The ISO 8573-1 specifies the purity of the compressed air. Purity of the compressed air is important for the processes used in a number of industry segments like Pharmaceutical industry, Food and Beverage,…

The ISO Class specifies the amount of oil (vapour and aerosols), water content or dewpoint and particles which can remain in the compressed air. 

Up to Class 1, the quantities of particles, oil content and dewpoint are specified. For Class 1, the oil-content can be maximum 0,008 ppm/<=0.01mg/m³ which is extremely low and barely measurable. This quantity is absolute and is the standard in for example the food industry. 

A standard Oil-flooded compressor has more than 2ppm in the compressed air. To reduce the oil content, filtration and drying techniques are required. These filtration techniques can achieve the Class 1 standard. However, more difficult is to guarantuee the stability of the purity during its lifetime.

One would assume that Class Zero means zero content of oil in the compressed air.  However, the Class Zero does not indicate any absolute value, but allows a specific value to be specified by the user of the ISO Class 8573-1.

If the customer specifies Class 0, the oil content could be specified at 0.0075ppm.

What means then the compressor is Class Zero certified.

The interpretation is by the Auditors, providing certification of Class Zero of a specific air-compressor, that zero oil is added to the air during compression. The quality of the compressed air coming out is at least equal to the air going into the compressor.

Different types of compressors are compressing without adding oil to the compression chamber.

The compressors can be classified into different types, complying with Class Zero, like a Centrifugal compressor, a Dry screw compressor, Oil-free piston compressor and Water-flooded compressor.

In the LOT31 study on Oil-free compressors for the European Union, the Class Zero is described as being more a marketing tool rather than a useful classification.