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 too 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. 

Igloo Ed ran some tests using a carbon monoxide meter and found the following: with an ISO-butane lantern and stove burning on simmer, the CO level was below 5 ppm and with the stove on a high heat setting, they saw a CO level of 15 ppm. When running a stove on simmer and using Coleman fuel, they had readings of 22 ppm but didn’t test for CO with the stove on a high heat setting. They had readings of 125 ppm when they primed the Coleman fuel stove inside the igloo and advise anyone using any stove that requires priming to prime it outside the igloo. This text is 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).

Vent location

Generally speaking, it is best to poke the vent above the door or trench. The main concern is getting snow on your gear when working on the vent. Poking the hole at top dead center puts it above the trench in all but the 7 foot igloo. In the 7 foot igloo, create the vent down the wall a little above eye level but still over the short door pit.

The air in the vent travels about 5 mph which is fast enough to blow falling snow away from the vent and keep it open. Snow actually collects on the windward side of the hole and blows away/melts on the leeward side of the igloo creating an angled vent hole. 

A concern with vents on the side(s) of the igloo is the vent getting plugged with fresh falling snow.  If the vent was on the side of the igloo, there is a chance that it would be where a drift collects on the igloo and it might overwhelm the air flow and plug the vent. That might be a reason to use two vents (one on each side) if you prefer the side vs the top, as snow would only form a drift on the leeward side of the igloo leaving the other vent open.

Snow could potentially blow into the igloo if the vent was on the windward side of the igloo and the wind was stronger than the air pressure inside the igloo due to heat rising.

The size of the vent controls the amount of air exchange one gets, not the number of vents.