The vent

[Igloo Ed]

If you are going to run a stove or lantern in the igloo, you will want to have a vent so you get an exchange of air to lessen the chance of carbon monoxide poisoning. If there is a chance of your door drifting shut through the night, a vent should also be used to lessen the chance of suffocation.
The vent will melt bigger with time and will need to be plugged and opened again so it is not to large. If the vent gets as big as a baseball, your heat will escape and you will feel a cold draft coming in the door. It is best to poke the vent above the door or trench so you don't get snow on your gear when working on the vent.
We ran some tests using a carbon monoxide meter and with a ISO-butane lantern and stove burning on simmer, the CO level was below 5 ppm and with the stove on a high heat setting, we saw a CO level of 15 ppm.
When running a stove on simmer and using Coleman fuel, we had readings of 22 ppm but we didn't test for CO with the stove on a high heat setting.
We had readings of 125 ppm when we primed the Coleman fuel stove inside the igloo and we advise anyone using any stove that requires priming to prime it outside the igloo.

This is some text copied from an OSHA document pointing out these levels labeled as hazardous:
The Occupational Safety and Health Administration (OSHA) standard for exposure to carbon monoxide prohibits workers' exposure to more than 35 parts of the gas per million parts of air (ppm), averaged over an 8-hour workday. There is also a ceiling limit of 200 ppm (as measured over a 15-minute period).