How to Read Air Quality Classes: ISO 8573-1 Particles, Water, and Oil Explained at Once

Pneumatic Technology Explained · Air Supply Series

Let’s get straight to the conclusion: whether an air supply is “clean” cannot be judged by feel — it must be judged by the standard. ISO 8573-1 classifies compressed air quality in three independent categories — particles, water, and oil — expressed as three numbers in the form “particles : water : oil” (e.g. 1:2:1). Fix on this class, and both selection and acceptance have a basis.

The Three Classifications of ISO 8573-1

The particle class is graded by the maximum number and size of particles per cubic meter of air; the water class by pressure dew point; the oil class by oil concentration. The three are independent of each other, so all three numbers must be read together — none can be omitted.

For example, “1:2:1” means particle class 1, dew point class 2, oil class 1. Requirements differ by industry: general pneumatics is fine at around 7:5:4, while food and electronics need 1:1:1 or stricter. The exact class thresholds follow the ISO 8573-1 classification tables.

Typical Air Quality Requirements by Industry (Indicative Examples)

Industry	Particles	Water (dew point)	Oil
General pneumatics	Class 7–8	Class 5–7	Class 4–5
Food / pharmaceutical	Class 1–2	Class 2–3	Class 1
Electronics / semiconductor	Class 1	Class 1–2	Class 1

Field Notes: Mistakes We Have Seen

1. Buying an “oil-free compressor” and assuming the air supply now complies. Oil-free only solves oil — particles and water still need treatment.
2. Reading the class numbers backwards. In ISO 8573, smaller numbers mean cleaner — a big number does not mean clean.
3. Quoting only one number. Always state all three as “particles : water : oil” — none can be missing.
4. Ignoring the measurement location. Air quality must be measured at the point of use, not at the compressor outlet.

Frequently Asked Questions (FAQ)

Q1: How do I read the ISO 8573-1 classes?

It is expressed as three numbers “particle class : water class : oil class”, e.g. 1:2:1, corresponding to particle content, pressure dew point, and oil concentration. Read all three together — never just one. Each number maps to the classification table in the standard: particles by maximum count and size, water by dew point temperature, oil by concentration in mg/m³. Because the air quality you need is three-dimensional — particles, water, and oil must all comply — selection and acceptance must be aligned on “all three numbers”.

Q2: Is a bigger number cleaner or a smaller one?

Smaller is cleaner. In ISO 8573-1, class 1 is the strictest (cleanest), and larger numbers are more lenient. For example, oil class 1 demands extremely low content, while class 4 is far more relaxed. This is the easiest thing to get backwards — many people assume a bigger number means “higher class, cleaner”. Remember: “1 is strictest; the smaller the number, the cleaner.” Only then can you correctly read the class a manufacturer states, and not mistake a lenient class for a premium specification — something to watch especially carefully at selection time.

Q3: How do I determine what class my process needs?

Work backwards from “the most sensitive link”. First identify which of particles, water, and oil your equipment or product is most sensitive to: for example, food touches the product directly, so oil must be class 1; electronics fears moisture, so the dew point must be very low; precision cylinders fear particle scratches, so the particle class must be high. List the requirements of every link and take the strictest value — that is the class the whole air supply must reach. When in doubt, refer to industry practice for similar processes, or have the equipment manufacturer or air-supply supplier confirm — avoiding both “over-configuration waste” and “under-configuration non-compliance”.

Q4: Does an oil-free compressor automatically mean oil class 1?

No. An oil-free compressor simply keeps lubricating oil out of the compression chamber, so outlet oil content is naturally very low — but “low” is not the same as “class 1”. Moreover, residual oil in the piping or oil-lubricated components downstream can still introduce oil into the system. To confirm oil class 1, measure the oil concentration at the point of use — do not judge by whether the compressor is oil-free. An oil-free compressor is the “near-oil-free” foundation; the final class must rest on measurement and the standard. Do not equate “oil-free model” with “class 1 compliant”.

Q5: Where should air quality be measured for accuracy?

At the point of use (the end of the line), not at the compressor outlet. Between the compressor and the point of use, air passes through the receiver tank, piping, and after-treatment, and can pick up moisture, oil, and corrosion particles along the way — a clean compressor outlet does not guarantee clean air at the end. So in acceptance and routine checks, place the sampling point just upstream of the key air-consuming equipment; only then does the measurement represent “the air actually delivered to the components”. This is also why the end-of-line FRL matters — it catches the secondary contamination picked up in the piping.

Conclusion

Fix on the three ISO 8573-1 classes “particles : water : oil”, and the cleanliness of the air supply gains a quantifiable yardstick — with a solid basis for selection, acceptance, and routine inspection.

Suggested Illustrations

1. ISO 8573-1 classification table: side-by-side class tables for particles / water / oil, so readers can check for themselves.
2. Industry requirement comparison: bars or color blocks showing how general pneumatics / food / electronics differ.

The above content was compiled by the LEANYEE pneumatic technical team from hands-on commissioning experience, for reference by industry peers.

Technical Disclaimer

The fitting selection, thread types, and technical parameters discussed in this article are based on applicable international standards, including ISO 4414 (Pneumatic Fluid Power — General Rules and Safety Requirements), ISO 8573 (compressed air quality), ISO 6358 (flow characteristics), and relevant thread standards such as ISO 7-1, ISO 228-1, and ANSI/ASME B1.20.1, together with publicly available technical information from international manufacturers such as SMC, Festo, and Parker.

This article is intended for general technical reference only. Actual performance may be affected by cylinder bore size, load, operating pressure, temperature, piping layout, and other application-specific factors.

On-site testing and commissioning are recommended where necessary. Always refer to the product documentation and technical parameters confirmed by the manufacturer for the specific product being used.

LEANYEE Technical Team

Topic: The ISO 8573-1 air quality standard | Target readers: junior designers, process engineers