It is interesting that SQF would choose to use the word “purity” because air is not pure. This creates some confusion with the requirement. However, the intent is clear. Clean dry air is needed to ensure quality.
Many members have made valid statements. Compressed air can contain all sorts of contaminants which could be problematic for a consumer to ingest. As mentioned particles, water, oil, and microbiological contaminants are the key suspects.
But what does that mean in lay terms?
Particles can be rust, pipe scale, foreign materials, glues, microbiological and more. None of which you would want to ingest.
Water contributes to microbiological contamination and can be damaging to some products.
Oil consists of oil aerosol and oil vapors, neither of which you would want in your food. Oil-free compressors DO NOT eliminate ambient oil pulled in by the intake. So it should still be tested.
Particles, water, and oil combined all create the perfect breeding ground for microbiological contaminants. Microbiological contaminants grow and continue to grow. Keep your PWO under control and you can manage your microbial contamination better.
You should start with the system you have installed to determine the appropriate testing. Do you use filtration that filters to say 0.5 microns or better? You should make sure the testing you do can measure that low. Otherwise you are not conducting testing that determines if your system is working as intended.
Determine the quality of air the system is designed to produce and find a lab that can test to those limits, classes or parameters.
Hope that info helps!