At one point cars had to have fresh air intakes due to law I believe. There is always some concern of "poisoning" a trapped volume of air inside a car, either with enough people using up the oxygen (unlikely, but I suppose technically possible) or an exhaust leak into the cabin that doesn't get flushed out with fresh air coming in. Old cars don't typically seal that well to begin with, so it likely not really an issue and a very small risk, but I suppose possible.