Two weekends ago went to the Ford OpenXC Hackathon at TechShop Menlo Park. I had fun and learned not to use my recycled air button unless the air outside is bad. What I tested was the CO2 concentration in the car cabin and found that it increase quite rapidly with the recycled air on. With two people in the car the CO2 level reached above 1000 ppm (this is the level where adverse affects start) in less than 3 minutes. You can look at some of the data in the presentation below.
Hackathon Presentation PDF