16 December, 2025
Estimated reading time: 7 minutes
Compressed air is essential for modern industry. Without it, factories would halt, trains would stop, and ships would drift. But not all compressed air is equal. Its quality depends on where and how it is used. In food and pharmaceutical production, the air must be completely clean to protect products and consumers. In other industries, such as automotive or construction, the focus is on reliability and lifetime of air-powered tools.
This is where ISO 8573-1:2010 comes in. It is the international standard that defines compressed air purity classes. The standard categorizes air quality based on the concentration of three main types of contaminants:
Solid particles such as dust, rust, or microplastics that can damage equipment and contaminate products.
Water, in liquid or vapor form, which can cause corrosion, blockages, and microbial growth.
Oil, present as liquid, vapor or aerosols, which can harm sensitive processes and compromise safety in sectors like food and healthcare.
By setting clear limits for each of these categories, ISO 8573 helps companies choose the right treatment equipment (filters, dryers, and separators) to achieve the purity level their applications require.
Understanding the classification system is crucial for two reasons. First, some industries are legally bound to meet strict air quality standards, and non-compliance can lead to fines or production shutdowns. Second, the higher the purity level, the more costly and energy-intensive it is to produce. Striking the right balance not only ensures compliance and product safety but also helps reduce costs and environmental impact.
What is ISO 8573-1:2010?
For those unfamiliar with compressed air systems, choosing the right air purity level can seem complicated. Different industries demand different standards, and contaminants like moisture, dust, and oil each affect performance in their own way. To make things comparable worldwide, the ISO 8573 series of standards was created.
First published in 1991 and revised in 2010, ISO 8573-1:2010 sets the rules for compressed air purity. The update refined the classification system and testing methods to match modern industry needs, ensuring that air quality can be measured consistently everywhere.
The series is made up of nine parts. ISO 8573-1 defines the purity classes, while the other parts cover testing methods for oil, water, particles, gases, and even microorganisms. Together they provide a complete framework for assessing compressed air quality.
Within ISO 8573-1, air purity is classified by three groups of contaminants: solid particles, water, and oil. Each group has several classes, with lower numbers meaning cleaner air. For instance, class 1 particle limits are extremely strict, while class 9 water levels permit up to 10 g/m³. Oil ranges from just 0.01 mg/m³ in class 1 to 5 mg/m³ in class 4.
This system helps companies select the right filtration and drying equipment for their needs. It avoids unnecessary costs while guaranteeing compliance, process safety, and long-term reliability.
ISO 8573-1 purity classes explained
The ISO 8573-1:2010 standard uses a system of purity classes to define compressed air quality. These classes cover three broad categories of contaminants: particles, water, and oil. Each category has its own class scale, with lower numbers indicating cleaner air and stricter limits. This numbering system makes it easy to compare requirements across industries.
For example, the highest purity classes are essential in sectors like electronics, food, and pharmaceuticals. Mid-range purity levels are typically used in general manufacturing, while lower classes are sufficient for basic pneumatic tools.
Overview table
By knowing how each ISO class is defined, you can select the right filtration level for your system, avoiding unnecessary costs and ensuring consistent air purity. See the table below for a quick overview of purity classes.
Note:
While the ISO 8573-1 standard defines several purity classes, Class 0 deserves special mention. In Atlas Copco terminology, Class 0 always refers to air produced by an oil-free compressor, not to achieving lower oil content through additional filtration stages. This distinction ensures true oil-free air at the source, reducing maintenance needs and the risk of contamination in sensitive applications.
Solid particles (P classes)
ISO 8573-1 defines particle classes (P) based on the number and size of solid contaminants in a cubic meter of compressed air. These particles can include dust, rust, pollen, or wear debris from pipes.
Class 1: Very high purity, maximum particle size ≤ 0.1 μm and extremely low particle count. Used in critical environments such as semiconductor manufacturing, pharmaceutical production, and medical applications where even microscopic particles can cause defects or contamination.
Class 2–3: Moderate filtration, particle size up to 1 μm (Class 2) or 5 μm (Class 3). Common in food & beverage, chemical processing, and precision engineering where clean air prevents product contamination and equipment wear.
Class 4–6: Minimal or no filtration, particle size up to 15 μm (Class 4), 40 μm (Class 5), or higher. Typically means no filter is installed, which is not recommended for pneumatic tools or any sensitive equipment, as particles can cause abrasion and premature failure.
Water content (W classes)
The water classes (W) are determined by the moisture level in compressed air, measured through pressure dew point or, in higher classes, by liquid water content.
Class 1: Pressure dew point ≤ −70 °C. Used in ultra-critical environments such as semiconductor manufacturing, pharmaceutical production, and high-precision electronics where even trace moisture can cause defects.
Class 2–3: Pressure dew point between −40 °C and −20 °C. Common in food & beverage processing, chemical plants, and medical applications where low moisture prevents contamination and corrosion.
Class 4–5: Pressure dew point between +3 °C and +7 °C. Typical for general industrial use, such as automotive, metal fabrication, and packaging, where moderate dryness is sufficient to protect tools and equipment.
