Did bit of research.
Found research on oil mist from machining operations, none on used motor oil yet.
Oil mist from machining operations is limited to
5 parts per million or
5 mg / cubic meter TWA (time weighted average) / OSHA.
This TWA or time weighted average is, I believe, over an 8-hour span.
The limits are also based upon:
10-hour work day or
40 hour work week
I have never run a chainsaw 10 hours straight, nor for a 40-hour workweek in my life. I'd say most saw users in the world do not. Those who do probably also shut the saw off, sharpen, and eat lunch, etc. so that saw would not be running in their hands the full 40 hours.
This is for perspective. I understand ANY exposure to used motor oil is not needed if you use real bar oil. However, good bar oil will result in some exposure, also, not zero.
As far as aerosolization of oil by the saw, this is actually more plausible now that I pound out a few rough numbers. There is a lot more churning at the drive sprocket than I originally thought. I know of no measurement of how much oil gets into the air from a bar and chain, but I am sure it is more than zero.
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Maybe those worried about breathing oil are not totally crazy.
I am still convinced it is a mild exposure level for the average user, but hey, I don't deny it is there at least.