Specifying the wrong compressed air quality class is one of the most common — and costly — mistakes in industrial plant design. Too lax a specification risks contamination of products, instruments, and processes; too stringent a specification drives up capital and operating costs unnecessarily. ISO 8573 gives plant engineers a structured, internationally recognised framework to get this right. This guide explains the standard series, its purity classes, and how to select the correct filtration stages to achieve them.
What Is ISO 8573?
ISO 8573 is a multi-part international standard that defines compressed air quality. It covers three primary contaminant groups — particles, water, and oil — and assigns each a numerical purity class from 0 (most stringent) to 9 (least stringent). The standard is widely adopted across Europe and is referenced in pharmaceutical GMP guidelines, food safety regulations, and industrial equipment specifications.
The series comprises several parts, each addressing a specific aspect of compressed air quality:
- ISO 8573-1 — Contaminants and purity classes (the classification table)
- ISO 8573-2 — Test methods for oil aerosol content
- ISO 8573-3 — Test methods for humidity and water content
- ISO 8573-4 — Test methods for solid particle content
- ISO 8573-6 — Test methods for gaseous contaminant content
Understanding the Three Contaminant Groups
1. Solid Particles
Particles in compressed air originate from the atmosphere, compressor wear, pipework corrosion, and filter media degradation. ISO 8573-1 classifies particle contamination by both particle size (0.1–0.5 µm, 0.5–1 µm, 1–5 µm) and concentration (particles per m³). Class 1 permits fewer than 20,000 particles per m³ at 0.1–0.5 µm; Class 5 allows up to 100,000 particles per m³ at 0.5–1 µm. For critical applications such as instrument air or food contact, Class 1 or Class 2 is typically required.
Achieving low particle classes requires a staged filtration approach. A compressed air filter train typically begins with a coarse pre-filter, followed by a high-efficiency coalescing stage, and optionally a particulate after-filter. R+F FilterElements offers the RF-C coalescing element series and RF-P particulate elements, both rated at 99.99% efficiency at ≥ 0.1 µm and ≥ 0.3 µm respectively, making them suitable for achieving ISO 8573-1 Class 1 and Class 2 particle ratings.
2. Water and Humidity
Water is the most damaging contaminant in compressed air systems. It causes corrosion, promotes microbial growth, and degrades lubricants and seals. ISO 8573-1 classifies water content by pressure dew point (PDP), ranging from Class 1 (PDP ≤ −70 °C) to Class 9 (no limit). For outdoor pipework in northern European climates, Class 4 (PDP ≤ +3 °C) is a common minimum; pharmaceutical and electronics applications typically demand Class 1 or Class 2.
Dew point is primarily controlled by the dryer type — refrigerant dryers achieve Class 4–5, while adsorption dryers reach Class 1–2. Filtration alone cannot reduce dew point, but coalescing filters are essential upstream of dryers to remove bulk liquid water and oil aerosols that would otherwise saturate or contaminate the desiccant bed.
3. Oil Content
Oil enters compressed air from lubricated compressors, atmospheric hydrocarbons, and pipework contamination. ISO 8573-1 classifies total oil content (aerosol + vapour + liquid) in mg/m³. Class 1 requires ≤ 0.01 mg/m³; Class 2 ≤ 0.1 mg/m³; Class 3 ≤ 1 mg/m³. Oil-free compressors still require filtration to remove atmospheric hydrocarbons, which can reach 0.05–0.5 mg/m³ in industrial environments.
ISO 8573-1 Purity Classes at a Glance
Identify the most critical end use
Which Filtration Stages Achieve Which Classes?
| Filtration Stage | Element Type | Particle Class Achievable | Oil Class Achievable |
|---|---|---|---|
| Pre-filter (bulk separation) | RF-P particulate | Class 3–4 | Class 3–4 |
| Coalescing filter | RF-C coalescing | Class 1–2 | Class 2 (≤ 0.1 mg/m³) |
| Activated carbon adsorber | RF-AC / RF-DIA | Class 1 (combined) | Class 1 (≤ 0.01 mg/m³) |
| Point-of-use inline filter | RF-DIL | Class 1–2 (final polish) | Supplementary |
Use our free Engineering Tool to get a filtration recommendation for your specific application in under 2 minutes.
Selecting the Right Class for Your Application
The correct ISO 8573-1 class depends on the end use of the compressed air. Below are common application categories and their typical requirements:
- Pharmaceutical manufacturing (direct product contact): Particle Class 1, Water Class 1–2, Oil Class 1. Validated filtration with documented change-out intervals is mandatory.
- Food and beverage (incidental contact): Particle Class 1–2, Water Class 2–3, Oil Class 1. Process gas filtration principles apply equally here.
- Instrument air (control valves, analysers): Particle Class 2, Water Class 2–4 (depending on ambient temperature), Oil Class 2. See our guide on coalescing vs particulate filter elements for element selection guidance.
- General pneumatics (cylinders, tools): Particle Class 3–5, Water Class 4–6, Oil Class 3–5. Standard pre-filter and coalescing stage is usually sufficient.
- Breathing air (EN 12021): Governed by a separate standard, but ISO 8573-1 Class 1 for all three contaminants is a common starting point.
For applications involving hydrogen, oxygen, or other process gases, the ISO 8573 framework still applies but must be supplemented with gas-specific purity standards. Our process gas filter range is designed for these demanding environments.
Practical Steps for Specifying Compressed Air Quality
Follow this sequence when specifying compressed air quality for a new or upgraded system:
- Identify the most critical end use — the most stringent application in the system sets the minimum class for the entire distribution network, or for that branch if zoned.
- Specify all three contaminant classes — write the full ISO 8573-1 class designation, e.g. "ISO 8573-1:2010 Class 1.2.1" (particles.water.oil).
- Select filtration stages — use the table above to map required classes to filter types. Always include a pre-filter upstream of coalescing stages to extend element life.
- Verify with ISO 8573-4 particle testing — commission-stage testing using a calibrated particle counter confirms compliance before handover.
- Plan element change-out intervals — coalescing elements should be replaced at least annually or when differential pressure exceeds 0.35 bar. Use the R+F Engineering Sizing Tool to calculate flow-based intervals.
For a deeper dive into how coalescing elements work and how to select between RF-C and RF-P types, see our article on coalescing vs particulate filter elements. For high-pressure or process gas applications where ISO 8573 classes must be met at elevated pressures, the R+F filter elements range includes K-type (sour gas) and sintered metal variants rated to 450 °C.
- ISO 8573 is a multi-part international standard that defines compressed air quality.
- Particles in compressed air originate from the atmosphere, compressor wear, pipework corrosion, and filter media degradation.
- Pharmaceutical manufacturing (direct product contact):
- Identify the most critical end use
Related Reading
- Coalescing vs Particulate Filter Elements — Which Do You Need?
- ISO 8573-1 Compressed Air Quality — A Practical Overview
- Oxygen Filtration Safety — What Plant Engineers Must Know
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