I can't see the CO thing being an issue - turbine bleed air is taken before combustion (typically 9th and 14th stage compressor). It's just outside air heated up by the energy of compression.
As suggested before - 10000' above ground or sea level would be good to know? Ok just read it!
Is it a dual instrument system ie two altimeters supplied by seperate pitot sources? If it's a single alti did the standby agree with the master? Does it have a FDR?
I can't think of anything contamination wise that would make the crew nearly pass out without feeling unwell first - oil, hydraulics, fuel are all very noticeable!
My 2p worth.
