Somebody hands you a drawing or a customer spec and it says "compressed air per ISO 8573-1:2010 Class 1.2.1" and that is the whole instruction. Three numbers, one colon-free string, no explanation. We field this question weekly, usually from a shop that just picked up food packaging or medical device work and suddenly has to prove something about its air.
The good news is the code is simple once you know the order. The ISO 8573-1 air quality classes use three digits, always in the same sequence: solid particles first, water second, oil third. Class 1.2.1 means Class 1 particles, Class 2 water, Class 1 oil. That is it. The hard part is knowing what each digit costs you in equipment.
Read it left to right: particles, water, oil
The standard splits contamination into three families because they come from different places and you remove them with different hardware.
- Particles are rust, pipe scale, desiccant dust, wear metal, and whatever the intake filter missed. You remove them with particulate and coalescing filters.
- Water is measured as pressure dew point, not as "wet" or "dry." You control it with a dryer, and the dryer type sets the class you can reach.
- Oil means total oil: liquid, aerosol, and vapor combined. Coalescing filters handle the first two. Only activated carbon handles vapor.
In every class, a lower number is cleaner. Zero is the tightest, and higher numbers get progressively dirtier.
The water classes are the ones people actually shop for
Nine times out of ten, the digit that decides what you buy is the middle one, because it maps directly onto a dryer type. Here are the pressure dew point limits:
| Water class | Max pressure dew point | What gets you there |
|---|---|---|
| 1 | -70°C (-94°F) | Heatless or heated desiccant dryer |
| 2 | -40°C (-40°F) | Desiccant dryer, the common industrial spec |
| 3 | -20°C (-4°F) | Desiccant dryer, or membrane in smaller flows |
| 4 | +3°C (37°F) | Refrigerated dryer, tight end of the range |
| 5 | +7°C (45°F) | Refrigerated dryer, typical rating |
| 6 | +10°C (50°F) | Refrigerated dryer, loose end |
| 7, 8, 9 | Rated by liquid water content | Separators and traps only, no dryer |
Read that table and the shopping decision falls out on its own. If your spec calls for water Class 4, 5, or 6, a refrigerated dryer does the job and costs you very little in energy. If it calls for Class 1, 2, or 3, you are buying a desiccant dryer, and you need to budget for purge air, which is typically 15 to 18 percent of your flow on a heatless unit.
That jump from Class 4 to Class 3 is the single most expensive step in the whole standard. If a drawing calls for -40 and the actual reason is "we do not want water in the line," it is worth asking whether a refrigerated dryer would satisfy the real requirement. Plenty of specs get copied forward from a job that had a genuine subfreezing outdoor run and then get applied to an indoor assembly line that never sees 50 degrees.
Oil classes and the vapor trap
Oil is where people get caught, because they buy a good coalescing filter, feel finished, and then fail a test.
| Oil class | Max total oil (mg/m³) | What gets you there |
|---|---|---|
| 1 | 0.01 | Coalescing filter plus activated carbon |
| 2 | 0.1 | High efficiency coalescing filter, carbon usually still needed |
| 3 | 1 | Coalescing filter |
| 4 | 5 | General purpose coalescing filter |
| X | Above 5 | Nothing specified |
The number is total oil, and vapor counts. A coalescing filter pulls out liquid droplets and aerosols brilliantly and does essentially nothing about vapor. On a hot day, the vapor fraction coming out of a lubricated screw is not small. If your spec is oil Class 1 or 2 and you do not have an activated carbon filter downstream of the coalescer, you will not hit it. We have tested plenty of systems where that was the only thing missing.
Also worth knowing: an oil-free compressor does not automatically give you oil Class 1. Ambient air carries hydrocarbons, and in an industrial building it can carry a lot of them. If the number matters, you still filter.
