ISO 8573-1 Compressed-Air Quality Classes Explained

ISO 8573-1 air quality classes define the maximum permitted contamination in compressed air. Read the three-part designation from left to right as particles : water : oil. For example, ISO 8573-1:2010 [2:4:1] means particle Class 2, water Class 4, and oil Class 1.

The classes describe the required air, not a universal equipment package. You must separately select filters, a dryer, drains, pipework, and monitoring that can deliver the requirement under the site's actual pressure, temperature, flow, inlet contamination, and operating cycle.

ISO lists ISO 8573-1:2010 as the published third edition, although it is marked “to be revised.” It applies to compressed-air purity at any location where the air is specified or measured and covers particles, water, and oil while also identifying gaseous and microbiological contaminants. ISO’s official overview explains that scope.

How to read an ISO 8573-1 designation#

The usual format is:

ISO 8573-1:2010 [particle class : water class : oil class]

Thus:

• [1:2:1] means particle Class 1, water Class 2, and oil Class 1.
• [2:4:2] means particle Class 2, water Class 4, and oil Class 2.
• [4:4:3] means particle Class 4, water Class 4, and oil Class 3.

The three numbers are independent. Particle Class 1 does not automatically require water Class 1 or oil Class 1. Define each contaminant limit from the process risk rather than selecting three matching numbers for convenience.

Class 0 also does not mean zero contamination. The user or supplier defines its limit, which must be more stringent than Class 1 and within the measurement capability of the applicable ISO 8573 method. A defensible Class 0 specification therefore includes the contaminant, numerical limit, units, and test method—not only the digit 0.

ISO 8573-1 particle classes#

Particle Classes 1 through 5 use maximum particle counts per cubic metre in defined size bands. Class 6 and Class 7 use mass concentration. A dash below means that the class does not define a limit in that column.

Particle class	0.1–0.5 µm, particles/m³	0.5–1.0 µm, particles/m³	1.0–5.0 µm, particles/m³	Mass concentration
0	User-defined; stricter than Class 1	User-defined	User-defined	User-defined
1	≤20,000	≤400	≤10	—
2	≤400,000	≤6,000	≤100	—
3	—	≤90,000	≤1,000	—
4	—	—	≤10,000	—
5	—	—	≤100,000	—
6	—	—	—	≤5 mg/m³
7	—	—	—	>5 to ≤10 mg/m³
X	—	—	—	>10 mg/m³; state the maximum

These limits are reproduced in Parker’s compressed-air catalog and purity-testing paper, which identify ISO 8573-4 for number concentration and ISO 8573-8 when mass concentration is required. The current ISO 8573-4:2019 page confirms that Part 4 covers particle size and number concentration, measurement limitations, sampling, evaluation, and uncertainty. ISO 8573-8:2004 covers solid-particle mass concentration.

What the particle class means for filtration#

A finer class usually requires staged treatment, not simply a filter with a matching micron label. A practical arrangement can include:

1. A separator or general-purpose filter for bulk liquid and coarse contamination.
2. A coalescing stage for fine liquid aerosols and particles.
3. A high-efficiency particle or coalescing stage where the specified count requires it.
4. A dry particulate after-filter downstream of a desiccant dryer when desiccant dust is a risk.
5. Point-of-use filtration where distribution piping or local equipment can reintroduce particles.

Nominal pore size, removal efficiency, and ISO 8573 outlet class are different claims. Ask for the test standard, challenge aerosol, inlet concentration, flow, pressure, and complete filter configuration behind any published performance statement. Our filtration supplier framework and modular filter verification framework can organize those questions, but they cannot determine an installed system’s ISO class without measurements.

ISO 8573-1 water classes#

Water Classes 1 through 6 specify a maximum pressure dew point. Classes 7 through 9 apply where liquid water is present and use liquid-water mass concentration.

