Also, is it safe to use the feature for long time? If the air inside the car is always recirculated, won’t it be saturated with carbon dioxide as we breath?
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The cold air that leaves the vent cools down the average temp in the car. Then the lower average temp air in the car gets sucked in and gets pumped back to the business end of the AC to get cooled down even lower to be blown out of the vent and repeating the cycle.
But no, CO2 poisoning is not possible that way. Your car isn’t completely sealed and the AC also does take in a bit of fresh air from outside.
If cars were sealed to the point that one could suffocate themselves by not opening a window, it would be difficult to open your door, and then when you closed it your ears would pop.
Setting the HVAC (Heating, Ventilation and AC) in your car to recirculate just means it will draw the majority of the air it is using to either heat, cool or ventilate the car from the cabin instead of just at the base of your windshield outside. This helps in certain situations. Mostly it’s good for when you’re using your AC because it reduces the ambient temperature of the incoming air, making chilling it easier for you system. It’s also good for when you’re behind a truck or some ‘bro’ rolling coal to stop the fumes from being drawn into the cabin of your car. Also on particularly humid days it can be used to speed up the defogging of your windshield (carry a shower squeegee for this in your car as well, you’ll thank me come this fall/winter), and any day with a pollen index warning to stop that from being drawn in. Other than those situations it’s fine to use either setting.
It’s reusing the air already in the car. This helps it cool down as others have said since the air the A/C is cooling is already cooler than the air outside the car.
This won’t cause you to get CO2 poisoning because fresh air is still getting into the car through other gaps and openings to the outside. It’s not an airtight environment. There probably isn’t much difference between the amount of Co2 you’re breathing from using outside air or recirculated air. In cartain situations, I might argue you would be better off brrathing the recirculated air. If you’re stuck in heavy traffic then you may be breathing in some of the exhaust and fumes coming off the other cars. If you use recirculated air the A/C can help filter some of that out.
>won’t it be saturated with carbon dioxide as we breath
If a car was sealed THAT airtight, then it would be impossible to run the AC on anything *but* recirculation mode, since adding outside air to a sealed vessel would pressurize it.
Since you can run AC without pressurizing your car, we can conclude that car cabins aren’t sealed anywhere close to airtight, and therefore CO2 accumulation isn’t a concern since there’s always some air exchanging with outside.
Sunhating’s answer covers how it works cooling-wise. By taking the air from the car and cooling that, the air coming from the vents is colder than intaking hot air from outside.
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