When asked in Quora, this is what someone said about the reason for getting sick while travelling in AC bus or car.
It’s the smell of the car seat covers.
Our nose can easily take on strong scents of the leather covers but it is the brain which processes them in an awkard and haphazard manner. It doesn’t help that the windows are closed.
Further once psychologically inprinted in our brain the next time when you sit in a closed car even a trace of the same smell works as an initiator and causes distress and vomiting.
Even if the smell is not so strong chances are you would still feel distress and vomit over your seat which would then exponentially rise up the distress level and then the cycle of vomiting and breathing would go on untill your intestines empties out of your stomach or somebody cracks a window.
And this is exactly right for many people, while some other might have problem with motion sickness. But for many it might be the smell inside the bus or car which triggers this situation. It's getting very difficult to travel in bus and car, as most of them have AC and windows are fixed there. How could we deal with such situation practically without getting sick?