Not sure if this topic has already been discussed elsewhere, just something that has unexpectedly been a bit of a source of conflict for me recently due to different cultural perception.
TLDR: In Taiwan, most people always keep the recirculation on which can increase CO2 levels in the car to levels which are known to significantly affect cognitive function. A non-issue or maybe a negative contributor to traffic safety…?
But let’s start with the basics first:
Pretty much every car with AC (so I’d guess 100% of cars sold in Taiwan) has a little button with an image which looks similar like this:
This button causes the ventilation system of the car to recirculate the indoor air instead of supplying “fresh” air from outside. The benefit is obviously that “bad”, stinky air from outside (PM 2.5 and others) won’t get inside and it can also increase AC efficiency.
So pretty much a no-brainer to keep it on continuously?
In fact, that’s basically what people in Taiwan (and in most of Asia from what I have seen) keep that button on all the time. In fact, most cars will automatically turn on this function when being started.
The situation is completely opposite in Europe: There, many car manufacturers place a warning about this particular button in the manuals of their cars (like this one):
Stale air can cause the driver to tire quickly and lose concentration, which can lead to collisions, accidents and serious injuries.
- Never use recirculation mode for a longer period of time, as no fresh air enters the passenger compartment.
- When the cooling system is switched off, the windows can mist up very quickly in recirculation mode and severely restrict visibility.
- Switch off recirculation mode if it is not required.
I also remember vaguely about being told this in driving school: Turn it on when needed (e.g. when driving through a tunnel), but don’t forget to turn it off after some time because the air will get stale eventually.
Also, I remember having a newish car in German which would turn off air recirculation after a couple of minutes (actually, I had to ask the dealer to recode that feature because the time was so short that I couldn’t even pass a medium-length tunnel without the recirculation turning off itself).
Even automobile magazines will always address the topic of air recirculation with warnings, such as:
Important to know: If you use the recirculation button, you should switch it off again after a certain period of time. This is because the carbon dioxide content in the air in the car increases, which can lead to tiredness, poor concentration or a reduced reaction speed.
The US seems to be mostly with Taiwan on that one (at least when taking Reddit as an indicator):
Q: Is it safe to have the air recirculation on for more than 5 minutes?
A: Wut
A: LOL. This might be the funniest thing I’ve read on Reddit in ages.
A: Not sure if this is a trolling post but automobile HVAC engineer here, you can run your air in recirc as much as you would like.
A: Consider fan death as well, since there is a fan blowing that air through the vents.
Then, there is the scientific / academic aspect:
There seems to be consensus that high CO2 levels in the air can negatively affect some areas of cognitive function (e.g. here or here):
The control CO2 levels used for comparison were below 1000 ppm, while the exposure concentrations were divided into three groups: 1000–1500 ppm, 1500–3000 ppm, and 3000–5000 ppm. The results indicated that CO2 exposure below 5000 ppm impacted human cognitive performance, with complex cognitive tasks being more significantly affected than simple tasks. The complex task performance declined significantly when exposed to additional CO2 concentrations of 1000–1500 ppm and 1500–3000 ppm
So CO2 concentrations somewhere above 1500-2500 ppm definitely seem to be bad for cognitive performance. Often, a level of 1000-1500 ppm is seen as a “threshold” level for acceptable CO2 content of the air.
And then there are studies that measure of model CO2 levels in cars while the recirculation mode is on, such as:
- Development of a Model to Predict Build-up of Cabin Carbon Dioxide Concentrations in Automobiles for Indoor Air Quality
- Monitoring of Carbon Dioxide (CO 2 ) Accumulation in Vehicle Cabin
- In-vehicle carbon dioxide concentration in commuting cars in Bangkok, Thailand
Result: With the air recirculation turned on and more than one occupant, levels of over 2000ppm (and even above 5000 ppm with four occupants) can be reached after less than one hour.
In-vehicle CO2 concentration in cars can reach levels that are higher than the safety values prescribed by standards, especially when the vehicle carries its full passenger capacity. When the vehicle is in REC mode, where no or minimal fresh air is taken into the vehicle cabin, there is a rapid increase in the CO2 level
Overall, cars seem to be much more air-tight than people generally believe:
When the vehicles are parked for long time, the CO2 concentration decays gradually to an acceptable level. Parking for a period of about 10 hr brings the concentration level to near ambient level
There is some research about optimizing recirculation by either shutting it one and off periodically or combining recirculation with fresh air, but so far it doesn’t seem that manufactures implement those measures widely (some Tesla cars seem to toggle recirc on and off).
So what should we take away from all of this?
Honestly, I am not sure: I do feel, though, that this topic seems to receive less attention that it maybe should:
While the “European” way of only using recirculation in “exceptional” situations does not seem to be ideal in a country like Taiwan because of increased pollution making its way into the cabin and the reduced efficiency of the AC, I still do have my doubts that keeping on air recirculation all the time (like it’s practiced in Taiwan) is really a good idea: If concentration of the driver really suffers when CO2 is building up in the car, that’s definitely a safety issue. Who knows, maybe this effect even explains some of the accidents or fatalities happening in Taiwan where the driver was “tired” or “distracted”…
Anyone else ever wondered about this?
Or put some thought into this topic…? 
