How does the ISO 8573-1 three-number purity class format work?

ISO 8573-1 expresses compressed air purity as three class numbers separated by colons in the format Particle Class : Water Class : Oil Class. Each number is independent and refers to a separate contamination category. Class 1:4:2 means: particle Class 1 (≤20,000 particles/m³ ≥0.1 µm), water Class 4 (pressure dewpoint ≤+3°C), and oil Class 2 (≤0.1 mg/m³ total oil). Lower class numbers represent cleaner air. Class 0 (highest purity) is not defined by the standard — it is application-specific and defined by the compressed air user and supplier through agreement. Class 0 requires oil-free compressor technology plus downstream treatment; Classes 1–4 are achievable with properly selected downstream filtration and drying equipment.

What are the ISO 8573-1 particle classes and what concentrations do they represent?

ISO 8573-1 defines particle classes based on particle concentration in particles per cubic metre at specified size ranges. Class 1: ≤20,000 particles/m³ at ≥0.1 µm (the cleanest defined class). Class 2: ≤400,000 particles/m³ at ≥0.1 µm and ≤6,000/m³ at ≥0.5 µm. Class 3: ≤90,000 particles/m³ at ≥0.5 µm and ≤1,000/m³ at ≥1.0 µm. Class 4: ≤10,000 particles/m³ at ≥1.0 µm. Class 5: ≤100,000 particles/m³ at ≥1.0 µm. Higher classes (6–9) cover less clean applications. Instrument air typically requires Class 2 or better; pneumatic actuators in process control require Class 3–4; blow-off air may accept Class 5–6.

What do the ISO 8573-1 water classes represent in terms of pressure dewpoint?

ISO 8573-1 water classes are defined by pressure dewpoint (PDP) — the temperature at which water vapour in compressed air at system pressure would begin to condense. Class 1: PDP ≤−70°C (ultra-dry, cryogenic dryer required). Class 2: PDP ≤−40°C (membrane or pressure-swing adsorption dryer). Class 3: PDP ≤−20°C (regenerative adsorption dryer). Class 4: PDP ≤+3°C (refrigeration dryer). Class 5: PDP ≤+7°C (refrigeration dryer at higher dew point). Class 6: liquid water content ≤5 g/m³ (partially dried or partially treated). For instrument air and pneumatic controls in environments where ambient temperature exceeds +3°C, Class 4 (PDP ≤+3°C) prevents condensation in distribution pipework. DRYCORE™ pneumatic brake-system air-dryers are sized to the water-class target specified for the approved vehicle application.

What are the ISO 8573-1 oil classes and what sources of oil contamination do they address?

ISO 8573-1 oil classes specify total oil content (liquid oil + oil aerosol + oil vapour) in mg/m³ at line pressure and temperature. Class 1: ≤0.01 mg/m³ (ultra-clean, requires activated carbon adsorber). Class 2: ≤0.1 mg/m³ (high-efficiency coalescing filter plus activated carbon). Class 3: ≤1 mg/m³ (coalescing filter alone achieves this for most lubricated compressors). Class 4: ≤5 mg/m³ (general industrial applications without strict oil requirements). Oil contamination sources include: compressor lubricant carried over as aerosol droplets (addressed by coalescing filters), oil vapour evaporated from lubricant in the compression stage (addressed by activated carbon adsorbers), and atmospheric hydrocarbon vapour drawn in through the compressor inlet. For food-contact and pharmaceutical applications, ISO 8573-1 Class 1 oil specification (≤0.01 mg/m³) is mandatory.

What compressed air treatment train is required to achieve ISO 8573-1 Class 1:4:1?

Achieving ISO 8573-1 Class 1:4:1 requires a four-to-five stage treatment train downstream of the air compressor. Stage 1: Bulk separator (removes liquid water and oil aerosol >10 µm). Stage 2: Pre-filter with coalescing element (removes oil aerosol to ≤1 mg/m³, particles >1 µm). Stage 3: Refrigeration dryer (achieves PDP ≤+3°C, satisfying water Class 4). Stage 4: High-efficiency coalescing filter (removes oil aerosol to ≤0.01 mg/m³, satisfying oil Class 1). Stage 5: Activated carbon adsorber (removes oil vapour to ≤0.003 mg/m³). An optional dust/carbon fines filter after the adsorber removes activated carbon particulate to satisfy particle Class 1.

