I have heard a couple of different explanations about what causes this and one night at a mine safety training class I attended there were two MSHA "experts" on mine air quality that nearly exchanged blows after a heated argument about the reason the Carbon Monoxide is present but there was no disagreement about whether or not it is there, the fact that the monitors are on the compressor systems bears that out. In most (but not ALL!) cases the levels are low enough to not be dangerous but without the monitor you would have no way of knowing. It is my understanding it forms when air containing contaminants (contaminants in the intake air plus oil, carbon build-up, etc from the pump) is compressed and heated by the pump. In any case it is there but the real question would be in what amounts? I thought I would do a quick search at OSHA and see what I could find and turned up a lot of info, here are a few from OSHA and some I just came up with on the net.
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