Regarding recirc mode vs fresh air mode, I seriously doubt that hazardous gas levels are going to be an issue with continuous use of recirculate mode. The Pathfinder is hardly air-tight! What if you don't turn on the vents at all? I mean, come on, do you really think you're gonna suffocate because you're not sucking fresh air from the outside?
I nearly always run with the vents set to "fresh" air because, well, fresh air is nicer.... right? It helps keep from making the air feel 'stuffy' and keeps humidity down. It's also quieter to use than recirc mode and it really helps when you've got some passengers with strong BO. j/k...
Anyway, on really hot days or when I have drive in very dusty conditions (such as off-road), I run with "recirc" full-time. On especially hot days, its actually better to run the AC in recirc mode because the interior air temp will be lower than the exterior air temp. Same goes for really cold days- your heater does a better job heating warmed-up interior air than outside air.
I've never felt dizzy, short-of-breath, etc. due to lack of oxygen or excess of CO or CO2 when using recirc for extended periods of time. Moreover, I notice that the air tends to feel "sticky" after a while, mostly due to buildup of humidity. But it sure beats having dust blow all throughout the cabin!
Just my .02 guys.