Icing was considered earlier in this thread. Now that CO poisoning has emerged, it follows that a failure in the alternate air supply may have been involved. There was an AD in Feb 2009 on the alternate air control linkage. I cannot conceive any other circumstance other than alt air selection which in such a short period of time may have led to cabin air contamination so as to prevent action such as actuating the cabin dump, switch and pull the cabin pressurisation control out, turn on vent fan and turn off cabin air recirculation fan.