccant dryer |
| 3 | Pressure dew point ≤ −20 °C | Desiccant or membrane dryer |
| 4 | Pressure dew point ≤ +3 °C | Refrigerated dryer |
| 5 | Pressure dew point ≤ +7 °C | Refrigerated dryer |
| 6 | Pressure dew point ≤ +10 °C | Refrigerated dryer |
| 7 | Liquid water Cw ≤ 0.5 g/m³ | Water separator and drains |
| 8 | Liquid water 0.5 < Cw ≤ 5 g/m³ | Water separator and drains |
| 9 | Liquid water 5 < Cw ≤ 10 g/m³ | Water separator and drains |
| X | Liquid water Cw > 10 g/m³ | No treatment |
The dryer column is a rule of thumb, not part of the standard: each dryer's own rating at your inlet temperature and pressure is what counts. A dew point is always quoted at a pressure, so measure it at the line pressure you actually run. For dryer sizing, see our guide to air dryers for oilless compressors.
What are the ISO 8573-1 oil classes?
The oil class covers total oil: liquid oil, oil aerosol (fine droplets) and oil vapour added together.
| Class | Total oil (aerosol + liquid + vapour) |
|---|---|
| 0 | As specified by the equipment user or supplier, and stricter than class 1 |
| 1 | ≤ 0.01 mg/m³ |
| 2 | ≤ 0.1 mg/m³ |
| 3 | ≤ 1 mg/m³ |
| 4 | ≤ 5 mg/m³ |
| X | > 5 mg/m³ |
Vapour is the part people forget. A coalescing filter catches droplets but lets vapour through; vapour is removed with activated carbon. A test that only measures aerosol cannot prove an oil class on its own.
What does ISO 8573-1 Class 0 mean?
Class 0 does not mean zero. In all three tables, Class 0 means a limit that is "as specified by the equipment user or supplier" and stricter than Class 1. The number only has meaning together with that written limit, for example a total oil limit below 0.01 mg/m³ agreed in a specification.
So when a compressor or filter is described as "Class 0", ask three questions:
- Which contaminant? Class 0 is usually quoted for oil only. It says nothing about particles or water.
- What is the stated limit? Without a number, "Class 0" cannot be checked.
- How was it tested? Which parts of ISO 8573 were used, at what inlet conditions, and was oil vapour included?
Does an oil-free compressor give you Class 0 air?
Not by itself. An oil-free compressor has no oil in the compression chamber, so the compressor is no longer a source of oil. That removes the biggest risk: in an oil-lubricated machine, a worn separator or a saturated filter can send oil downstream.
But the compressor still breathes the air around it, and that air can carry hydrocarbons from traffic, engines, solvents and nearby processes. Those pass straight through an oil-free compressor. The oil class at your outlet therefore depends on the intake air and on the treatment after the compressor, not only on the compressor type. Particles and water are not affected by the compressor being oil-free at all; they still need filters and a dryer.
| Oil-lubricated compressor | Oil-free compressor | |
|---|---|---|
| Oil from the compressor | Yes; removed by filters | None |
| Hydrocarbons from intake air | Yes | Yes |
| Effect of a failed or saturated filter | Compressor oil can reach the process | Only intake hydrocarbons pass |
| Can it reach oil class 1? | Yes, with coalescing and carbon filtration | Yes; filtration still needed for intake hydrocarbons |
| Particles and water | Need filters and a dryer | Need filters and a dryer |
For a closer look at the design, see what makes an oil-free air compressor truly oil-free.
Which purity class do you need?
ISO 8573-1 defines the classes but does not say which class an application needs. That comes from your own process risk, your customer or a sector guideline. Decide each contaminant separately, for each point of use:
- Does the air touch the product or the patient? Direct contact needs tighter oil and particle classes than air that only drives a cylinder.
- Can the line get cold? The pressure dew point must stay below the lowest temperature the pipe or the product will see, or water condenses.
- What does your customer or regulator ask for? In the UK, for example, the British Compressed Air Society's food and beverage guideline is widely cited for [2:2:1] where air contacts food and [2:4:2] where it does not. Check the current edition yourself.
