CO reporting levels

At what level of CO (in ppms) do you report a safety hazard? The OSHA level is 50 ppm in an enclosed space for 8 hours - but when testing a furnace or the area of the furnace/HW room usually for about 10-15 minutes - at what number would you call it out as a hazard ?
I have been told that firefighters must use an air tank if the level is above 15 ppm. Can’t seem to find a definitive number for a short test.
Any help?

This has limits that alarms are set to activate.

Linus, does that mean if my detector picks up less than 400ppm in the 15-20 minute test, I don’t report it? - seems fuzzy and not quite right (and maybe still dangerous).

Found this at

I know that no standard has been established and the ambient air will measure up to 9 ppm and most people wont feel any effects up to 70 ppm (according to USCPSC) -Makes me think that if I get a reading of 16ppm over 20 minutes, that will easily equates to 50ppm over an hour - toxic over time -
Just trying to get a line on what level gets reported and what doesn’t - Want to be safe but not cry wolf -