Air flow is the most crucial factor in these vehicles, says Lisa Lee, an epidemiologist and public health ethicist at Virginia Tech University. Whether you’re travelling on a train, bus, or plane, you really want to be out of range of people’s exhalations. Ideally, the vehicle you are in will have a lot of air exchange with the outside, and minimal stale air circulation. For travel modes like airplanes, it isn’t so easy to pop open a window to get a bit of fresh air if the person next to you is breathing too much in your direction. Luckily, airplanes these days actually have quite good technology that can bring outside air in to circulate inside the cabin, Lee says. When it comes to a bus or train, you might have a little more leeway in taking circulation into your own hands.