Water class	Pressure dew point	Liquid-water concentration
0	User-defined; stricter than Class 1	User-defined
1	≤−70°C	—
2	≤−40°C	—
3	≤−20°C	—
4	≤+3°C	—
5	≤+7°C	—
6	≤+10°C	—
7	—	≤0.5 g/m³
8	—	>0.5 to ≤5 g/m³
9	—	>5 to ≤10 g/m³
X	—	>10 g/m³; state the maximum

ISO 8573-3:1999 covers water-vapour humidity measurement, including sampling, method selection, limitations, uncertainty, and reporting. It does not cover liquid water. ISO 8573-9:2004 provides the corresponding methods for liquid-water mass concentration.

Pressure dew point is not atmospheric dew point#

Pressure dew point, or PDP, is the temperature at which water vapour starts to condense at the compressed-air pressure. Atmospheric dew point is the condensation temperature after the air expands to atmospheric pressure.

Expansion reduces the water-vapour partial pressure, so atmospheric dew point is normally lower than the PDP of the same moisture load. A dryer described as producing a −40°C PDP is therefore not making the same statement as one producing a −40°C atmospheric dew point.

Every moisture requirement and test report should state:

• Whether the value is PDP or atmospheric dew point.
• The measurement pressure.
• The measurement location.
• Gas temperature and flow conditions.
• Instrument method, range, calibration status, and uncertainty.
• Whether the system was at stable operating conditions.

What the water class means for dryer selection#

A Class 4 target of +3°C PDP commonly points toward a correctly sized refrigerated dryer. Classes 1 through 3 generally call for adsorption or another low-dew-point drying technology. That is a starting point, not an automatic selection rule.

The dryer must maintain the required PDP at the worst credible inlet temperature, ambient temperature, pressure, and flow. Account for compressor unload cycles, seasonal humidity, pressure drop, condensate drainage, and bypass leakage. For low-PDP applications, compare documented purge demand, controls, prefiltration, after-filtration, and rated conditions in our guide to the best desiccant air dryers for point-of-use service.

ISO 8573-1 oil classes#

Oil class concerns total oil: liquid oil, oil aerosol, and oil vapour combined.

Oil class	Maximum total oil concentration
0	User-defined; 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; state the maximum

The limits appear in Parker’s compressed-air and gas filtration catalog. ISO divides the measurement work between two parts: ISO 8573-2:2018 covers liquid oil and oil aerosols, while ISO 8573-5:2025 covers pressurized sampling and gas-chromatography measurement of oil vapour.

What the oil class means for treatment#

Coalescing filters remove entrained liquid and aerosols but do not, by themselves, establish a low total-oil result because oil vapour is part of the total. A stringent oil class can require:

• Bulk-liquid separation and reliable condensate drains.
• One or more coalescing stages.
• Controlled air temperature, because vapour can condense downstream.
• An adsorption stage, such as activated carbon, for oil vapour.
• Protection against contaminated intake air, pipe deposits, compressor carryover, and bypass leakage.

An “oil-free” compressor does not by itself prove an oil class at the use point. Ambient hydrocarbon vapour and downstream contamination remain possible. Conversely, the presence of an oil-lubricated compressor does not define the measured outlet class; the complete treatment system and actual test result do.

The ISO 8573 test-method parts#

ISO 8573-1 defines classifications. The other parts define measurement scopes:

Part	Measurement scope
ISO 8573-2:2018	Liquid oil and oil aerosol
ISO 8573-3:1999	Humidity and water vapour
ISO 8573-4:2019	Particle size and concentration by number
ISO 8573-5:2025	Oil vapour
ISO 8573-6:2003	Gaseous contaminants
ISO 8573-7:2003	Viable microbiological contaminants
ISO 8573-8:2004	Solid particles by mass concentration
ISO 8573-9:2004	Liquid-water mass concentration

The applicable parts depend on the specified contaminants and classes. Notice that the current oil-vapour method is ISO 8573-5:2025; the 2001 edition cited in older literature has been withdrawn and replaced.

Component performance versus system-level air quality#

A filter, dryer, or packaged treatment skid can have documented performance under defined inlet and operating conditions. That evidence supports component selection. It does not establish purity at an arbitrary production outlet.

The installed system can differ because of:

• Higher inlet contamination or flow than the rated condition.
• Dryer or filter bypass leakage.
• Saturated elements or adsorbent.
• Failed or incorrectly installed drains.
• Oil, rust, scale, or microbial material in receivers and distribution pipes.
• Moisture condensation in a cold downstream section.
• Contamination added by hoses, regulators, lubricators, or process equipment.

