On long flights, with more than four hours of flight time, staying sited the entire time may increase the risks of deep vein thrombosis (DVT), consisting of blood clots forming in the veins of your legs. It is recommended that you exercise in your seat or that you walk in the aisles from time to time when the “seat belt” sign is switch off, to reduce the risks. For passengers with higher risks of contracting DVT, it is recommended that you speak with your doctor before you travel, to be better informed about additional precautions as compression socks or medication.
The air circulating in the aircraft cabin is often dry, due to the low humidity and it may cause a slight discomfort, causing dryness on the lips, nose, eyes (specially the passengers using contact lenses) and skin but the dry atmosphere does not provoke dehydration.
If you feel any discomfort or illness during a flight, you must communicate it to the crew members. They will assist you and take the necessary precautions in order for you the be as safe and comfortable possible.