Hi.. I'm not sure how relevant or valid it may be, but 'breathing quality' air is a slightly different matter to supplying air ( compressed or otherwise ) through lines and fittings usually used for things like air tools or spraying etc.
I'm no expert on the subject... but for anyone rigging up their own 'custom-made' setup, please ensure you're not introducing more contaminants into your respiratory system, than you may be curing.. compressed air from a standard compressor can potentially be laden with oil vapours etc, and without the correct filtration levels downstream of the equipment ( i.e. - on its way to the user ), its possible that all manner of 'nastys' could be finding their way to you.
There is, dependent on the specified filtration levels and/or max allowable ppm contaminant content in many breathing-air applications, sometimes the need to utilise chemically de-greased lines, valves and fittings, which are fairly specialised and very expensive. ( think - hospital breathing apparatus, diving equipment etc... )
In theory, if the compressor has its air intake from the same environment in which the contaminants prevail, its quite possible that all one would be achieving, is to compress the already contaminated air, and effectively re-introduce it back to the operative - which probably is somewhat counterproductive, regardless of the filtration...... filters themselves can indeed be misleading, - in 'particulate terms', one could be filtering microbes or spacehoppers, molecularly speaking.
Don't want to be alarmist... but even more 'dont want' anyone coming to any harm !
(See here -->
http://ezinearticles.com/?Air-Filters-a ... &id=517851 and note the last paragraph of the article)