Oil contamination (O classes)
Oil classes (O) include both aerosol and vapor forms of oil that can enter the system from lubricated compressors or the surrounding environment.
Class 0: Achieve the highest possible purity, 100% oil-free, in accordance with Atlas Copco standards. Essential for industries with the most stringent purity requirements.
Class 1: Allows a maximum of 0.01 mg/m³, required in industries like food, beverage, or healthcare.
Class 2-3: Up to 0.1 mg/m³, often used in general industrial processes.
Class 4-5: Up to 1 mg/m³ and 5 mg/m³ respectively, sufficient for less demanding pneumatic applications.
How to specify compressed air quality
When defining the required compressed air quality, it is important to specify it according to ISO 8573-1:2010. The standard uses a three-part code that indicates the chosen purity class for particles (P), water (W), and oil (O). For example, an air quality requirement of 2.4.1 means Class 2 for particles, Class 4 for water, and Class 1 for oil. This format avoids misunderstandings between suppliers, operators, and inspectors by making the requirement both simple and precise.
Specifying compressed air quality starts with understanding the application. Industries such as pharmaceuticals, food, and electronics often require stricter classes, while general manufacturing or workshop tools can work with mid-range classes. It is also important to balance purity with cost: producing very clean air requires additional filtration, drying, and energy consumption. Defining only the level of purity that is truly needed helps ensure compliance, process safety, and energy efficiency.
Choosing the right compressed air treatment
Once you know the required compressed air ISO class, the next step is choosing the right air treatment equipment. Filters and dryers work together to remove all main contaminants: particles, water, and oil, ensuring the air quality matches your ISO 8573-1 specification.
- Particulate filters remove dust, rust, and other solids, helping achieve the correct particle (P) class.
- Coalescing filters capture oil and water aerosols as well as fine particles, ensuring both oil (O) and particle classes are met.
- Activated carbon filters eliminate oil vapor, odors, and hydrocarbons, required when very pure air is needed, such as in food or pharmaceutical production.
- Dryers reduce the moisture content (W class) by lowering the pressure dew point.
Refrigerant dryers are ideal for general industrial use.
Desiccant dryers provide extremely dry air for sensitive or low-temperature environments.
For example, Atlas Copco’s UD+ coalescing filter is rated [1:-:2], meaning it ensures class 1 particle removal and class 2 oil removal. The dash indicates that it does not treat water, so a dryer or separator would be required if moisture must also be controlled.
When designing your system, always match filters and dryers to your target ISO 8573-1 code.
This ensures compliance, protects your equipment, and avoids unnecessary energy and maintenance costs.
It is vitally important to the user that the compressed air be of the right quality.
Industry applications and requirements
- Food and Beverage Production: Requires Class 1:2:1 air quality to avoid contamination.
- Pharmaceuticals: Needs Class 1:2:1 for critical processes where contaminants could compromise product quality.
- Textile Industry: Class 4:4:3 air is often sufficient for textile machinery, ensuring efficient and uninterrupted operation.
- Industrial Tools: Pneumatic tools in general manufacturing usually perform well with Class 3:4:4 air quality to avoid wear or malfunction.
Note:
ISO 8573-1 purity is expressed as [Particles : Water : Oil].
For example, “Class 1:2:1” means:
- Class 1 for particles
- Class 2 for water (dew point around –40 °C)
- Class 1 for oil (≤ 0.01 mg/m³)
Actual requirements vary by process, climate, and compressor type.
Find the right filtration solution for your ISO requirements
Unsure which ISO class your compressed air system currently meets or should target? Poor air quality can damage equipment, compromise products, and increase operational costs.
Frequently asked questions
What is an example of class 3 air?
Class 3 air allows a moderate level of contamination. For particles, this means up to 90,000 particles in the 0.5–1 micron range per cubic meter. For water, class 3 corresponds to a pressure dew point of –20 °C, and for oil, it allows up to 1 mg/m³. This quality is typically suitable for general industrial use where absolute purity is not critical.
What is the importance of ISO 8573-1 air quality standards?
The standard provides a universal way to define and measure compressed air purity. Without it, manufacturers and users would have no common reference for what “clean air” means. ISO 8573-1 ensures safety, product quality, and equipment reliability, while also helping companies avoid overspending on unnecessary purification.
What is the cleanest ISO 8573 air quality class?
The cleanest level is Class 0, which is stricter than Class 1 and typically defined by the user or manufacturer for highly sensitive applications. It is used where even the tiniest trace of contamination could compromise safety or product quality, such as in pharmaceuticals, electronics, or food packaging.
What does ISO 8573-1:2010 [1,2,1] mean?
This code shows the required purity classes for each contaminant group: Class 1 for particles, Class 2 for water, and Class 1 for oil. It means the air must be extremely clean in terms of solid particles and oil, with moderately dry air for water.
How often should compressed air filters be replaced?
Filter replacement depends on operating conditions, but as a rule, most filters should be changed every 12 months or after 4,000–8,000 operating hours. In harsh environments with heavy contamination, more frequent replacement may be needed. Following the manufacturer’s maintenance schedule helps maintain compliance with your required air filter class and ensures reliable performance.
This article was updated in October 2025; it was originally published in June 2021.
