I've just returned from guiding a group from SUSS around the gated section in Water Icicle, and the air quality in the shaft, the North West Passage and the gated section was very benign by WICC's standards.
I measured the oxygen levels using a QRAE 4-gas monitor, and the readings were:-
Foot of Shaft 20.00%
Top of ladder (North West Aven) 19.80%
Three Way Aven 19.50%
Elevator Junction 18.90%
End of Urchin Passage 18.70%
Cherty 2 Choke 19.10%
End of Cherty 2 Extensions 18.50%
If you subtract these figures from the surface reading, you get an approximate value for the CO2 percentage.
These levels wouldn't normally hinder cavers to a great extent in the short term, but prolonged exposure to any high level of CO2 could have an effect eventually - remember the "normal" reading for CO2 in fresh air is 0.08%. And it should be noted that other parts of the system (Volcanic Bugpusher extensions, Batty Farber etc) have been known to have higher concentrations of CO2 than the parts I measured today.