17.) Breathe In, Breathe Out
The oxygen levels we operate at are at 21% of the air when we are on the ground and get lower the higher up we go. The oxygen levels are kept at the right levels in planes artificially, but many people still find it a little bit hard to breathe, especially if they have a lung condition.
While many travelers just need to relax and do deep breathing exercises to get the oxygen pumping in their blood, some others might need additional oxygen. You can take a hypoxic challenge (or fit to fly test) at the doctor to see how your body responds to air at higher altitudes and in pressured cabins.