Swelling of the feet during a flight, also known as "flight-related edema," can occur due to several factors:

Prolonged Sitting: When you're seated for extended periods, the circulation in your lower legs can become restricted. This causes fluid to pool in the feet and ankles, leading to swelling.
Cabin Pressure: The lower air pressure inside the cabin can affect your blood circulation, which might also contribute to fluid buildup in your feet and ankles.
Dehydration: Airplane cabins often have low humidity levels, which can lead to dehydration. This can cause your body to retain more fluid, resulting in swelling.
Restricted Movement: Limited movement during a flight prevents your muscles from contracting and helps to circulate blood. This can also contribute to the swelling.

Move Regularly: Stand up, walk around, or perform in-seat exercises like ankle circles and toe lifts to promote circulation. Try to do this every hour or so.
Stay Hydrated: Drink plenty of water throughout the flight to avoid dehydration. Avoid alcohol and caffeinated drinks, as they can lead to dehydration.
Wear Compression Socks: Compression socks can help reduce swelling by improving circulation in your legs and feet. They’re especially useful on long flights.
Elevate Your Feet: If possible, elevate your feet by placing them on a footrest or under your seat to reduce swelling.
Avoid Tight Clothing: Tight socks, shoes, or clothing around your feet and legs can restrict circulation and worsen swelling.
Massage Your Legs: Lightly massaging your feet and calves can help stimulate circulation and reduce fluid retention.