Share 'Carbon Monoxide Levels'

Hey all, I was just wondering if anyone had any data on carbon monoxide.  In particular, at what level of CO present should a firefighther have an SCBA on?  We have always used that if it is less than 35 PPM then no SCBA needs to be used.  I have recently seen that "Acceptable Atmospheric Conditions" are Less than 30 PPM.  What do you use as a basis and where did you get the information to suppor…