ISO 8573-1 is the international standard that defines compressed air quality. If you are specifying compressed air filtration for a new installation, responding to a customer's air purity requirement, or trying to understand what "Class 1" means on a filter data sheet, this guide gives you a complete, practical explanation — with a direct mapping from purity class to the filter grades you need.
What ISO 8573-1 Defines
ISO 8573-1 classifies compressed air quality by three separate contaminant categories: solid particles, water, and oil. Each category is assigned a class number from 0 (the most stringent) to Class X (beyond the scope of the standard — meaning extremely contaminated). The standard does not specify how to achieve a given class — it only defines what the output air must contain (or not contain). The filtration and drying equipment you select determines which class you achieve.
It is important to understand that a Class 1 system requires Class 1 performance across all three contaminant categories simultaneously — particles, water, and oil. You cannot declare Class 1 if you have excellent particle filtration but poor moisture control. The class designation refers to the combination.
The Three Contaminant Categories
| Class | Solid Particles (μm) | Water (PDP °C) | Oil (mg/m³) |
|---|---|---|---|
| 0 | Better than Class 1 | Better than Class 1 | <0.01 |
| 1 | ≤0.1 μm, ≤20,000 particles/m³ | ≤−70°C | ≤0.01 mg/m³ |
| 2 | ≤1 μm, ≤400,000 particles/m³ | ≤−40°C | ≤0.1 mg/m³ |
| 3 | ≤5 μm, ≤90 million particles/m³ | ≤−20°C | ≤1 mg/m³ |
| 4 | ≤15 μm | ≤+3°C | ≤5 mg/m³ |
| 5 | ≤40 μm | ≤+7°C | ≤25 mg/m³ |
| 6 | ≤5 μm quantity limit | ≤+10°C | n/a |
How to Read a Multi-Class Specification
A compressed air quality specification is written as three class numbers separated by colons: Particles : Water : Oil. For example:
- 1:4:1 — Excellent particle filtration, refrigerated drying (+3°C PDP), excellent oil removal. Typical for food contact packaging.
- 2:2:1 — Good particle filtration, desiccant drying (−40°C PDP), excellent oil removal. Typical for pharmaceutical manufacturing.
- 3:4:3 — Moderate filtration, refrigerated drying, moderate oil removal. Typical for general engineering where some contamination is acceptable.
- 1:2:1 — Maximum filtration, desiccant drying to −40°C, trace oil. Typical for electronics cleanrooms and instrument air.
Which Filter Grades Achieve Which Class?
Filter grades in the Omega Air system are designated by letter codes that correspond to removal efficiency. Here is how they map to ISO 8573-1 oil content classes:
| Filter Grade | Removal Rating | Oil Aerosol Residual | ISO 8573-1 Oil Class |
|---|---|---|---|
| P (3 μm particulate) | General particulate | Does not remove oil | — |
| M (0.1 μm coalescing) | 0.1 μm @ ≥99.9% | ≤0.1 mg/m³ | Class 2 |
| S (0.01 μm coalescing) | 0.01 μm @ ≥99.9999% | ≤0.01 mg/m³ | Class 1 |
| ACS (activated carbon) | Oil vapour adsorption | ≤0.003 mg/m³ | Class 0 (oil vapour) |
Filter Sequences for Common Applications
The ISO standard defines the output quality requirement. To achieve it, you need to select the right combination of filters. Always filter in stages — coarser filter before finer filter — to protect each downstream element and maximise element life.
| Application | ISO Class Target | Recommended Filter Train | Dryer Required |
|---|---|---|---|
| Food contact packaging | 1:4:1 | P → S → ACS | Refrigerated (+3°C PDP) |
| Pharmaceutical manufacturing | 2:2:1 | P → M → S → ACS | Desiccant (−40°C PDP) |
| Electronics / cleanroom | 1:2:1 | P → S → ACS | Desiccant (−40°C PDP) |
| Instrument air | 2:2:2 | P → M → S | Desiccant (−40°C PDP) |
| General engineering | 3:4:3 | P → M | Refrigerated (+3°C PDP) |
| Spray painting (automotive) | 2:4:1 | P → M → ACS | Refrigerated (+3°C PDP) |
| Breathing air (SCBA fill) | 1:2:1 | P → M → S → ACS | Desiccant (−40°C PDP) |
| Nitrogen generation feed air | 2:2:1 | P → M → ACS | Desiccant (−40°C PDP) |
The "Class 1" Filter Misconception
The compressed air industry sometimes loosely refers to a high-efficiency coalescing filter as a "Class 1 filter." This is misleading. A single filter element can only address one or two contaminant categories. No single filter achieves ISO 8573-1 Class 1:X:X across particles, water, and oil simultaneously. Achieving Class 1 overall requires a properly designed system with the right filter train, an appropriate dryer, and correctly maintained condensate drains and separators throughout.
When a customer or auditor requires "Class 1 air," clarify which of the three categories they mean — or whether they mean the full 1:1:1 specification. In most food and pharma applications, the practical requirement is 1:4:1 or 1:2:1, not 1:1:1 (which would require −70°C PDP, achievable only with heated desiccant dryers and not necessary for most applications).
- ISO 8573-1 class = Particles : Water : Oil — all three categories together
- Food and pharma typically need 1:4:1 to 2:2:1 — not necessarily 1:1:1
- Oil class 1 (0.01 mg/m³) requires a 0.01 μm coalescing filter (Grade S)
- Water class 2 (−40°C PDP) requires a desiccant dryer — refrigerated is insufficient
- Always filter in sequence: P → coalescing → carbon — never skip the coarser pre-filter
Nitrogenium supplies the full range of Omega Air filter grades — P, M, S and ACS — with all series from screwed BSP (AAF) to high-pressure (HF, CHP). Tell us your application and required ISO class and we will specify the correct filter train.