The International Standard for compressed air quality introduces a simple system of classification for the three main contaminants present in any compressed air system – DIRT, WATER and OIL.

To specify the quality class required for a particular application, simply list the class for each contaminant in turn.
For Example:

Compressed air to Quality Class: 2.2.2. (Dirt: 1 micron. Water: -40°C PDP. Oil: 0.1 mg/m3)

ISO8573.1 : 2001 (E)

Quality Class Solid Particles
maximum number of particles per m3 
WATER Pressure Dewpoint °C (ppm. vol.)

at 7 bar g

OIL

(Including vapour)

mg/m3

0.1-0.5
micron 
0.5-1.0
micron 
1.0-5.0
micron 
1 100 1 0 -70 (0.3) 0.01
2 100,000 1,000 10 -40 (16) 0.1
3 10,000 500 -20 (128) 1.0
4 1,000 +3 (940) 5
5 20,000 +7 (1240)
6 +10 (1500)