How does air recirculation with increasing co2 compare to having to breath/smell car exhaust, cigarette smoke, etc through vents? I started doing recirculation as my default years ago due to that.
Some sources suggest recirc for heavy traffic -- I'd prefer a good filtration system.
And for rapidly heating or cooling a car, recirc can help. My preference is to open windows & sunroof (where present) to flush out hot air first, in hot weather. In cold weather, the heater won't kick in until the engine has warmed up, so recirc before that buys you little. And recirc + heat can result in fogged windows, so you may want to turn in the AC as well to rduce humidity. (Many cars now activate AC when defrost is selected.)
I rarely keep recirculated air set for more than a minute or two, most often far less.
I've questioned why fresh is even there (seems unlikely co2 would be the reason) - it's always going to be less efficient and you can open a window if you want fresh air.
Anecdotally I feel fine driving for 2 hours straight in recirculated air with closed windows.
Also in heating mode, there's little reason not to use fresh, as we can heat up cars really easily.
It sounds like you misunderstood my original comment.
Edit: welp sometimes you learn English as your first language for the second time and today is that day for me
I think the verb you're looking for is breathe. Breath is a noun.
Leave it on recirculate unless you are defrosting your windows.