Treating only the points that need it is usually cheaper than treating the whole plant to the strictest class, because every filter and dryer adds pressure drop and running cost. Application guides go deeper on food-grade pneumatic conveying, medical air compressors, dental air compressors and cleanroom and semiconductor labs.
How is compressed air purity tested?
ISO 8573-1 only sets the classes. The test methods are in the other parts of the ISO 8573 series:
| Part | What it measures |
|---|---|
| ISO 8573-2 | Oil aerosol content |
| ISO 8573-3 | Humidity (dew point) |
| ISO 8573-4 | Solid particle content (counts) |
| ISO 8573-5 | Oil vapour and organic solvent content |
| ISO 8573-6 | Gaseous contaminant content |
| ISO 8573-7 | Viable microbiological contaminant content |
| ISO 8573-8 | Solid particle content by mass concentration |
| ISO 8573-9 | Liquid water content |
To prove an oil class you need both oil aerosol (part 2) and oil vapour (part 5). Test at the point of use, not only at the compressor, so that piping and filters are included in the result, and record the pressure and temperature during the test.
Which oil-free compressor pumps can feed a clean air system?
Our oil-free piston compressor pumps are made for OEM equipment makers. They add no oil to the air; the purity class at your outlet still depends on the filters and dryer you fit after them. Datasheet values:
| Model | Supply | Flow at pressure | Power | Noise | Weight |
|---|---|---|---|---|---|
| HC280 | 220 V/50 Hz or 110 V/60 Hz | 20–25 L/min at 700 kPa | ≤340–370 W | ≤63–67 dB(A) | 6.8 kg |
| HC480 | 220 V/50 Hz or 110 V/60 Hz | 40–45 L/min at 700 kPa | ≤480–560 W | ≤65–66 dB(A) | 9.8 kg |
| HC680 | 220 V/50 Hz or 110 V/60 Hz | 60 L/min at 700 kPa | ≤610 W | ≤60–61 dB(A) | 9.9 kg |
| HC1100 | 220 V/50 Hz or 110 V/60 Hz | 100 L/min at 800 kPa | ≤1100 W | ≤70 dB(A) | 17.2 kg |
| HC1500 | 220 V/50 Hz or 110 V/60 Hz | 120 L/min at 800 kPa | ≤1500 W | ≤70 dB(A) | 17.5 kg |
More models are in our AC air compressor pumps and DC air compressor pumps for battery and vehicle equipment. To compare flows quoted in other units, see what CFM means or use our CFM to L/min converter.
Common questions
Is ISO 8573-1 Class 0 better than Class 1?
It is stricter, but only to the limit that is written down. Class 0 without a stated limit cannot be verified, so a specification with a clear Class 1 can be worth more than a vague Class 0.
Does Class 1 oil mean the compressor is oil-free?
No. Class 1 is a limit on the air (0.01 mg/m³ total oil), not on the machine. An oil-lubricated compressor with coalescing and activated carbon filters can meet it, and an oil-free compressor drawing polluted air may not meet it without filtration.
Does ISO 8573-1 cover bacteria?
The purity classes cover particles, water and oil only. Viable microorganisms are measured with ISO 8573-7; where they matter, as in food, pharmaceutical and medical use, the requirement is set separately, usually with sterile filtration at the point of use.
What dew point does a refrigerated dryer give?
Typically a pressure dew point of about +3 °C, which is water class 4. For class 2 (−40 °C) or class 1 (−70 °C) you need a desiccant dryer.
Do I need to keep testing once the system passes?
Yes, if a customer or auditor relies on the class. Filters load up, drains fail and dryers drift, so purity is checked again at intervals set by your quality system. Keeping filters and drains serviced is part of keeping the class; see our oil-free compressor maintenance guide.
Browse our oil-free air compressor pumps, or send us your flow, pressure, supply voltage, duty cycle and the ISO 8573-1 class you need at the outlet, and our engineers will suggest a model and the treatment to fit after it, including OEM versions for your machine.