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.

ISO 8573-1 classifies compressed air quality using a single, numerical system. Each contaminant group is assigned its own class, based on the concentration or dew point measured in the system. The lower the number, the cleaner the air. For example, Class 1 air is significantly purer than Class 5, with stricter limits on allowable contaminants. This structure enables engineers and operators to specify precisely the purity level required for each application, whether that’s a pharmaceutical filling line or a general manufacturing process.

Below is a simplified representation of Class 1 to 9:

Note: Exact values should be verified against a full ISO 8573-1 table – this simplified version helps illustrate how stringency decreases as the class number increases.

These three measurements are then combined into a three-digit designation, such as 1.4.2 or 4.4.4. Each digit corresponds to a specific contaminant category in a fixed order:

• First digit = particles
• Second digit = water
• Third digit = oil

A specification of 1.4.2 indicates extremely clean air in terms of particles (Class 1), moderate moisture control (Class 4) and fairly stringent oil limits (Class 2).

This provides a common language that helps engineers, quality managers and suppliers precisely communicate air quality expectations, as well as select and validate compressors, dryers, filtration systems or monitoring equipment.

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.

Choosing the correct compressed air purity level is an operational decision that affects production quality, system reliability and long-term cost control. Whether your facility requires Class 0 air or a well-defined standard class, the consequences of getting it wrong can be significant.

Contaminants such as particles, oil and moisture are among the most common causes of failure in compressed air driven systems. Even small amounts of particulate matter can block valves, erode seals and damage precision pneumatic tools. Moisture accelerates corrosion, creates sludge in distribution lines and can freeze in outdoor or low-temperature environments. Oil aerosols may foul instrumentation, damage membranes or compromise sensitive processes. Across industries, these risks ultimately translate into higher defect rates, reduced throughput and unplanned downtime – all of which erode operational resilience.

For many sectors, achieving a higher ISO class (or specifying a Class 0 threshold) is essential for meeting quality control obligations:

• Food and beverage production must ensure no oil or moisture enter product contact areas.
• Pharmaceutical and clean room environments rely on tightly controlled air purity to protect sterile processes and maintain regulatory accreditation.
• Electronics and semiconductor manufacturers require ultra clean air to prevent microscopic contamination that could ruin high-value components.
• Data centres increasingly depend on high-purity air in cooling systems to safeguard uptime and equipment integrity.

For these industries, air quality is tied to safety, auditability and brand reputation.

Not every process requires the extreme stringency of Class 0. Some applications operate perfectly well Class 2, 3 or even higher classes for water, particles or oil. The power of the ISO 8573-1 framework lies in its flexibility: it enables operators to tailor air purity to the actual sensitivity of each process. This prevents over-specification, which can drive up equipment cost and energy usage, while ensuring that critical steps in the operation are protected by the appropriate level of air cleanliness. Defining the right class (or a custom Class 0 specification) ensures purity where it matters most and cost efficiency where it doesn’t.

By aligning air purity with operational needs, organisations can balance safety, reliability and financial performance, creating a compressed air strategy that is resilient and sustainable.

Achieving and sustaining the required level of compressed air purity demands a complete system approach. ISO 8573-1 provides the framework, but it is the correct combination of equipment, monitoring and maintenance that ensures ongoing compliance. Whether your goal is to meet a standard ISO class or implement a custom Class 0 specification, the following elements are essential.

System Design: Build Purity in from the Start

Compliance begins with selecting the right air generation and air treatment equipment:

• Compressor selection: oil-free compressors are often the preferred choice for Class 0 or high-purity applications, as they eliminate a major source of oil contamination at the source. Oil-lubricated compressors remain suitable for many industrial processes, provided they are paired with appropriately rated filtration and separators.
• Filtration: a staged approach is typical – particulate filters to remove solid contaminants, coalescing filters to capture oil aerosols and activated carbon or specialised vapour filters where ultra-low oil vapour limits are required.
• Dryers: refrigerated dryers are suitable for moderate dew-point requirements (typically Class 4 or above), while desiccant dryers are necessary when achieving very low moisture content (Class 1 or 2) or when Class 0 moisture limits are specified.

Designing the system around the required purity class, rather than retrofitting solutions later, yields more stable performance and lower operating costs.

Measurement and Verification: Prove That the System Performs

ISO 8573’s sub-standards outline precise testing methods for measuring particle concentration, humidity and oil content. To confirm that your system meets its defined class or Class 0 specification, periodic sampling and laboratory analysis is important. Testing intervals vary based on application criticality, but industries such as pharmaceuticals, food processing or electronics typically require more frequent verification.

Continuous Monitoring: Maintain Stability Between Audits

Periodic testing verifies compliance at a point in time, but many industries require assurance that high-purity performance is being maintained continuously. This is where real-time visibility becomes invaluable.

On-board sensors and analytical devices can monitor dew point, oil vapour and particulate levels in critical applications.

Aggreko Remote Management (ARM) adds an additional layer of protection by providing continuous live data on system performance, including pressure, temperature, load, dryer efficiency and filter differential pressures. ARM’s early-warning alerts help operators catch developing issues, such as dryer drifting out of specification or a filter approaching saturation, before they impact air purity.

While ARM does not directly measure contaminants, it safeguards the conditions required to maintain ISO 8573-1 compliance, particularly in Class 0 or near-Class 1 environments where process stability is essential.

