In a car
Your biggest problem in a car is having the air conditioning or heating on recirculate, because it means you are not taking in any fresh air. The volume of air in a car is tiny and with each exhaled breath containing 40,000ppm of CO₂ the CO₂ level can build up very quickly.
5000ppm in half an hour with only two people in a small car:
With two adults, three children and a dog in an average, family-sized car you can easily get to levels of 8000ppm in a very short time. Quite apart from being unhealthy from an infection point of view, these are very dangerous levels in terms of your cognitive function and sleepiness.
Here at CO₂ Radical, we believe that unrecognised high CO₂ levels in vehicles may well be a significant cause of accidents through causing sleepiness and poor concentration.
In a taxi or an Uber, you should insist on switching recirculate to OFF. Also, it is worth noting that on most modern cars selecting 'Max AC' automatically switches recirculate to ON.
Seriously toxic levels of CO₂ can build up in a car in a very short period of time: