Should I leave the car's air-conditioning recirculation button switched on or off? I always get it mixed up - whether "on" or "off" lets in outside air. When driving in a built-up area like Singapore, you can leave the recirculation button in the "on" position (the button is lit).
In this mode, the ventilation system does not take in outside air, or does so minimally.
On a hot day, this eases the load on your car's air-conditioning system, which would otherwise have to work doubly hard to cool the cabin.
Car journeys in Singapore are typically short, so there is little chance of you suffering the effects of oxygen depravation if the recirculation button is "on".
But on long drives, up to Malaysia for example, it is good to leave the recirculation button in the "off" position (unlit). This allows fresh air to enter the cabin, which is important if you are in the car for more than an hour at a stretch.
Otherwise, you might feel drowsy.
Some premium cars come with an automatic recirculation switch. It opens the vents intermittently to ensure the cabin air is well-circulated. Typically, but not always, these systems are equipped with air filters too, which mitigate the effects of smells from vehicle exhaust and other unpleasant odours.