Field Air Compressors - Blog Post Article - ISO 8573-1 Class zero oil-free

Air Quality Standards & ISO 8573

The History of ISO 8573

ISO 8573 is the group of international standards relating to the quality (or purity) of compressed air or air quality standards.

The standard consists of nine separate parts:

Part 1 specifies the quality requirements of the compressed air
Parts 2 – 9 specifies the methods of testing for a range of contaminants.

Forms of oil in Pipelines

When oil is present in pipelines it is always in these three forms. These impact air quality standards.

These are minute droplets oil suspended in the air stream. Aerosols are partially removed by coalescing filters and appear as condensate.

Wall Flow
This is oil in liquid form, which creeps along the pipe wall. Wall flow either appears in condensate or travels to the process.

Vapours or oil mist
This is vapourised oil in a cloud form. Vapours are not removed by coalescing filters.

ISO 8573-1 (1991)

ISO 8573-1:2010 is the international standard the specifies the amount of contamination allowable in each cubic metre of compressed air.

The original version of 1991 designed 5 classes on oil concentration.
The table below shows maximum oil content per class

Class Maximum Concentration mg/m3
1 0.01
2 0.1
3 1
4 5
5 25
At 1 bar absolute pressure +20 degree celsius and a relative vapour pressure of 0.6. It should be noted that at pressure above atmposhperic pressure, the contaminant concentration is correspondingly higher.

Conformance to Class 1 was sometimes called a “technically oil-free solution”. This initial air quality standard recommended methods to remove oil from the compressed air and cautioned users about using oil-free compressors. The provisions in this initial standard were not adequate to assure the purity levels demanded by the industry and a new edition of the standard evolved – and so, edition 2 was developed.

ISO 8573-1 (2001)

In 2001 the updated standard was released and consisted of the following major changes

  • The standard now spoke about the total oil content (aerosol, liquid and vapour)
  • A standard was introduced on measurement of oil vapour, ISO 8573 part 5
  • A new class (Class Zero) was introduced to cover more stringent quality requirements
  • The clause which recommended ways to remove oil was deleted
Class Concentration Total Oil (aerosol, liquid, vapour) mg/m3
0 As specified by the equipment user or supplier and more
1 <0.01
2 <0.2
3 <1
4 < 5


Temperature has an impact on oil carry over. Higher temperatures will cause a greater oil carry over.

Selection of Purification Equipment to Comply with ISO8573-1: 2010 Air Quality Standards

Below is a 10 point guideline to assist in the selection of purification equipment to comply with the current air quality standards.

  1. Purification equipment is installed to provide air quality and you must first of all identify the quality of compressed air required for your system. Each usage point in the system may require a different quality of compressed air dependent upon the application. Using the quality classification’s shown in ISO8573-1:2010 will assist your equipment supplier to quickly and easily select the correct purification equipment necessary for each part of the system.
  2. ISO8573-1:2010 is the latest edition of the standard. Ensure it is written in full when contacting suppliers. Specifying air quality as ISO8573-1, ISO8573-1:1991 or ISO8573-1:2001 refers to the previous editions of the standard and may result in a different quality of delivered compressed air.
  3. Ensure that the equipment under consideration will actually provide delivered air quality in accordance with the quality classifications you have selected from ISO8573-1:2010.
  4. When comparing coalescing filters, ensure that they have been tested in accordance with both the ISO8573-2, ISO8573-4 and ISO12500-1 standards.
  5. Ask for independent validation of product performance by a third party.
  6. For peace of mind, ensure the manufacturer provides a written guarantee of delivered air quality.
  7. Oil-free compressor installations require the same filtration considerations as oil lubricated compressor installations.
  8. When considering the operational costs of coalescing filters, only compare the initial saturated pressure loss as dry pressure loss is not representative of performance in a normally wet compressed air system. ISO12500-1 requires pressure losses for coalescing filters to be recorded when the element is saturated.
  9. Look at the blockage characteristics of the filter. Just because it has a low starting dp, doesn’t mean it will remain low throughout the filter element’s lifetime. Energy costs should always be calculated based upon the blockage characteristics of the filter, not just initial saturated dp. Ask supplier for verification of blockage characteristics.
  10. Look at the total cost of ownership for purification equipment (purchase cost, operational costs and maintenance costs), a low initial purchase price, may

In short, air quality standards are vital and as technology and standards progress these standards will evolve and become enforced more stringently by government.