Fresh Air vs. Recirculation
The reason you should never set your air to recirculation in the summertime when you first turn on your air conditioner, is because it draws the majority of air from the interior, which is of course where the hottest air is. Once the air inside the vehicle cools, you can adjust the fan back to recirculate.
The recirculation setting always works best when a vehicle is already at its desired temperature. This allows the air-conditioner to work less (use less energy) to maintain that temperature. If you live in the congested city, the recirculation setting is also suggested, as the CO2 emissions from other vehicles can find their way inside your vehicle when pulling the majority of air from the outside.
To reduce your fuel consumption and CO2 emissions even more, don't forget to turn your air-conditioner off a few minutes before you reach your destination.