It has something to do with how the Air is circulated. How exactly do they work?

(sorry for the shitty phone camera pic)

 

If it's on the left the air gets recirculated. Just a very minor amount of fresh air is going through from the outside.
On the right position it is doing it automatically when it detects that the air outside is too much polluted.

Recirculation / Automatic recirculation

 

what if both are off? (no green light under either of them)

 

Then you only get the air from the outside, polluted or not.

You can manually switch to recirculation, or let the car measure the air quality and let the system decide for you. If you never want recirculation, leave it out.

 

Ok, thanks for the info.

Whats the best setting? Auto?

 

I allways use Auto and never experienced any problem.

I only use the manual mode when I approach (for example) a construction side and I see dust clouds in the sky. Then I use the manual mode. But right after going throught he dust I switch back to Auto.

Auto works perfect.

 

Okay, I've been waiting for this one....

 

http://en.wikipedia.org/wiki/RTFM

For those who still don't know...

 

I normally have it off. I only turn it on to avoid bad smells from the outside to gets in the car.

 

And that's exactly what Auto is for, because you never know when it's going to smell abd.