CO2 buildup in vehicle cabins becoming a safety issue

High carbon dioxide concentration in a small area, such as a passenger-car cabin, is a health hazard. For many years, the outside-air flap on most HVAC systems was notched so that in recirculation there always was some “fresh” air flowing into the cabin. Even without the notch, the vehicle body was relatively leaky and the blower switch didn't have an off position, just a low speed to purge stale air.

To improve A/C performance, the recirculation switch (or Max A/C position) in newer vehicles permits shutting off outside air. Now there's even a U.S. EPA fuel economy credit because this approach improves fuel economy. However, for passenger comfort in regions with high ambient temperatures, shutting off outside air is common.