Generally, use recirc only briefly when driving through heavy exhaust, smoke, dust, chemicals, smells. Return to fresh air once you've cleared 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.

Fresh just doesn't work well enough compared to recirculated for me. If the car isn't getting cold enough, recirculation being off is always the culprit.

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.

Fresh is there at least partly because cars didn't used to have AC. If you are driving in-town or stop-and-go having a fan on your face with fresh air takes a bit of the edge off.

Also in heating mode, there's little reason not to use fresh, as we can heat up cars really easily.

Insufficient cooling capacity is likely an AC issue. Might care to have that serviced.

I will if it becomes more of a problem. Also considering putting some kind of transparent barrier between the front and back.

Most cars should be leaky enough to get fresh air in at the same time. This is why you smell skunks even on recirculate.

Isn't that just... what outside smells like? What do you do about smelling people's breath in the air (??) whenever you exit your car?

A side street would see a few cars per minute. Compare that to a freeway where you are continuously surrounded by multiple cars doing high rpm.

I can imagine there is a non-trivial increase in certain gas concentrations, but smelling people's breath? That just seems ridiculous.

I'm going to guess English is not your first language. Breath is also a verb. Example: "I don't want to breath in all the exhaust fumes".

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

> "Breath is also a verb."

I think the verb you're looking for is breathe. Breath is a noun.

I stand corrected. Thank you

Cars are built to mostly recirculate the cabin air when the recirculate option is turned on. Note that cabins are not fully sealed from the outside. You have multiple vents located throughout the car whose job is to keep the outside elements away and let the cabin "breathe".

Leave it on recirculate unless you are defrosting your windows.

You don't need fresh on all the time, but for long trips I will switch to fresh for at least five minutes each hour. You just want to cycle the air in the cabin for less re-breathed air.

My tests with my meter don't imply that is correct. Almost immediately on recirculate the PPM goes up and stays there.

This is almost certainly correct. An unventilated space as small as a car will increase within 30-60 seconds, based on my testing of small indoor rooms.

Wow I wonder if anyone has done a study on any correlation between cabin CO2 levels and driver safety?