Maintenance Best Practices: Keep the System Performing as Designed

Even the best-designed compressed air system will drift out of compliance without disciplined upkeep. Key practices include:

• Regular filter replacement based on manufacturer guidelines and pressure drop readings.
• Scheduled maintenance of dryers, including desiccant regeneration or replacement.
• Effective condensate management to prevent moisture carryover and microbial growth.
• Routine inspections and air audits to check for leaks, pressure drop and equipment wear.

A proactive maintenance strategy, supported by live insights provided by ARM, protects system integrity, stabilises air quality and ensures long-term conformity with ISO 8573-1, including any defined Class 0 thresholds.

Even with clear framework like ISO 8573-1, organisation often fall into avoidable traps that compromise air quality, increase costs or create unnecessary operational risk. Understanding these pitfalls – and how to avoid them – is essential for maintaining, reliable, compliant compressed air systems.

Misinterpreting Class 0 as Zero Contamination

Class 0 does not mean absolute purity. It is a user-defined standard that must be stricter than Class 1 but does not imply the total absence of particles, moisture or oil. Misunderstanding this can lead to unrealistic expectations or poorly defined specifications.

Assuming Compressor or Filter Specifications Alone Guarantee ISO-Class Compliance

No compressor, filter or dryer is inherently ISO 8573-1 compliant on its own. Compliance is the outcome of a correctly designed and maintained system and must be confirmed through proper testing.

Over-Specifying Purity When It’s Not Required

Specifying Class 0 or Class 1 purity for processes that do not truly require it can drive up capital and operating costs. Higher purity demands more intensive drying, filtration and energy use. A risk-based assessment ensures purity is applied where it has real operational, safety or compliance value.

Relying Only on Period Tests

Periodic sampling is essential, but it only provides a snapshot. Contamination events can occur between tests, especially in systems with fluctuating demand or environmental conditions. For high-value or high-risk applications, real-time monitoring helps detect early deviations before they impact product quality or uptime.

Overlooking Contaminants Not Covered by ISO 8573-1

ISO 8573-1 focuses on particles, water and oil, but some applications face additional risks. Microbial contamination, for example, is a known concern in food, beverage, pharmaceutical and sterile manufacturing environments. These risks must be addressed through broader hygiene controls, sterilisation practices and, where necessary, enhanced filtration or air treatment steps.

By recognising and avoiding these common mistakes, organisations can build compressed air strategies that are more robust, more efficient and better aligned with their operational and regulatory demands.

Achieving Class 0 compressed air quality is a structured process that ensures your air system is designed, operated and validated to meet the purity your application demands. The following workflow provides a clear, step-by-step approach to defining and delivering Class 0 performance with confidence.

1. Identify Application Sensitivity and Required Purity Level

Start by understanding what your process requires. Consult production teams, quality managers, regulatory bodies and equipment suppliers to determine how air purity affects product safety, compliance or reliability. High-risk environments often justify Class 0-level control.

2. Define Clear Contaminant Limits

Class 0 is not a fixed value; it is a user-defined specification that must exceed Class 1 across particles, moisture and oil. Set measurable, application-specific limits for:

• Maximum allowable particle concentration
• Required pressure dew point
• Total oil content (liquid, aerosol and vapour)

These become your formal Class 0 thresholds and should be documented as part of your quality or engineering standards.

3. Specify the Right Equipment and System Design

Build the system around the purity level required:

• Oil-free or correctly lubricated compressors, depending on risk tolerance
• Multi-stage filtration, including particulate, coalescing and vapour filters
• Appropriate drying, using refrigerated or desiccant dryers based on dew-point needs
• Condensate management to prevent moisture carryover
• Monitoring instruments, supported by technologies such as ARM for visibility of system performance

A properly designed system is the foundation of stable, repeatable air quality.

4. Establish a Maintenance and Monitoring Regime

Define the ongoing activities that keep the system in compliance:

• Regular filter changes and dryer servicing
• Condensate management checks
• Scheduled ISO 8573 sampling
• Continuous monitoring for critical applications to detect deviations between audits

A proactive maintenance strategy ensures day-to-day stability, not just point-in-time compliance.

5. Document and Validate Compliance

Finally, ensure your Class 0 specification is auditable and verifiable. Keep clear records of equipment selection, test results, maintenance activity and monitoring data. For regulated industries, third-party testing or certification adds an additional layer of assurance, helping demonstrate compliance to customers, auditors and regulators.

ISO 8573-1 provides the framework for defining and measuring compressed air purity and understanding it is essential for any organisation that relies on clean, reliable air to protect product quality, equipment performance or regulatory compliance. Within that framework, Class 0 represents the highest level of control: a user-defined specification that exceed Class 1 and is tailored to the most sensitive, high-risk applications.

But achieving that level of purity is never about a single piece of equipment. It is the result of a complete system approach: selecting the right compressor technology, applying effective filtration and drying, maintaining stable performance through monitoring, and validating results with consistent testing and maintenance.

For operators in sensitive environments, getting this right is non-negotiable. The first step is to define, with clarity, the purity your processes require. From there, choose equipment aligned to that requirement, including oil-free air compressors where oil contamination risk must be eliminated, and implement robust monitoring and verification practices to ensure ongoing compliance.

If you’re unsure whether your current system meets the purity level your operations demand, now is the ideal moment to act. Our team is on hand to provide expertise in high-purity, oil-free compressed air. Contact us today to discuss your requirements.