Particle classes
Particles are counted per cubic meter in three size bands. The tighter classes are specified as counts, and the looser ones switch over to a mass concentration.
| Particle class | 0.1 to 0.5 µm | 0.5 to 1.0 µm | 1.0 to 5.0 µm |
|---|---|---|---|
| 1 | 20,000 | 400 | 10 |
| 2 | 400,000 | 6,000 | 100 |
| 3 | Not specified | 90,000 | 1,000 |
| 4 | Not specified | Not specified | 10,000 |
| 5 | Not specified | Not specified | 100,000 |
| 6 and above | Rated by mass concentration in mg/m³ rather than particle count | ||
In practice a properly sized particulate filter after the dryer covers most industrial particle specs. The thing that quietly wrecks particle counts is a desiccant dryer with a failing outlet filter, because the desiccant beads break down into dust and send it downstream. If you run desiccant, an afterfilter is not optional.
What Class 0 really means
This one gets misused constantly in marketing. Class 0 in ISO 8573-1:2010 does not have fixed numbers in the standard. It means "stricter than Class 1, with the actual limits specified and documented by the equipment supplier or the user." So a Class 0 claim is only meaningful if it comes with the numbers attached and, ideally, third party validation. If a datasheet says Class 0 and gives you nothing else, that is a sales word, not a specification.
Building a system to hit a spec
Order matters as much as the parts list. A typical layout for a lubricated rotary screw hitting something like Class 2.2.1:
- Aftercooler and water separator, to knock out the bulk liquid while the air is still hot and wet. Every gallon you drop here is a gallon your filters do not have to fight.
- General purpose coalescing prefilter, to protect the dryer.
- Dryer, sized for actual inlet conditions rather than nameplate CFM. Inlet temperature and pressure both move the rating.
- High efficiency coalescing filter.
- Activated carbon filter, if the oil class is 1 or 2.
- Particulate afterfilter, mandatory behind desiccant.
Two things that ruin an otherwise correct system. First, filters that never get changed. A plugged element does not just cost you pressure drop, it can collapse and dump everything it caught downstream. Second, undersized filters. Filter ratings are given at a reference flow and pressure, and running 200 CFM through a 100 CFM housing does not just double the pressure drop, it drops the removal efficiency too.
Frequently Asked Questions
What does ISO 8573-1 Class 1.4.1 mean?
Class 1 for solid particles, Class 4 for water, and Class 1 for oil. In equipment terms that is high efficiency particulate filtration, a refrigerated dryer holding about a 37 degree F pressure dew point, and a coalescing filter followed by activated carbon for the oil.
Which ISO class do I need for food packaging?
It depends on whether the air contacts the product, and the governing document is usually ISO 8573-1 combined with your customer's or auditor's requirement rather than the standard alone. Direct contact air commonly lands around Class 1 or 2 for particles and oil with a subfreezing dew point. Get the requirement in writing from whoever is auditing you, because guessing high is expensive and guessing low fails.
Does an oil-free compressor mean oil Class 1?
No. Oil-free removes the compressor as an oil source, but the ambient air the compressor inhales still contains hydrocarbon vapor, and in a shop with forklifts or solvents it contains a fair amount. If the oil class is contractual, filter for it and test for it.
How do I prove my air meets a class?
You test it. Particle counts, dew point, and oil content each need their own measurement, and oil vapor in particular requires lab analysis rather than a shop gauge. Dew point you can monitor continuously with an inline instrument, which is worth doing anyway because it tells you when a dryer is drifting.
What is a realistic general shop air spec?
Most machine shops and general manufacturing operations run happily around Class 2.4.2 or looser: a refrigerated dryer, a coalescing filter, and a particulate filter. You do not need desiccant and carbon to run impact wrenches and cylinders, and paying for them is money spent on nothing.
The short version
Three digits, always particles, water, oil, and lower is cleaner. The middle digit tells you which dryer to buy and is where the money is. The third digit tells you whether you need carbon on top of coalescing, and it is the one people miss. If you have a spec in hand and are not sure what it translates to in hardware, send us the three numbers along with your CFM and pressure and we will lay out the filtration and drying that hits the ISO 8573-1 air quality classes you are being held to, without selling you a step you do not need.