How does ISO 8573-2 complement ISO 8573-1 for particle measurement?

ISO 8573-1 defines the purity classes — the acceptable maximum contamination levels. ISO 8573-2 (Compressed Air — Part 2: Test Methods for Aerosol Oil and Particle Content) defines the measurement methods for determining whether the air meets those classes. ISO 8573-2 specifies the use of optical particle counters (OPC), laser particle counters, and gravimetric methods for measuring solid particle content at the size ranges relevant to ISO 8573-1 particle classes. Without ISO 8573-2 compliant measurement, there is no technically valid means to verify that the compressed air system meets its specified ISO 8573-1 particle class — specification alone without measurement verification provides no guarantee of actual air quality.

What ISO 8573-1 class is required for food and beverage packaging applications?

Food and beverage packaging applications require ISO 8573-1 Class 1:2:1 as a minimum for direct product contact air. Class 1 particles (≤20,000/m³ at ≥0.1 µm) prevents particulate contamination of packaging surfaces. Class 2 water (PDP ≤−40°C) prevents condensation inside packaging lines operating at variable temperature. Class 1 oil (≤0.01 mg/m³) prevents oil contamination of food contact surfaces, which would violate food safety regulations. Additional requirements beyond ISO 8573-1 may apply under specific national food safety regulations (FDA 21 CFR in the USA; EC 1935/2004 in Europe) for compressor lubricants and materials of construction in the air treatment system contacting the product stream.

What is the consequence of under-specifying ISO 8573-1 purity class for pneumatic instrument air?

Pneumatic instrument air powers control valves, positioners, and pneumatic transmitters in process plants. Under-specifying purity class causes predictable failure modes. Excess particle contamination (Class 4 specified where Class 2 needed): plugging of 100–500 µm valve orifices and bleed ports in positioners, causing erratic valve position. Excess water (Class 6 or 5 where Class 4 needed): condensate accumulation in instrument tubing, causing freeze blockage below 0°C ambient and bacterial/algae growth in hot climates. Excess oil (Class 3 where Class 1 needed): sticky deposits on positioner internals that cause valve hysteresis and permanent valve offset errors. These failure modes cause process upsets and safety instrument system (SIS) degradation — the cost of correct ISO 8573-1 specification is typically less than 5% of one unplanned process shutdown.

Does ISO 8573-1 address microbiological contamination in compressed air?

ISO 8573-1:2010 does not include a microbiological contamination class — it addresses solid particles, water, and oil only. Microbiological compressed air quality for pharmaceutical and medical device manufacturing is addressed by ISO 8573-7 (Part 7: Test method for viable microbiological contaminant content) and by GAMP-5 quality system requirements. For pharmaceutical compressed air applications (EU GMP Annex 1 cleanrooms), viable microbiological contamination limits are specified in the pharmaceutical product manufacturing authorization rather than in ISO 8573-1 directly. Validated microbiological sampling per ISO 8573-7 using impingement samplers at point-of-use is required to demonstrate compliance with pharmaceutical manufacturing authority (EMA, FDA) requirements.

How does altitude affect ISO 8573-1 compressed air purity classification?

ISO 8573-1 purity classes are defined at compressed air system pressure conditions (typically 7 bar gauge for industrial systems). The water class (pressure dewpoint) is specified at the system pressure — when air expands to atmospheric pressure at point of use, the dewpoint shifts relative to the compressed system dewpoint by the pressure ratio. A system achieving PDP −40°C at 7 bar gauge would have an atmospheric-equivalent dewpoint of approximately −12°C. At high-altitude facilities (above 2,000 m), atmospheric pressure is lower, which reduces the pressure ratio relative to sea-level calculations and shifts the atmospheric-equivalent dewpoint. For high-altitude compressed air applications, dewpoint calculations must account for local barometric pressure to verify that the specified PDP prevents condensation at actual atmospheric conditions.

Knowledge Center›Standards›ISO 8573-1

INDUSTRIAL STANDARD · 2010

ISO 8573-1

Compressed Air — Contaminant Classes and Purity Requirements

Classification of compressed air purity by contamination class for solid particles, water (liquid and vapor), and total oil (liquid, aerosol, and vapor).

