Understanding ISO 8573-1 and What ‘Class 0’ Means

Compressed air is a fundamental part of industry, and its purity affects the quality, safety and continuity of your operations. When air carries contaminants, it can impact product integrity and equipment reliability. For instance, in pharmaceuticals and food and beverage, a single trace of oil vapour can compromise products, while excess moisture in data centre cooling can threaten uptime.

That’s why global industries rely on ISO 8573, and specifically ISO 8573-1, the internationally recognised standard for defining compressed air purity. This framework sets clear, measurable limits for three primary contaminant groups – particles, water and oil – allowing businesses to specify, validate and maintain the level of air quality their processes demand.

This guide explains ISO 8573-1 Class 0, including what it means, how it differs from standard purity classes and what organisations should look for when they see ‘Class 0’ on compressors, filtration systems or air treatment equipment.

ISO 8573 is a comprehensive, multi-part standard developed to bring clarity and consistency to compressed air quality across industries. Rather than treating air purity as a vague concept, the standard breaks it into measurable parameters and defines how each should be tested. Each part of ISO 8573 focuses on a different aspect of air quality, from measurement methods to test equipment, giving operators a complete framework for specifying, evaluating and verifying the performance of their compressed air systems.

At the heart of the series is ISO 8573-1:2010, the section most frequently referenced by engineers, OEMs and quality managers. ISO 8573-1 defines the purity classes used worldwide to classify compressed air. These numerical classes set allowable limits for the three major contaminant groups, enabling organisations to clearly specify what level of cleanliness their processes require.

The standard focuses on three primary categories of contaminants:

1. Solid particles: such as dust, rust or scale resulting from corrosion or wear within the compressed air system. These particles may block valves, damage tools or compromise product quality in sensitive manufacturing environments.
2. Water: present as liquid water, moisture or vapour, and typically assessed through pressure dew point. Excess water promotes corrosion, microbial growth and freezing in outdoor lines, making moisture control essential for reliability and compliance.
3. Oil: including liquid oil, oil aerosols and oil vapours. Trace amounts of oil contamination can damage downstream equipment or jeopardise safety in food, pharmaceutical or breathable-air applications.

Together, these contaminant groups form the backbone of the ISO 8573-1 classification system.

Many organisations use Class 0 when they require an air purity level beyond what the standard classes typically offer. In practice, Class 0 is chosen for high-risk, high-sensitivity environments where even minimal contamination could jeopardise product safety, system reliability or regulatory compliance.

However, a common misconception is that Class 0 means zero contamination. It is not a predefined purity within the ISO 8573-1 tables, and the standard does not prescribe any fixed numeral limits for it. In other words, Class 0 does not promise the absence of particles, moisture or oil.

Instead, Class 0 requires the user, operator or supplier to define their own maximum allowable contaminant thresholds, and those limits must be stricter than Class 1 for each relevant category. This makes Class 0 a custom specification, tailored to the unique risk profile of a given application.

This flexibility exists because certain industries demand purity levels that exceed standard benchmarks, for instance:

• Pharmaceutical production may require extremely low oil vapour levels to protect sterile manufacturing.
• Electronics and semiconductor facilities often need ultra-low particle counts to avoid defects.
• Medical environments rely on exceptionally dry air to protect sensitive equipment.
• Data centre cooling systems may specify lower dew points to minimise moisture risks. 

In all these scenarios, even the stringent limits of Class 1 may not be sufficient, resulting in the need for a Class 0 definition.