ISO 8573-1 Oil Classes: What It Requires for Compressed Air

This answer covers the oil classes of ISO 8573-1:2010 only, not its particle, water, gas or microbiological classes. It does not set breathing air or medical air limits, which follow their own rules in each jurisdiction, and it does not rate filters or compressors.

What and whom does ISO 8573-1 cover?

ISO 8573-1 covers the purity of compressed air anywhere in a system, and it applies to anyone who writes or claims an ISO 8573-1 class: plant engineers, compressor and filter suppliers, and auditors. ISO describes the scope in one sentence: "ISO 8573-1:2010 specifies purity classes of compressed air with respect to particles, water and oil independent of the location in the compressed air system at which the air is specified or measured" (ISO 8573-1:2010, abstract).

As of 2026-10-14, ISO lists ISO 8573-1:2010 as edition 3, published in April 2010, prepared by committee ISO/TC 118/SC 4 and last reviewed and confirmed in 2017 (ISO, 2026). National standards bodies adopt it under their own numbers; check whether a national adoption is identical to the 2010 edition or modified.

The standard also names gaseous and microbiological contaminants and points to the other parts of ISO 8573 for measurement (ISO 8573-1:2010, abstract). It sets classes, not test methods, so an oil class in a purchase specification needs its test method beside it.

What are the ISO 8573-1 oil purity classes?

ISO 8573-1:2010 has six oil purity classes, 0, 1, 2, 3, 4 and X, and each class sets a ceiling on total oil in mg/m3, counting liquid oil, oil aerosol and oil vapor together (Parker domnick hunter, 2010). The table below shows each oil class, its total oil limit and what it means when you choose a compressor and filter train.

The row most buyers need is class 1: total oil must stay at 0.01 mg/m3 or less, vapor included, at the standard's reference conditions (Parker domnick hunter, 2010).

The filter column is guidance from one filter maker's 2010 selection guide and one compressor maker's page, not text from ISO 8573-1, which sets limits and leaves equipment choice to the user. In that guide, classes 3 and 4 share one filter grade, class 2 adds a second grade and class 1 adds an oil vapor removal stage, listed as AO + AA + OVR, AO + AA + ACS or AO + AC (Parker domnick hunter, 2010). Class X only reports air above the class 4 limit, and public charts disagree on its bound.

What is the oil limit for ISO 8573-1 class 1 compressed air?

The oil limit for ISO 8573-1:2010 class 1 compressed air is 0.01 mg of total oil per cubic meter, oil vapor included. Parker states it this way: "In each cubic metre of compressed air, not more than 0.01mg of oil is allowed. This is a total level for liquid oil, oil aerosol and oil vapour" (Parker domnick hunter, 2010).

Because vapor counts, coalescing filters alone do not reach class 1 behind a lubricated compressor. A compressor maker that promotes oil-free machines writes that class 1 air from an oil-flooded compressor needs "at a minimum, three filters" including "activated carbon filtration for oil vapours" (Hitachi Global Air Power, 2026). The same source states that in-line filter performance depends on compressed air temperature, and that the air "can jump between Class 1 and Class 2, depending on the temperature" (Hitachi Global Air Power, 2026). That claim comes from a supplier, so treat it as a reason to measure, not as a rule.

An oil-free compressor does not settle the question either. Parker advises that "Oil-free compressor installations require the same filtration considerations as oil lubricated compressor installations" (Parker domnick hunter, 2010).

What does ISO 8573-1 class 0 mean for oil?

ISO 8573-1:2010 class 0 for oil means a total oil limit that the equipment user or supplier specifies and that is more stringent than class 1, so it has no fixed number until someone writes one (Parker domnick hunter, 2010). Parker puts it plainly: "Class 0 does not mean zero contamination" (Parker domnick hunter, 2010).

Parker lists what a valid class 0 needs. The user and the equipment manufacturer agree contamination levels as part of a written specification, the agreed levels sit within the measurement capability of the test methods in ISO 8573 parts 2 to 9, and the agreed specification is written on all documentation (Parker domnick hunter, 2010). Without that number, Parker writes, "Stating Class 0 without the agreed specification is meaningless and not in accordance with the standard" (Parker domnick hunter, 2010).

A class 0 claim is also tied to where it was measured. Parker notes that a compressor tested in clean room conditions can lose its class 0 result once installed where the intake air carries contamination (Parker domnick hunter, 2010). A usable class 0 requirement states a number, a test method and a measurement point.

How is oil in compressed air measured under ISO 8573?

Oil in compressed air is measured under ISO 8573 in two parts: ISO 8573-2:2018 measures liquid oil and oil aerosol, and ISO 8573-5:2025 measures oil vapor, and the two results add up to the total oil that ISO 8573-1 classifies. ISO 8573-2:2018, edition 3, describes Method A, which collects oil with inline coalescing filters, and Method B, which uses sampling discs whose oil is extracted with a solvent and analyzed by infrared spectrometry or gas chromatography with flame ionization detection (ISO, 2018). It also describes indicator devices as an alternative in Annex E (ISO, 2018).

