My new car has "automatic" air conditioning. Unlike all previous cars I have owned, it's default setting when temperatures are warm is to recirculate the air in the car, not to bring in fresh outside air. (It can be manually set to outside, but it would be annoying have to do this all the time.) It's my understanding that recirculating the air for a short while can cool down a hot car faster, but it's unhealthy to not switch to fresh outside air after a few minutes. Has the recirculating method been changed on newer cars so that the air does not get stale as it did on older cars, or should I manually switch to outside air and not let the system decide? – Hugh
You are correct – on older or manual systems, using the recirculate feature can lead to a real and dangerous problem.
Since the air has already been used by occupants, it is heavily laden with moisture and depleted of oxygen. The result is a gradual lessening of awareness or grogginess and fogged-up windows if the ambient conditions are right.
But this is one area where automatic systems have the edge. Sensors monitor moisture and oxygen levels and introduce enough fresh outside air to keep things on a safe level.
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