KEY PARAMETERS

Particle:Water:Oil class numbers

Format

Class 2:4:1

Instrument air minimum

<20,000/m³ at ≥0.1 µm

Class 1 particles

01 /

Purity Class Structure

ISO 8573-1 specifies compressed air purity using three independent class numbers in the format X:Y:Z — where X is the particle class (1–9 or 0), Y is the water class (1–9 or 0), and Z is the oil class (1–4 or 0). Lower numbers represent higher purity. Class 1:4:1 represents particle concentration <20,000 per m³ at ≥0.1 µm, pressure dewpoint ≤+3°C, and total oil <0.01 mg/m³. Class 0 (highest purity) is application-specific and defined by the equipment supplier and end user.

02 /

Application Requirements

Typical application requirements: pneumatic general service Class 5:4:3; instrument air Class 2:4:1; food contact Class 1:2:1; pharmaceutical filling Class 1:2:1. ISO 8573-1 is used in conjunction with ISO 8573-2 (particle measurement), ISO 8573-3 (humidity and water measurement), and ISO 12500 (coalescing filter test). DRYCORE™ pneumatic brake-system air-dryers are selected against the water-class (pressure dewpoint) target specified for the approved vehicle application.

03 /

Treatment Stage Requirements

Achieving Class 1:4:1 requires a multi-stage compressed air treatment train: pre-filter (bulk liquid and >3 µm particles), refrigeration dryer (pressure dewpoint 2–5°C), coalescing filter (oil aerosol to 0.01 mg/m³), activated carbon (oil vapor to 0.005 mg/m³), and post-filter (carbon fines removal). Each stage is tested and classified individually against the applicable ISO 8573 part.

COMMON ENGINEERING MISTAKES

Specifying ISO 8573-1 Class 0 (oil-free) for applications that only require Class 1 (≤0.01 mg/m³ oil aerosol). Class 0 requires an oil-free compressor; Class 1 is achievable with downstream coalescing filtration on a lubricated compressor — a significant cost difference.

Not distinguishing between ISO 8573-1 oil aerosol content (measured by 8573-2) and total hydrocarbon content (aerosol + vapour). A system meeting Class 1 oil aerosol may still contain oil vapour requiring activated carbon treatment.

Applying ISO 8573-1 purity class specifications at a single point in the system without accounting for recontamination from distribution pipework. Point-of-use air quality must be measured at the application, not at the dryer outlet.

ENGINEERING REFERENCES

STANDARD

ISO 8573-1:2010, Compressed Air — Part 1: Contaminants and Purity Classes, ISO Geneva

Primary compressed air purity classification standard defining particle, water (pressure dewpoint), and oil content classes; the water-class portion of this framework is relevant to specifying DRYCORE™ pneumatic brake-system air-dryer requirements.

STANDARD

ISO 8573-2:2018, Compressed Air — Part 2: Test Methods for Aerosol Oil and Particle Content, ISO Geneva

Measurement method standard complementing ISO 8573-1; specifies optical particle counter and gravimetric methods for verifying compressed air meets specified particle and oil aerosol purity classes.

STANDARD

ISO 8573-4:2019, Compressed Air — Part 4: Test Methods for Solid Particle Content, ISO Geneva

Gravimetric and optical test methods for measuring solid particle content in compressed air at the concentration levels relevant to ISO 8573-1 Class 1–4 particle specifications.

STANDARD

ISO 12500-1:2007, Filters for Compressed Air — Test Methods — Part 1: Oil Aerosol, ISO Geneva

Test standard for measuring oil aerosol removal efficiency of coalescing compressed air filters; required for confirming that installed coalescing filter elements achieve ISO 8573-1 oil class requirements at point of use.

FREQUENTLY ASKED QUESTIONS — ISO 8573-1

ENGINEERING ARTICLES REFERENCING ISO 8573-1

Compressed Air Dryer Selection: Refrigeration, Desiccant, and Membrane Drying Technologies vs ISO 8573-1 Dew Point Classes

12 min read

Compressed Air Purity

10 min read

Compressed Air Quality Verification — ISO 8573-4, -5, and -7

12 min read

ISO 29463 High-Efficiency Particulate Air Filters: HEPA and ULPA Classification, Testing, and Application

11 min read

ISO 8573-1

Purity Class Structure

Application Requirements

Treatment Stage Requirements

Related Engineering Content

Connect the test method or standard to the correct filtration application.