ISO 8573-1 explicitly permits purity to be specified or measured at different system locations. State the required location in the purchase specification. If the process risk exists at the tool or product-contact outlet, a compressor-room reading alone does not answer the system-level question.

Use our compressed-air treatment systems guide to identify equipment categories and documented configurations. Use sampling at the specified boundary to determine delivered-air purity.

How to write a defensible requirement#

A useful procurement requirement should include more than three class numbers. A concise example is:

Compressed air at outlet POA-12 shall meet ISO 8573-1:2010 [2:2:1] at normal production flow and the stated maximum demand. Measure particles to ISO 8573-4:2019, humidity to ISO 8573-3:1999, oil aerosol and liquid oil to ISO 8573-2:2018, and oil vapour to ISO 8573-5:2025. Record pressure, temperature, flow condition, sampling arrangement, calibration, uncertainty, and stabilization period.

Then define:

1. Sampling point: Name the physical boundary and whether the sample represents full flow or an isokinetic partial flow.
2. Operating envelope: State minimum and maximum pressure, flow, inlet temperature, ambient temperature, and relevant compressor modes.
3. Timing: Define stabilization, sampling duration, production state, and any seasonal or load cases.
4. Test methods: Cite the applicable edition for every contaminant.
5. Acceptance rule: State how measurement uncertainty affects pass or fail.
6. Frequency: Set commissioning, periodic, post-maintenance, and event-triggered tests from process risk.
7. Class 0 or X limits: Add explicit numerical limits and units.
8. Report contents: Require instrument identification, calibration traceability, sampling configuration, raw results, conditions, and deviations.

Parker’s purity-testing technical paper explains why convenient tee fittings and non-standard instruments can produce only indicative results. It states that classification requires the sampling methods and equipment specified by the relevant ISO 8573 parts.

Practical specification checklist#

Before buying treatment equipment or accepting a compliance statement, confirm:

• The notation is in particle-water-oil order.
• Each selected class follows the process or product risk assessment.
• PDP is not confused with atmospheric dew point.
• Total oil includes liquid, aerosol, and vapour.
• Class 0 has a stated numerical limit.
• Class X has a stated maximum concentration.
• Equipment ratings cover the worst operating conditions.
• Filter and dryer combinations are documented as complete configurations.
• The measurement point matches the contractual point of use.
• Sampling and measurement use the applicable ISO 8573 parts.
• The report states conditions, calibration, uncertainty, and deviations.
• Retesting is required after treatment changes or contamination events.

Frequently asked questions#

What does ISO 8573-1 Class 1:2:1 mean?#

The figures identify particle, water, and oil classes in that order. Particle Class 1 allows no more than 20,000 particles/m³ from 0.1–0.5 µm, 400 particles/m³ from 0.5–1.0 µm, and 10 particles/m³ from 1.0–5.0 µm. Water Class 2 requires a PDP of −40°C or lower. Oil Class 1 limits total oil to 0.01 mg/m³.

Does Class 0 mean completely contaminant-free air?#

No. Class 0 is a specified numerical limit that is more stringent than Class 1 and remains within the applicable measurement capability. Define it separately for particles, water, or oil. “Class 0” without a limit and test basis is incomplete.

Does an ISO-rated filter make the complete system compliant?#

No. It can support a treatment design when its documented test conditions match the application. Compliance at a use point requires representative sampling and measurement of the installed system under defined conditions.

Sources#

• Current standard, class order, scope, and measurement-location principle: ISO 8573-1:2010
• Particle, water, and oil class-limit tables: Parker compressed-air and gas filtration catalog
• Sampling requirements, classification limitations, and component-versus-system context: Parker compressed-air purity testing
• Liquid oil and oil-aerosol methods: ISO 8573-2:2018
• Humidity and water-vapour methods: ISO 8573-3:1999
• Particle number concentration: ISO 8573-4:2019
• Current oil-vapour method: ISO 8573-5:2025
• Particle mass concentration: ISO 8573-8:2004
• Liquid-water concentration: ISO 8573-9:2004