Oil vapor sits outside ISO 8573-2. ISO 8573-5:2025, edition 2, published in July 2025, replaced ISO 8573-5:2001, which ISO now lists as withdrawn (ISO, 2026). ISO describes the new edition this way: "This document specifies the test method for determining the content of oil vapour in compressed air using pressurized sampling and gas chromatography" (ISO 8573-5:2025, abstract). It also covers sampling, evaluation, uncertainty and reporting for the ISO 8573-1 oil class (ISO 8573-5:2025, abstract).

A test report that cites ISO 8573-5:2001 after July 2025 uses a withdrawn edition. A report with only an aerosol result cannot support a total oil class.

What is changing in the next edition of ISO 8573-1?

The next edition of ISO 8573-1 is edition 4, registered as the approved work item ISO/AWI 8573-1, and as of 2026-10-14 it is at stage 20.00, new project registered in the work program, on 2025-12-04 (ISO, 2026). ISO states that it "Will replace ISO 8573-1:2010" (ISO, 2026), and lists the 2010 edition at stage 90.92, to be revised (ISO, 2026).

No draft content is public. The ISO project page shows the 2010 abstract and the life cycle, not the planned changes, so any claim about new oil limits for edition 4 is unconfirmed as of 2026-10-14. The draft still has to pass the committee, enquiry and approval stages before publication at stage 60.60 (ISO, 2026).

Two related parts have already moved. ISO 8573-2 was reissued in 2018 as "Contaminant measurement, Part 2: Oil aerosol content" (ISO, 2018), and ISO 8573-5 was reissued in 2025 as "Contaminant measurement, Part 5: Oil vapour content" (ISO, 2026). Until edition 4 is published, ISO 8573-1:2010 remains the edition to cite in specifications, and its classes stay as listed in the table above.

Why do supplier pages describe ISO 8573-1 class 0 differently?

Supplier pages describe ISO 8573-1 class 0 differently because the 2010 class sets no number, so each supplier fills the gap with its own definition, often tied to what it sells: the point of use for filter makers, the compressor outlet for compressor makers.

A filter maker's guide holds that class 0 needs an agreed written limit and does not mean zero contamination (Parker domnick hunter, 2010). A compressor maker writes that "In Atlas Copco terminology, Class 0 always refers to air produced by an oil-free compressor, not to achieving lower oil content through additional filtration stages" (Atlas Copco, 2026). Another compressor maker states that its oil-free lines "are certified to meet the Class 0 standard (as per ISO 8573-1:2010)" (Hitachi Global Air Power, 2026). Its page also places "(0.01 mg/m3 of oil as a contaminant)" right after the words "more stringent than Class 1", which a reader can take as the class 0 limit; 0.01 mg/m3 is the class 1 ceiling (Hitachi Global Air Power, 2026; Parker domnick hunter, 2010).

Public charts also differ on class X: Parker's combined chart prints more than 10 mg/m3 (Parker domnick hunter, 2010), while other charts give only the class 4 limit of 5 mg/m3. Check class X against the purchased ISO text.

How does ISO 8573-1 differ from ISO 12500 filter ratings?

ISO 8573-1 classifies the air in a system, while ISO 12500-1:2007 tests a coalescing filter under fixed conditions, so a filter rating does not by itself prove an ISO 8573-1 oil class at your use point. ISO 12500-1:2007, edition 1, last confirmed in 2020, presents filter performance "as outlet oil aerosol concentration stated in milligrams per cubic metre from results obtained under standard rating parameters" (ISO, 2020).

Two gaps follow. First, ISO 12500-1 covers oil aerosol, not vapor; Parker points to ISO 12500-2 for adsorption filters that remove oil vapor, and notes that "Adsorption filters have a finite ability to remove oil vapour and when their capacity is used up, they must be replaced" (Parker domnick hunter, 2010). Second, the rating holds only at the standard rating parameters, not at your line conditions. Parker writes that the ISO 8573 test methods "were primarily developed to verify air quality in a compressed air system, not test purification equipment" (Parker domnick hunter, 2010).

A working rule: use ISO 12500 ratings to compare filters, and use ISO 8573-2 and ISO 8573-5 measurements at the use point to prove the ISO 8573-1 oil class.

When to call a compressed air specialist about ISO 8573-1 oil classes

Call a compressed air or lubrication specialist about ISO 8573-1 oil classes when a class 0 or class 1 claim has no measured total oil value behind it, or when the claim and your use point do not match.

A supplier quotes class 0 without the agreed number, which Parker calls not in accordance with the standard (Parker domnick hunter, 2010). A class 1 result covers aerosol only, with no ISO 8573-5:2025 vapor result. A test report cites the withdrawn ISO 8573-5:2001 (ISO, 2026). A class 1 train relies on an activated carbon stage with no replacement interval tied to its capacity. The compressor is oil-free but its intake draws contaminated air, which Parker warns can invalidate a class 0 claim (Parker domnick hunter, 2010).

A specialist can compare the compressor and filter documents with the measured results and the class written in your specification. Lubinpla works this kind of case from a site's own technical data sheets, certificates and maintenance history and returns one cited document for review.