Why Compressed Air Fails Food and Pharma Audits (ISO 8573-1)

Published on 12 November 2026

Why compressed air fails audits in food and pharma: oil, moisture and ISO 8573-1

Compressed air fails a food or pharma audit for one of four reasons: too much oil, too much moisture, too many particles, or microbes growing in wet pipework. ISO 8573-1 is the standard that puts a number on the first three. When a test fails, the compressor is often not the only cause. A dryer that can't keep up, a filter element that is months overdue, or an old pipe run after the filters is just as likely.

This article explains what ISO 8573-1 measures, how to read the class your auditor or customer asks for, the usual reasons air fails, and what a sound air treatment chain looks like.

What ISO 8573-1 measures

ISO 8573-1 sorts compressed air into purity classes for three contaminants:

• Solid particles, counted by size per cubic metre of air.
• Water, measured as the pressure dew point: the temperature at which moisture in the air would start to condense at line pressure.
• Total oil, meaning oil droplets, liquid oil and oil vapour together, in milligrams per cubic metre.

Each contaminant gets its own class number. The lower the number, the cleaner the air. Here are the oil and water classes most often asked for in food, beverage and pharma plants:

Microbes are not part of ISO 8573-1's classes, but they have their own test method in the same family of standards, and pharma auditors in particular often ask for them.

How to read a class like [1:4:1]

A purity specification is written as three numbers in the order particles : water : oil. For example, ISO 8573-1 [1:4:1] means Class 1 particles, Class 4 water and Class 1 oil.

Your required class is set by your customer, your auditor, your food safety scheme or your own validation, not by the compressor supplier. The usual pattern is:

• Air that touches the product or the inside of the packaging (PET bottle blowing, product conveying, tablet coating, nitrogen or air flushing of packs): tight oil and particle classes, and often a low dew point.
• Air that only runs machines near the product (cylinders, valves, actuators): looser classes, but still dry and filtered.

If you aren't sure which class applies to which line, ask your quality team for the written specification before you buy treatment equipment. It is cheaper to design for the right class once than to add a dryer after an audit.

The usual reasons air fails a test

1. Oil carried over from a lubricated compressor. A lubricated machine puts a small amount of oil into the air. Coalescing filters remove most of it, but only while the elements are in date. A saturated or bypassed filter lets oil through, and the test result follows.

2. Oil vapour that filters don't catch. Coalescing filters stop oil droplets, not vapour. Vapour needs an activated carbon stage, and carbon gets used up. Oil vapour can also come in through the compressor intake, from diesel exhaust, a nearby process or a generator room. So the intake location matters even with an oil-free compressor.

3. Moisture. This is the most common failure. The dryer may be too small for the flow, set up for a lower inlet temperature than a summer afternoon in the compressor room, or bypassed during maintenance and never put back in line. Blocked condensate drains on the receiver or filters let water build up and carry forward.

4. Particles and rust after the filters. Air can leave the filters clean and pick up rust and scale from old galvanised or carbon-steel pipework on the way to the machine. Point-of-use filters close to the product protect against this.

5. Microbes. Bacteria and moulds need moisture. Wet air, low points where condensate sits, and dead legs in the pipework give them a place to grow. Keeping the air dry is the main control.

6. Sampling in the wrong place. Air tested at the compressor outlet tells you about the compressor. The auditor cares about the air at the point of use. Test where the air meets the product.

Oil-free compressor or filtered lubricated air?

Both can meet a tight oil class. The difference is where the risk sits.

With a lubricated compressor, air purity depends on the filters and carbon working every hour of every day. That means element changes on time, a carbon stage that hasn't run out, and condensate that has to go through an oil–water separator before it can be discharged.

With an oil-free compressor, no oil goes into the compression chamber at all, so there is no oil from the compressor for the filters to catch. Colt's oil-free reciprocating compressors seal the pistons with dry-running PTFE piston and rider rings, and keep crankcase oil away from the cylinders. An oil-free compressor still needs a dryer and particle filters, and an activated carbon stage is still sensible if the intake air may carry vapour.

We explain the trade-offs in running cost and maintenance in oil-free vs lubricated reciprocating compressors. For 40 bar PET blowing air, see our PET bottling air compressor page.

A sound air treatment chain

For product-contact air, the order of equipment matters as much as the equipment itself:

1. Compressor, ideally oil-free for product-contact air, with its intake away from exhaust and process fumes.
2. Aftercooler and moisture separator, to drop out most of the water before it reaches anything else.
3. Air receiver, sized for the compressor, with an automatic drain.
4. Dryer. A refrigerated dryer gives a pressure dew point of about +3 °C, which is Class 4 water. Colt's refrigerated air dryers cover working pressures from 10 bar to 40 bar, with higher pressures on request; the standard model has a 3 °C pressure dew point, a dew point indicator and an electric timed drain valve with an anti-blocking mechanism. Where your specification asks for Class 2 or Class 1 water, Colt also supplies desiccant air dryers.
5. Filters, in stages. Colt's line filters come in four grades: a 3 µm pre-filter, a 0.1 µm filter, a 0.001 µm fine filter, and a 0.003 µm activated carbon filter that takes out oil vapour and odour. Each stage protects the next, so fit them in that order. Change the elements when the pressure drop across them reaches 0.7 bar, or sooner on a schedule. These line filters are rated up to 16 bar; high-pressure lines need filters rated for their pressure.
6. Point-of-use filters close to where the air meets the product.

See dryers, filters and receivers on our air accessories page.

Before your next audit

• Get the written ISO 8573-1 class for each air use, and keep it on file.
• Check the dryer's dew point reading on a hot day, not only in winter.
• Write down the filter element change dates and pressure drop readings.
• Test every receiver and filter drain.
• Walk the pipework for low points, dead legs and old pipe after the filters.
• Have air tested at the point of use, by a lab that tests to the ISO 8573 test methods, and keep the reports.

Frequently Asked Questions

Tell us what the air is used for, the class your auditor or customer asks for, and your flow and pressure through our contact page. Our engineers will look at your compressor, dryer and filters and suggest what to change.