<human>:  What are the rules for international air travel?
<bot>: All international air travel requires a valid passport and, depending on the destination, may also require a visa. 2 All airlines must comply with the International Air Transport Association's safety standards.3 Passengers must adhere to security measures imposed by the airports, such as going through metal detectors and submitting to personal inspections.4 All passengers must meet the airline's requirements for physical and mental health.5 Some airlines may impose limitations on luggage amount, size and weight per passenger.6 Many airlines put limitations on the amount of liquids in carry-on luggage, usually requiring any containers with over 100ml to be placed in checked baggage. 7 Some destinations may require additional immunizations and health certification.8 Commercial flights usually require the booking of a seat in advance, particularly on popular routes. 9 Regular ticket prices may be charged excesses for certain times of travel, such as holidays or peak periods. 10 For international flights crossing the borders of two countries, the laws of both countries may be applicable.